zshmisc - Online in the Cloud

PROGRAM:

NAME

zshmisc - everything and then some

SIMPLE COMMANDS & PIPELINES

A simple command is a sequence of optional parameter assignments followed by
blank-separated words, with optional redirections interspersed. For a description of
assignment, see the beginning of zshparam(1).

The first word is the command to be executed, and the remaining words, if any, are
arguments to the command. If a command name is given, the parameter assignments modify
the environment of the command when it is executed. The value of a simple command is its
exit status, or 128 plus the signal number if terminated by a signal. For example,

echo foo

is a simple command with arguments.

A pipeline is either a simple command, or a sequence of two or more simple commands where
each command is separated from the next by `|' or `|&'. Where commands are separated by
`|', the standard output of the first command is connected to the standard input of the
next. `|&' is shorthand for `2>&1 |', which connects both the standard output and the
standard error of the command to the standard input of the next. The value of a pipeline
is the value of the last command, unless the pipeline is preceded by `!' in which case the
value is the logical inverse of the value of the last command. For example,

echo foo | sed 's/foo/bar/'

is a pipeline, where the output (`foo' plus a newline) of the first command will be passed
to the input of the second.

If a pipeline is preceded by `coproc', it is executed as a coprocess; a two-way pipe is
established between it and the parent shell. The shell can read from or write to the
coprocess by means of the `>&p' and `<&p' redirection operators or with `print -p' and
`read -p'. A pipeline cannot be preceded by both `coproc' and `!'. If job control is
active, the coprocess can be treated in other than input and output as an ordinary
background job.

A sublist is either a single pipeline, or a sequence of two or more pipelines separated by
`&&' or `||'. If two pipelines are separated by `&&', the second pipeline is executed
only if the first succeeds (returns a zero status). If two pipelines are separated by
`||', the second is executed only if the first fails (returns a nonzero status). Both
operators have equal precedence and are left associative. The value of the sublist is the
value of the last pipeline executed. For example,

dmesg | grep panic && print yes

is a sublist consisting of two pipelines, the second just a simple command which will be
executed if and only if the grep command returns a zero status. If it does not, the value
of the sublist is that return status, else it is the status returned by the print (almost
certainly zero).

A list is a sequence of zero or more sublists, in which each sublist is terminated by `;',
`&', `&|', `&!', or a newline. This terminator may optionally be omitted from the last
sublist in the list when the list appears as a complex command inside `(...)' or `{...}'.
When a sublist is terminated by `;' or newline, the shell waits for it to finish before
executing the next sublist. If a sublist is terminated by a `&', `&|', or `&!', the shell
executes the last pipeline in it in the background, and does not wait for it to finish
(note the difference from other shells which execute the whole sublist in the background).
A backgrounded pipeline returns a status of zero.

More generally, a list can be seen as a set of any shell commands whatsoever, including
the complex commands below; this is implied wherever the word `list' appears in later
descriptions. For example, the commands in a shell function form a special sort of list.

PRECOMMAND MODIFIERS

A simple command may be preceded by a precommand modifier, which will alter how the
command is interpreted. These modifiers are shell builtin commands with the exception of
nocorrect which is a reserved word.

- The command is executed with a `-' prepended to its argv[0] string.

builtin
The command word is taken to be the name of a builtin command, rather than a shell
function or external command.

command [ -pvV ]
The command word is taken to be the name of an external command, rather than a
shell function or builtin. If the POSIX_BUILTINS option is set, builtins will
also be executed but certain special properties of them are suppressed. The -p flag
causes a default path to be searched instead of that in $path. With the -v flag,
command is similar to whence and with -V, it is equivalent to whence -v.

exec [ -cl ] [ -a argv0 ]
The following command together with any arguments is run in place of the current
process, rather than as a sub-process. The shell does not fork and is replaced.
The shell does not invoke TRAPEXIT, nor does it source zlogout files. The options
are provided for compatibility with other shells.

The -c option clears the environment.

The -l option is equivalent to the - precommand modifier, to treat the replacement
command as a login shell; the command is executed with a - prepended to its argv[0]
string. This flag has no effect if used together with the -a option.

The -a option is used to specify explicitly the argv[0] string (the name of the
command as seen by the process itself) to be used by the replacement command and is
directly equivalent to setting a value for the ARGV0 environment variable.

nocorrect
Spelling correction is not done on any of the words. This must appear before any
other precommand modifier, as it is interpreted immediately, before any parsing is
done. It has no effect in non-interactive shells.

noglob Filename generation (globbing) is not performed on any of the words.

COMPLEX COMMANDS

A complex command in zsh is one of the following:

if list then list [ elif list then list ] ... [ else list ] fi
The if list is executed, and if it returns a zero exit status, the then list is
executed. Otherwise, the elif list is executed and if its status is zero, the then
list is executed. If each elif list returns nonzero status, the else list is
executed.

for name ... [ in word ... ] term do list done
where term is at least one newline or ;. Expand the list of words, and set the
parameter name to each of them in turn, executing list each time. If the in word
is omitted, use the positional parameters instead of the words.

More than one parameter name can appear before the list of words. If N names are
given, then on each execution of the loop the next N words are assigned to the
corresponding parameters. If there are more names than remaining words, the
remaining parameters are each set to the empty string. Execution of the loop ends
when there is no remaining word to assign to the first name. It is only possible
for in to appear as the first name in the list, else it will be treated as marking
the end of the list.

for (( [expr1] ; [expr2] ; [expr3] )) do list done
The arithmetic expression expr1 is evaluated first (see the section `Arithmetic
Evaluation'). The arithmetic expression expr2 is repeatedly evaluated until it
evaluates to zero and when non-zero, list is executed and the arithmetic expression
expr3 evaluated. If any expression is omitted, then it behaves as if it evaluated
to 1.

while list do list done
Execute the do list as long as the while list returns a zero exit status.

until list do list done
Execute the do list as long as until list returns a nonzero exit status.

repeat word do list done
word is expanded and treated as an arithmetic expression, which must evaluate to a
number n. list is then executed n times.

The repeat syntax is disabled by default when the shell starts in a mode emulating
another shell. It can be enabled with the command `enable -r repeat'

case word in [ [(] pattern [ | pattern ] ... ) list (;;|;&|;|) ] ... esac
Execute the list associated with the first pattern that matches word, if any. The
form of the patterns is the same as that used for filename generation. See the
section `Filename Generation'.

Note further that, unless the SH_GLOB option is set, the whole pattern with
alternatives is treated by the shell as equivalent to a group of patterns within
parentheses, although white space may appear about the parentheses and the vertical
bar and will be stripped from the pattern at those points. White space may appear
elsewhere in the pattern; this is not stripped. If the SH_GLOB option is set, so
that an opening parenthesis can be unambiguously treated as part of the case
syntax, the expression is parsed into separate words and these are treated as
strict alternatives (as in other shells).

If the list that is executed is terminated with ;& rather than ;;, the following
list is also executed. The rule for the terminator of the following list ;;, ;& or
;| is applied unless the esac is reached.

If the list that is executed is terminated with ;| the shell continues to scan the
patterns looking for the next match, executing the corresponding list, and applying
the rule for the corresponding terminator ;;, ;& or ;|. Note that word is not
re-expanded; all applicable patterns are tested with the same word.

select name [ in word ... term ] do list done
where term is one or more newline or ; to terminate the words. Print the set of
words, each preceded by a number. If the in word is omitted, use the positional
parameters. The PROMPT3 prompt is printed and a line is read from the line editor
if the shell is interactive and that is active, or else standard input. If this
line consists of the number of one of the listed words, then the parameter name is
set to the word corresponding to this number. If this line is empty, the selection
list is printed again. Otherwise, the value of the parameter name is set to null.
The contents of the line read from standard input is saved in the parameter REPLY.
list is executed for each selection until a break or end-of-file is encountered.

( list )
Execute list in a subshell. Traps set by the trap builtin are reset to their
default values while executing list.

{ list }
Execute list.

{ try-list } always { always-list }
First execute try-list. Regardless of errors, or break, continue, or return
commands encountered within try-list, execute always-list. Execution then
continues from the result of the execution of try-list; in other words, any error,
or break, continue, or return command is treated in the normal way, as if
always-list were not present. The two chunks of code are referred to as the `try
block' and the `always block'.

Optional newlines or semicolons may appear after the always; note, however, that
they may not appear between the preceding closing brace and the always.

An `error' in this context is a condition such as a syntax error which causes the
shell to abort execution of the current function, script, or list. Syntax errors
encountered while the shell is parsing the code do not cause the always-list to be
executed. For example, an erroneously constructed if block in try-list would cause
the shell to abort during parsing, so that always-list would not be executed, while
an erroneous substitution such as ${*foo*} would cause a run-time error, after
which always-list would be executed.

An error condition can be tested and reset with the special integer variable
TRY_BLOCK_ERROR. Outside an always-list the value is irrelevant, but it is
initialised to -1. Inside always-list, the value is 1 if an error occurred in the
try-list, else 0. If TRY_BLOCK_ERROR is set to 0 during the always-list, the error
condition caused by the try-list is reset, and shell execution continues normally
after the end of always-list. Altering the value during the try-list is not useful
(unless this forms part of an enclosing always block).

Regardless of TRY_BLOCK_ERROR, after the end of always-list the normal shell status
$? is the value returned from try-list. This will be non-zero if there was an
error, even if TRY_BLOCK_ERROR was set to zero.

The following executes the given code, ignoring any errors it causes. This is an
alternative to the usual convention of protecting code by executing it in a
subshell.

{
# code which may cause an error
} always {
# This code is executed regardless of the error.
(( TRY_BLOCK_ERROR = 0 ))
}
# The error condition has been reset.

An exit command (or a return command executed at the outermost function level of a
script) encountered in try-list does not cause the execution of always-list.
Instead, the shell exits immediately after any EXIT trap has been executed.

function word ... [ () ] [ term ] { list }
word ... () [ term ] { list }
word ... () [ term ] command
where term is one or more newline or ;. Define a function which is referenced by
any one of word. Normally, only one word is provided; multiple words are usually
only useful for setting traps. The body of the function is the list between the {
and }. See the section `Functions'.

If the option SH_GLOB is set for compatibility with other shells, then whitespace
may appear between the left and right parentheses when there is a single word;
otherwise, the parentheses will be treated as forming a globbing pattern in that
case.

In any of the forms above, a redirection may appear outside the function body, for
example

func() { ... } 2>&1

The redirection is stored with the function and applied whenever the function is
executed. Any variables in the redirection are expanded at the point the function
is executed, but outside the function scope.

time [ pipeline ]
The pipeline is executed, and timing statistics are reported on the standard error
in the form specified by the TIMEFMT parameter. If pipeline is omitted, print
statistics about the shell process and its children.

[[ exp ]]
Evaluates the conditional expression exp and return a zero exit status if it is
true. See the section `Conditional Expressions' for a description of exp.

ALTERNATE FORMS FOR COMPLEX COMMANDS

Many of zsh's complex commands have alternate forms. These are non-standard and are
likely not to be obvious even to seasoned shell programmers; they should not be used
anywhere that portability of shell code is a concern.

The short versions below only work if sublist is of the form `{ list }' or if the
SHORT_LOOPS option is set. For the if, while and until commands, in both these cases the
test part of the loop must also be suitably delimited, such as by `[[ ... ]]' or `(( ...
))', else the end of the test will not be recognized. For the for, repeat, case and
select commands no such special form for the arguments is necessary, but the other
condition (the special form of sublist or use of the SHORT_LOOPS option) still applies.

if list { list } [ elif list { list } ] ... [ else { list } ]
An alternate form of if. The rules mean that

if [[ -o ignorebraces ]] {
print yes
}

works, but

if true { # Does not work!
print yes
}

does not, since the test is not suitably delimited.

if list sublist
A short form of the alternate if. The same limitations on the form of list apply
as for the previous form.

for name ... ( word ... ) sublist
A short form of for.

for name ... [ in word ... ] term sublist
where term is at least one newline or ;. Another short form of for.

for (( [expr1] ; [expr2] ; [expr3] )) sublist
A short form of the arithmetic for command.

foreach name ... ( word ... ) list end
Another form of for.

while list { list }
An alternative form of while. Note the limitations on the form of list mentioned
above.

until list { list }
An alternative form of until. Note the limitations on the form of list mentioned
above.

repeat word sublist
This is a short form of repeat.

case word { [ [(] pattern [ | pattern ] ... ) list (;;|;&|;|) ] ... }
An alternative form of case.

select name [ in word ... term ] sublist
where term is at least one newline or ;. A short form of select.

RESERVED WORDS

The following words are recognized as reserved words when used as the first word of a
command unless quoted or disabled using disable -r:

do done esac then elif else fi for case if while function repeat time until select coproc
nocorrect foreach end ! [[ { } declare export float integer local readonly typeset

Additionally, `}' is recognized in any position if neither the IGNORE_BRACES option nor
the IGNORE_CLOSE_BRACES option is set.

ERRORS

Certain errors are treated as fatal by the shell: in an interactive shell, they cause
control to return to the command line, and in a non-interactive shell they cause the shell
to be aborted. In older versions of zsh, a non-interactive shell running a script would
not abort completely, but would resume execution at the next command to be read from the
script, skipping the remainder of any functions or shell constructs such as loops or
conditions; this somewhat illogical behaviour can be recovered by setting the option
CONTINUE_ON_ERROR.

Fatal errors found in non-interactive shells include:

· Failure to parse shell options passed when invoking the shell

· Failure to change options with the set builtin

· Parse errors of all sorts, including failures to parse mathematical expressions

· Failures to set or modify variable behaviour with typeset, local, declare, export,
integer, float

· Execution of incorrectly positioned loop control structures (continue, break)

· Attempts to use regular expression with no regular expression module available

· Disallowed operations when the RESTRICTED options is set

· Failure to create a pipe needed for a pipeline

· Failure to create a multio

· Failure to autoload a module needed for a declared shell feature

· Errors creating command or process substitutions

· Syntax errors in glob qualifiers

· File generation errors where not caught by the option BAD_PATTERN

· All bad patterns used for matching within case statements

· File generation failures where not caused by NO_MATCH or similar options

· All file generation errors where the pattern was used to create a multio

· Memory errors where detected by the shell

· Invalid subscripts to shell variables

· Attempts to assign read-only variables

· Logical errors with variables such as assignment to the wrong type

· Use of invalid variable names

· Errors in variable substitution syntax

· Failure to convert characters in $'...' expressions

If the POSIX_BUILTINS option is set, more errors associated with shell builtin commands
are treated as fatal, as specified by the POSIX standard.

COMMENTS

In non-interactive shells, or in interactive shells with the INTERACTIVE_COMMENTS option
set, a word beginning with the third character of the histchars parameter (`#' by default)
causes that word and all the following characters up to a newline to be ignored.

ALIASING

Every eligible word in the shell input is checked to see if there is an alias defined for
it. If so, it is replaced by the text of the alias if it is in command position (if it
could be the first word of a simple command), or if the alias is global. If the
replacement text ends with a space, the next word in the shell input is always eligible
for purposes of alias expansion. An alias is defined using the alias builtin; global
aliases may be defined using the -g option to that builtin.

A word is defined as:

· Any plain string or glob pattern

· Any quoted string, using any quoting method (note that the quotes must be part of
the alias definition for this to be eligible)

· Any parameter reference or command substitution

· Any series of the foregoing, concatenated without whitespace or other tokens
between them

· Any reserved word (case, do, else, etc.)

· With global aliasing, any command separator, any redirection operator, and `(' or
`)' when not part of a glob pattern

It is not presently possible to alias the `((' token that introduces arithmetic
expressions, because until a full statement has been parsed, it cannot be distinguished
from two consecutive `(' tokens introducing nested subshells.

When POSIX_ALIASES is set, only plain unquoted strings are eligible for aliasing. The
alias builtin does not reject ineligible aliases, but they are not expanded.

Alias expansion is done on the shell input before any other expansion except history
expansion. Therefore, if an alias is defined for the word foo, alias expansion may be
avoided by quoting part of the word, e.g. \foo. Any form of quoting works, although there
is nothing to prevent an alias being defined for the quoted form such as \foo as well.
Also, if a separator such as && is aliased, \&& turns into the two tokens \& and &, each
of which may have been aliased separately. Similarly for \<<, \>|, etc.

For use with completion, which would remove an initial backslash followed by a character
that isn't special, it may be more convenient to quote the word by starting with a single
quote, i.e. 'foo; completion will automatically add the trailing single quote.

There is a commonly encountered problem with aliases illustrated by the following code:

alias echobar='echo bar'; echobar

This prints a message that the command echobar could not be found. This happens because
aliases are expanded when the code is read in; the entire line is read in one go, so that
when echobar is executed it is too late to expand the newly defined alias. This is often
a problem in shell scripts, functions, and code executed with `source' or `.'.
Consequently, use of functions rather than aliases is recommended in non-interactive code.

Note also the unhelpful interaction of aliases and function definitions:

alias func='noglob func'
func() {
echo Do something with $*
}

Because aliases are expanded in function definitions, this causes the following command to
be executed:

noglob func() {
echo Do something with $*
}

which defines noglob as well as func as functions with the body given. To avoid this,
either quote the name func or use the alternative function definition form `function
func'. Ensuring the alias is defined after the function works but is problematic if the
code fragment might be re-executed.

QUOTING

A character may be quoted (that is, made to stand for itself) by preceding it with a `\'.
`\' followed by a newline is ignored.

A string enclosed between `$'' and `'' is processed the same way as the string arguments
of the print builtin, and the resulting string is considered to be entirely quoted. A
literal `'' character can be included in the string by using the `\'' escape.

All characters enclosed between a pair of single quotes ('') that is not preceded by a `$'
are quoted. A single quote cannot appear within single quotes unless the option RC_QUOTES
is set, in which case a pair of single quotes are turned into a single quote. For
example,

print ''''

outputs nothing apart from a newline if RC_QUOTES is not set, but one single quote if it
is set.

Inside double quotes (""), parameter and command substitution occur, and `\' quotes the
characters `\', ``', `"', and `$'.

REDIRECTION

If a command is followed by & and job control is not active, then the default standard
input for the command is the empty file /dev/null. Otherwise, the environment for the
execution of a command contains the file descriptors of the invoking shell as modified by
input/output specifications.

The following may appear anywhere in a simple command or may precede or follow a complex
command. Expansion occurs before word or digit is used except as noted below. If the
result of substitution on word produces more than one filename, redirection occurs for
each separate filename in turn.

< word Open file word for reading as standard input.

<> word
Open file word for reading and writing as standard input. If the file does not
exist then it is created.

> word Open file word for writing as standard output. If the file does not exist then it
is created. If the file exists, and the CLOBBER option is unset, this causes an
error; otherwise, it is truncated to zero length.

>| word
>! word
Same as >, except that the file is truncated to zero length if it exists, even if
CLOBBER is unset.

>> word
Open file word for writing in append mode as standard output. If the file does not
exist, and the CLOBBER option is unset, this causes an error; otherwise, the file
is created.

>>| word
>>! word
Same as >>, except that the file is created if it does not exist, even if CLOBBER
is unset.

<<[-] word
The shell input is read up to a line that is the same as word, or to an
end-of-file. No parameter expansion, command substitution or filename generation
is performed on word. The resulting document, called a here-document, becomes the
standard input.

If any character of word is quoted with single or double quotes or a `\', no
interpretation is placed upon the characters of the document. Otherwise, parameter
and command substitution occurs, `\' followed by a newline is removed, and `\' must
be used to quote the characters `\', `$', ``' and the first character of word.

Note that word itself does not undergo shell expansion. Backquotes in word do not
have their usual effect; instead they behave similarly to double quotes, except
that the backquotes themselves are passed through unchanged. (This information is
given for completeness and it is not recommended that backquotes be used.) Quotes
in the form $'...' have their standard effect of expanding backslashed references
to special characters.

If <<- is used, then all leading tabs are stripped from word and from the document.

<<< word
Perform shell expansion on word and pass the result to standard input. This is
known as a here-string. Compare the use of word in here-documents above, where
word does not undergo shell expansion.

<& number
>& number
The standard input/output is duplicated from file descriptor number (see dup2(2)).

<& -
>& - Close the standard input/output.

<& p
>& p The input/output from/to the coprocess is moved to the standard input/output.

>& word
&> word
(Except where `>& word' matches one of the above syntaxes; `&>' can always be used
to avoid this ambiguity.) Redirects both standard output and standard error (file
descriptor 2) in the manner of `> word'. Note that this does not have the same
effect as `> word 2>&1' in the presence of multios (see the section below).

>&| word
>&! word
&>| word
&>! word
Redirects both standard output and standard error (file descriptor 2) in the manner
of `>| word'.

>>& word
&>> word
Redirects both standard output and standard error (file descriptor 2) in the manner
of `>> word'.

>>&| word
>>&! word
&>>| word
&>>! word
Redirects both standard output and standard error (file descriptor 2) in the manner
of `>>| word'.

If one of the above is preceded by a digit, then the file descriptor referred to is that
specified by the digit instead of the default 0 or 1. The order in which redirections are
specified is significant. The shell evaluates each redirection in terms of the (file
descriptor, file) association at the time of evaluation. For example:

... 1>fname 2>&1

first associates file descriptor 1 with file fname. It then associates file descriptor 2
with the file associated with file descriptor 1 (that is, fname). If the order of
redirections were reversed, file descriptor 2 would be associated with the terminal
(assuming file descriptor 1 had been) and then file descriptor 1 would be associated with
file fname.

The `|&' command separator described in Simple Commands & Pipelines in zshmisc(1) is a
shorthand for `2>&1 |'.

The various forms of process substitution, `<(list)', and `=(list)' for input and
`>(list)' for output, are often used together with redirection. For example, if word in
an output redirection is of the form `>(list)' then the output is piped to the command
represented by list. See Process Substitution in zshexpn(1).

OPENING FILE DESCRIPTORS USING PARAMETERS

When the shell is parsing arguments to a command, and the shell option IGNORE_BRACES is
not set, a different form of redirection is allowed: instead of a digit before the
operator there is a valid shell identifier enclosed in braces. The shell will open a new
file descriptor that is guaranteed to be at least 10 and set the parameter named by the
identifier to the file descriptor opened. No whitespace is allowed between the closing
brace and the redirection character. For example:

... {myfd}>&1

This opens a new file descriptor that is a duplicate of file descriptor 1 and sets the
parameter myfd to the number of the file descriptor, which will be at least 10. The new
file descriptor can be written to using the syntax >&$myfd.

The syntax {varid}>&-, for example {myfd}>&-, may be used to close a file descriptor
opened in this fashion. Note that the parameter given by varid must previously be set to
a file descriptor in this case.

It is an error to open or close a file descriptor in this fashion when the parameter is
readonly. However, it is not an error to read or write a file descriptor using <&$param
or >&$param if param is readonly.

If the option CLOBBER is unset, it is an error to open a file descriptor using a parameter
that is already set to an open file descriptor previously allocated by this mechanism.
Unsetting the parameter before using it for allocating a file descriptor avoids the error.

Note that this mechanism merely allocates or closes a file descriptor; it does not perform
any redirections from or to it. It is usually convenient to allocate a file descriptor
prior to use as an argument to exec. The syntax does not in any case work when used
around complex commands such as parenthesised subshells or loops, where the opening brace
is interpreted as part of a command list to be executed in the current shell.

The following shows a typical sequence of allocation, use, and closing of a file
descriptor:

integer myfd
exec {myfd}>~/logs/mylogfile.txt
print This is a log message. >&$myfd
exec {myfd}>&-

Note that the expansion of the variable in the expression >&$myfd occurs at the point the
redirection is opened. This is after the expansion of command arguments and after any
redirections to the left on the command line have been processed.

MULTIOS

If the user tries to open a file descriptor for writing more than once, the shell opens
the file descriptor as a pipe to a process that copies its input to all the specified
outputs, similar to tee, provided the MULTIOS option is set, as it is by default. Thus:

date >foo >bar

writes the date to two files, named `foo' and `bar'. Note that a pipe is an implicit
redirection; thus

date >foo | cat

writes the date to the file `foo', and also pipes it to cat.

If the MULTIOS option is set, the word after a redirection operator is also subjected to
filename generation (globbing). Thus

: > *

will truncate all files in the current directory, assuming there's at least one. (Without
the MULTIOS option, it would create an empty file called `*'.) Similarly, you can do

echo exit 0 >> *.sh

If the user tries to open a file descriptor for reading more than once, the shell opens
the file descriptor as a pipe to a process that copies all the specified inputs to its
output in the order specified, similar to cat, provided the MULTIOS option is set. Thus

sort <foo <fubar

or even

sort <f{oo,ubar}

is equivalent to `cat foo fubar | sort'.

Expansion of the redirection argument occurs at the point the redirection is opened, at
the point described above for the expansion of the variable in >&$myfd.

Note that a pipe is an implicit redirection; thus

cat bar | sort <foo

is equivalent to `cat bar foo | sort' (note the order of the inputs).

If the MULTIOS option is unset, each redirection replaces the previous redirection for
that file descriptor. However, all files redirected to are actually opened, so

echo Hello > bar > baz

when MULTIOS is unset will truncate `bar', and write `Hello' into `baz'.

There is a problem when an output multio is attached to an external program. A simple
example shows this:

cat file >file1 >file2
cat file1 file2

Here, it is possible that the second `cat' will not display the full contents of file1 and
file2 (i.e. the original contents of file repeated twice).

The reason for this is that the multios are spawned after the cat process is forked from
the parent shell, so the parent shell does not wait for the multios to finish writing
data. This means the command as shown can exit before file1 and file2 are completely
written. As a workaround, it is possible to run the cat process as part of a job in the
current shell:

{ cat file } >file >file2

Here, the {...} job will pause to wait for both files to be written.

REDIRECTIONS WITH NO COMMAND

When a simple command consists of one or more redirection operators and zero or more
parameter assignments, but no command name, zsh can behave in several ways.

If the parameter NULLCMD is not set or the option CSH_NULLCMD is set, an error is caused.
This is the csh behavior and CSH_NULLCMD is set by default when emulating csh.

If the option SH_NULLCMD is set, the builtin `:' is inserted as a command with the given
redirections. This is the default when emulating sh or ksh.

Otherwise, if the parameter NULLCMD is set, its value will be used as a command with the
given redirections. If both NULLCMD and READNULLCMD are set, then the value of the latter
will be used instead of that of the former when the redirection is an input. The default
for NULLCMD is `cat' and for READNULLCMD is `more'. Thus

< file

shows the contents of file on standard output, with paging if that is a terminal. NULLCMD
and READNULLCMD may refer to shell functions.

COMMAND EXECUTION

If a command name contains no slashes, the shell attempts to locate it. If there exists a
shell function by that name, the function is invoked as described in the section
`Functions'. If there exists a shell builtin by that name, the builtin is invoked.

Otherwise, the shell searches each element of $path for a directory containing an
executable file by that name. If the search is unsuccessful, the shell prints an error
message and returns a nonzero exit status.

If execution fails because the file is not in executable format, and the file is not a
directory, it is assumed to be a shell script. /bin/sh is spawned to execute it. If the
program is a file beginning with `#!', the remainder of the first line specifies an
interpreter for the program. The shell will execute the specified interpreter on
operating systems that do not handle this executable format in the kernel.

If no external command is found but a function command_not_found_handler exists the shell
executes this function with all command line arguments. The function should return status
zero if it successfully handled the command, or non-zero status if it failed. In the
latter case the standard handling is applied: `command not found' is printed to standard
error and the shell exits with status 127. Note that the handler is executed in a
subshell forked to execute an external command, hence changes to directories, shell
parameters, etc. have no effect on the main shell.

FUNCTIONS

Shell functions are defined with the function reserved word or the special syntax
`funcname ()'. Shell functions are read in and stored internally. Alias names are
resolved when the function is read. Functions are executed like commands with the
arguments passed as positional parameters. (See the section `Command Execution'.)

Functions execute in the same process as the caller and share all files and present
working directory with the caller. A trap on EXIT set inside a function is executed after
the function completes in the environment of the caller.

The return builtin is used to return from function calls.

Function identifiers can be listed with the functions builtin. Functions can be undefined
with the unfunction builtin.

AUTOLOADING FUNCTIONS

A function can be marked as undefined using the autoload builtin (or `functions -u' or
`typeset -fu'). Such a function has no body. When the function is first executed, the
shell searches for its definition using the elements of the fpath variable. Thus to
define functions for autoloading, a typical sequence is:

fpath=(~/myfuncs $fpath)
autoload myfunc1 myfunc2 ...

The usual alias expansion during reading will be suppressed if the autoload builtin or its
equivalent is given the option -U. This is recommended for the use of functions supplied
with the zsh distribution. Note that for functions precompiled with the zcompile builtin
command the flag -U must be provided when the .zwc file is created, as the corresponding
information is compiled into the latter.

For each element in fpath, the shell looks for three possible files, the newest of which
is used to load the definition for the function:

element.zwc
A file created with the zcompile builtin command, which is expected to contain the
definitions for all functions in the directory named element. The file is treated
in the same manner as a directory containing files for functions and is searched
for the definition of the function. If the definition is not found, the search
for a definition proceeds with the other two possibilities described below.

If element already includes a .zwc extension (i.e. the extension was explicitly
given by the user), element is searched for the definition of the function without
comparing its age to that of other files; in fact, there does not need to be any
directory named element without the suffix. Thus including an element such as
`/usr/local/funcs.zwc' in fpath will speed up the search for functions, with the
disadvantage that functions included must be explicitly recompiled by hand before
the shell notices any changes.

element/function.zwc
A file created with zcompile, which is expected to contain the definition for
function. It may include other function definitions as well, but those are neither
loaded nor executed; a file found in this way is searched only for the definition
of function.

element/function
A file of zsh command text, taken to be the definition for function.

In summary, the order of searching is, first, in the parents of directories in fpath for
the newer of either a compiled directory or a directory in fpath; second, if more than one
of these contains a definition for the function that is sought, the leftmost in the fpath
is chosen; and third, within a directory, the newer of either a compiled function or an
ordinary function definition is used.

If the KSH_AUTOLOAD option is set, or the file contains only a simple definition of the
function, the file's contents will be executed. This will normally define the function in
question, but may also perform initialization, which is executed in the context of the
function execution, and may therefore define local parameters. It is an error if the
function is not defined by loading the file.

Otherwise, the function body (with no surrounding `funcname() {...}') is taken to be the
complete contents of the file. This form allows the file to be used directly as an
executable shell script. If processing of the file results in the function being
re-defined, the function itself is not re-executed. To force the shell to perform
initialization and then call the function defined, the file should contain initialization
code (which will be executed then discarded) in addition to a complete function definition
(which will be retained for subsequent calls to the function), and a call to the shell
function, including any arguments, at the end.

For example, suppose the autoload file func contains

func() { print This is func; }
print func is initialized

then `func; func' with KSH_AUTOLOAD set will produce both messages on the first call, but
only the message `This is func' on the second and subsequent calls. Without KSH_AUTOLOAD
set, it will produce the initialization message on the first call, and the other message
on the second and subsequent calls.

It is also possible to create a function that is not marked as autoloaded, but which loads
its own definition by searching fpath, by using `autoload -X' within a shell function.
For example, the following are equivalent:

myfunc() {
autoload -X
}
myfunc args...

and

unfunction myfunc # if myfunc was defined
autoload myfunc
myfunc args...

In fact, the functions command outputs `builtin autoload -X' as the body of an autoloaded
function. This is done so that

eval "$(functions)"

produces a reasonable result. A true autoloaded function can be identified by the
presence of the comment `# undefined' in the body, because all comments are discarded from
defined functions.

To load the definition of an autoloaded function myfunc without executing myfunc, use:

autoload +X myfunc

ANONYMOUS FUNCTIONS

If no name is given for a function, it is `anonymous' and is handled specially. Either
form of function definition may be used: a `()' with no preceding name, or a `function'
with an immediately following open brace. The function is executed immediately at the
point of definition and is not stored for future use. The function name is set to
`(anon)'.

Arguments to the function may be specified as words following the closing brace defining
the function, hence if there are none no arguments (other than $0) are set. This is a
difference from the way other functions are parsed: normal function definitions may be
followed by certain keywords such as `else' or `fi', which will be treated as arguments to
anonymous functions, so that a newline or semicolon is needed to force keyword
interpretation.

Note also that the argument list of any enclosing script or function is hidden (as would
be the case for any other function called at this point).

Redirections may be applied to the anonymous function in the same manner as to a
current-shell structure enclosed in braces. The main use of anonymous functions is to
provide a scope for local variables. This is particularly convenient in start-up files as
these do not provide their own local variable scope.

For example,

variable=outside
function {
local variable=inside
print "I am $variable with arguments $*"
} this and that
print "I am $variable"

outputs the following:

I am inside with arguments this and that
I am outside

Note that function definitions with arguments that expand to nothing, for example `name=;
function $name { ... }', are not treated as anonymous functions. Instead, they are
treated as normal function definitions where the definition is silently discarded.

SPECIAL FUNCTIONS

Certain functions, if defined, have special meaning to the shell.

Hook Functions
For the functions below, it is possible to define an array that has the same name as the
function with `_functions' appended. Any element in such an array is taken as the name of
a function to execute; it is executed in the same context and with the same arguments as
the basic function. For example, if $chpwd_functions is an array containing the values
`mychpwd', `chpwd_save_dirstack', then the shell attempts to execute the functions
`chpwd', `mychpwd' and `chpwd_save_dirstack', in that order. Any function that does not
exist is silently ignored. A function found by this mechanism is referred to elsewhere as
a `hook function'. An error in any function causes subsequent functions not to be run.
Note further that an error in a precmd hook causes an immediately following periodic
function not to run (though it may run at the next opportunity).

chpwd Executed whenever the current working directory is changed.

periodic
If the parameter PERIOD is set, this function is executed every $PERIOD seconds,
just before a prompt. Note that if multiple functions are defined using the array
periodic_functions only one period is applied to the complete set of functions, and
the scheduled time is not reset if the list of functions is altered. Hence the set
of functions is always called together.

precmd Executed before each prompt. Note that precommand functions are not re-executed
simply because the command line is redrawn, as happens, for example, when a
notification about an exiting job is displayed.

preexec
Executed just after a command has been read and is about to be executed. If the
history mechanism is active (regardless of whether the line was discarded from the
history buffer), the string that the user typed is passed as the first argument,
otherwise it is an empty string. The actual command that will be executed
(including expanded aliases) is passed in two different forms: the second argument
is a single-line, size-limited version of the command (with things like function
bodies elided); the third argument contains the full text that is being executed.

zshaddhistory
Executed when a history line has been read interactively, but before it is
executed. The sole argument is the complete history line (so that any terminating
newline will still be present).

If any of the hook functions returns status 1 (or any non-zero value other than 2,
though this is not guaranteed for future versions of the shell) the history line
will not be saved, although it lingers in the history until the next line is
executed, allowing you to reuse or edit it immediately.

If any of the hook functions returns status 2 the history line will be saved on the
internal history list, but not written to the history file. In case of a conflict,
the first non-zero status value is taken.

A hook function may call `fc -p ...' to switch the history context so that the
history is saved in a different file from the that in the global HISTFILE
parameter. This is handled specially: the history context is automatically
restored after the processing of the history line is finished.

The following example function works with one of the options INC_APPEND_HISTORY or
SHARE_HISTORY set, in order that the line is written out immediately after the
history entry is added. It first adds the history line to the normal history with
the newline stripped, which is usually the correct behaviour. Then it switches the
history context so that the line will be written to a history file in the current
directory.

zshaddhistory() {
print -sr -- ${1%%$'\n'}
fc -p .zsh_local_history
}

zshexit
Executed at the point where the main shell is about to exit normally. This is not
called by exiting subshells, nor when the exec precommand modifier is used before
an external command. Also, unlike TRAPEXIT, it is not called when functions exit.

Trap Functions
The functions below are treated specially but do not have corresponding hook arrays.

TRAPNAL
If defined and non-null, this function will be executed whenever the shell catches
a signal SIGNAL, where NAL is a signal name as specified for the kill builtin. The
signal number will be passed as the first parameter to the function.

If a function of this form is defined and null, the shell and processes spawned by
it will ignore SIGNAL.

The return status from the function is handled specially. If it is zero, the
signal is assumed to have been handled, and execution continues normally.
Otherwise, the shell will behave as interrupted except that the return status of
the trap is retained.

Programs terminated by uncaught signals typically return the status 128 plus the
signal number. Hence the following causes the handler for SIGINT to print a
message, then mimic the usual effect of the signal.

TRAPINT() {
print "Caught SIGINT, aborting."
return $(( 128 + $1 ))
}

The functions TRAPZERR, TRAPDEBUG and TRAPEXIT are never executed inside other
traps.

TRAPDEBUG
If the option DEBUG_BEFORE_CMD is set (as it is by default), executed before each
command; otherwise executed after each command. See the description of the trap
builtin in zshbuiltins(1) for details of additional features provided in debug
traps.

TRAPEXIT
Executed when the shell exits, or when the current function exits if defined inside
a function. The value of $? at the start of execution is the exit status of the
shell or the return status of the function exiting.

TRAPZERR
Executed whenever a command has a non-zero exit status. However, the function is
not executed if the command occurred in a sublist followed by `&&' or `||'; only
the final command in a sublist of this type causes the trap to be executed. The
function TRAPERR acts the same as TRAPZERR on systems where there is no SIGERR
(this is the usual case).

The functions beginning `TRAP' may alternatively be defined with the trap builtin: this
may be preferable for some uses. Setting a trap with one form removes any trap of the
other form for the same signal; removing a trap in either form removes all traps for the
same signal. The forms

TRAPNAL() {
# code
}

('function traps') and

trap '
# code
' NAL

('list traps') are equivalent in most ways, the exceptions being the following:

· Function traps have all the properties of normal functions, appearing in the list
of functions and being called with their own function context rather than the
context where the trap was triggered.

· The return status from function traps is special, whereas a return from a list trap
causes the surrounding context to return with the given status.

· Function traps are not reset within subshells, in accordance with zsh behaviour;
list traps are reset, in accordance with POSIX behaviour.

JOBS

If the MONITOR option is set, an interactive shell associates a job with each pipeline.
It keeps a table of current jobs, printed by the jobs command, and assigns them small
integer numbers. When a job is started asynchronously with `&', the shell prints a line
to standard error which looks like:

[1] 1234

indicating that the job which was started asynchronously was job number 1 and had one
(top-level) process, whose process ID was 1234.

If a job is started with `&|' or `&!', then that job is immediately disowned. After
startup, it does not have a place in the job table, and is not subject to the job control
features described here.

If you are running a job and wish to do something else you may hit the key ^Z (control-Z)
which sends a TSTP signal to the current job: this key may be redefined by the susp
option of the external stty command. The shell will then normally indicate that the job
has been `suspended', and print another prompt. You can then manipulate the state of this
job, putting it in the background with the bg command, or run some other commands and then
eventually bring the job back into the foreground with the foreground command fg. A ^Z
takes effect immediately and is like an interrupt in that pending output and unread input
are discarded when it is typed.

A job being run in the background will suspend if it tries to read from the terminal.

Note that if the job running in the foreground is a shell function, then suspending it
will have the effect of causing the shell to fork. This is necessary to separate the
function's state from that of the parent shell performing the job control, so that the
latter can return to the command line prompt. As a result, even if fg is used to continue
the job the function will no longer be part of the parent shell, and any variables set by
the function will not be visible in the parent shell. Thus the behaviour is different
from the case where the function was never suspended. Zsh is different from many other
shells in this regard.

The same behaviour is found when the shell is executing code as the right hand side of a
pipeline or any complex shell construct such as if, for, etc., in order that the entire
block of code can be managed as a single job. Background jobs are normally allowed to
produce output, but this can be disabled by giving the command `stty tostop'. If you set
this tty option, then background jobs will suspend when they try to produce output like
they do when they try to read input.

When a command is suspended and continued later with the fg or wait builtins, zsh restores
tty modes that were in effect when it was suspended. This (intentionally) does not apply
if the command is continued via `kill -CONT', nor when it is continued with bg.

There are several ways to refer to jobs in the shell. A job can be referred to by the
process ID of any process of the job or by one of the following:

%number
The job with the given number.
%string
Any job whose command line begins with string.
%?string
Any job whose command line contains string.
%% Current job.
%+ Equivalent to `%%'.
%- Previous job.

The shell learns immediately whenever a process changes state. It normally informs you
whenever a job becomes blocked so that no further progress is possible. If the NOTIFY
option is not set, it waits until just before it prints a prompt before it informs you.
All such notifications are sent directly to the terminal, not to the standard output or
standard error.

When the monitor mode is on, each background job that completes triggers any trap set for
CHLD.

When you try to leave the shell while jobs are running or suspended, you will be warned
that `You have suspended (running) jobs'. You may use the jobs command to see what they
are. If you do this or immediately try to exit again, the shell will not warn you a
second time; the suspended jobs will be terminated, and the running jobs will be sent a
SIGHUP signal, if the HUP option is set.

To avoid having the shell terminate the running jobs, either use the nohup command (see
nohup(1)) or the disown builtin.

SIGNALS

The INT and QUIT signals for an invoked command are ignored if the command is followed by
`&' and the MONITOR option is not active. The shell itself always ignores the QUIT
signal. Otherwise, signals have the values inherited by the shell from its parent (but
see the TRAPNAL special functions in the section `Functions').

Certain jobs are run asynchronously by the shell other than those explicitly put into the
background; even in cases where the shell would usually wait for such jobs, an explicit
exit command or exit due to the option ERR_EXIT will cause the shell to exit without
waiting. Examples of such asynchronous jobs are process substitution, see the section
PROCESS SUBSTITUTION in the zshexpn(1) manual page, and the handler processes for multios,
see the section MULTIOS in the zshmisc(1) manual page.

ARITHMETIC EVALUATION

The shell can perform integer and floating point arithmetic, either using the builtin let,
or via a substitution of the form $((...)). For integers, the shell is usually compiled
to use 8-byte precision where this is available, otherwise precision is 4 bytes. This can
be tested, for example, by giving the command `print - $(( 12345678901 ))'; if the number
appears unchanged, the precision is at least 8 bytes. Floating point arithmetic always
uses the `double' type with whatever corresponding precision is provided by the compiler
and the library.

The let builtin command takes arithmetic expressions as arguments; each is evaluated
separately. Since many of the arithmetic operators, as well as spaces, require quoting,
an alternative form is provided: for any command which begins with a `((', all the
characters until a matching `))' are treated as a quoted expression and arithmetic
expansion performed as for an argument of let. More precisely, `((...))' is equivalent to
`let "..."'. The return status is 0 if the arithmetic value of the expression is
non-zero, 1 if it is zero, and 2 if an error occurred.

For example, the following statement

(( val = 2 + 1 ))

is equivalent to

let "val = 2 + 1"

both assigning the value 3 to the shell variable val and returning a zero status.

Integers can be in bases other than 10. A leading `0x' or `0X' denotes hexadecimal and a
leading `0b' or `0B' binary. Integers may also be of the form `base#n', where base is a
decimal number between two and thirty-six representing the arithmetic base and n is a
number in that base (for example, `16#ff' is 255 in hexadecimal). The base# may also be
omitted, in which case base 10 is used. For backwards compatibility the form `[base]n' is
also accepted.

An integer expression or a base given in the form `base#n' may contain underscores (`_')
after the leading digit for visual guidance; these are ignored in computation. Examples
are 1_000_000 or 0xffff_ffff which are equivalent to 1000000 and 0xffffffff respectively.

It is also possible to specify a base to be used for output in the form `[#base]', for
example `[#16]'. This is used when outputting arithmetical substitutions or when
assigning to scalar parameters, but an explicitly defined integer or floating point
parameter will not be affected. If an integer variable is implicitly defined by an
arithmetic expression, any base specified in this way will be set as the variable's output
arithmetic base as if the option `-i base' to the typeset builtin had been used. The
expression has no precedence and if it occurs more than once in a mathematical expression,
the last encountered is used. For clarity it is recommended that it appear at the
beginning of an expression. As an example:

typeset -i 16 y
print $(( [#8] x = 32, y = 32 ))
print $x $y

outputs first `8#40', the rightmost value in the given output base, and then `8#40 16#20',
because y has been explicitly declared to have output base 16, while x (assuming it does
not already exist) is implicitly typed by the arithmetic evaluation, where it acquires the
output base 8.

The base may be replaced or followed by an underscore, which may itself be followed by a
positive integer (if it is missing the value 3 is used). This indicates that underscores
should be inserted into the output string, grouping the number for visual clarity. The
following integer specifies the number of digits to group together. For example:

setopt cbases
print $(( [#16_4] 65536 ** 2 ))

outputs `0x1_0000_0000'.

The feature can be used with floating point numbers, in which case the base must be
omitted; grouping is away from the decimal point. For example,

zmodload zsh/mathfunc
print $(( [#_] sqrt(1e7) ))

outputs `3_162.277_660_168_379_5' (the number of decimal places shown may vary).

If the C_BASES option is set, hexadecimal numbers are output in the standard C format, for
example `0xFF' instead of the usual `16#FF'. If the option OCTAL_ZEROES is also set (it
is not by default), octal numbers will be treated similarly and hence appear as `077'
instead of `8#77'. This option has no effect on the output of bases other than
hexadecimal and octal, and these formats are always understood on input.

When an output base is specified using the `[#base]' syntax, an appropriate base prefix
will be output if necessary, so that the value output is valid syntax for input. If the #
is doubled, for example `[##16]', then no base prefix is output.

Floating point constants are recognized by the presence of a decimal point or an exponent.
The decimal point may be the first character of the constant, but the exponent character e
or E may not, as it will be taken for a parameter name. All numeric parts (before and
after the decimal point and in the exponent) may contain underscores after the leading
digit for visual guidance; these are ignored in computation.

An arithmetic expression uses nearly the same syntax and associativity of expressions as
in C.

In the native mode of operation, the following operators are supported (listed in
decreasing order of precedence):

+ - ! ~ ++ --
unary plus/minus, logical NOT, complement, {pre,post}{in,de}crement
<< >> bitwise shift left, right
& bitwise AND
^ bitwise XOR
| bitwise OR
** exponentiation
* / % multiplication, division, modulus (remainder)
+ - addition, subtraction
< > <= >=
comparison
== != equality and inequality
&& logical AND
|| ^^ logical OR, XOR
? : ternary operator
= += -= *= /= %= &= ^= |= <<= >>= &&= ||= ^^= **=
assignment
, comma operator

The operators `&&', `||', `&&=', and `||=' are short-circuiting, and only one of the
latter two expressions in a ternary operator is evaluated. Note the precedence of the
bitwise AND, OR, and XOR operators.

With the option C_PRECEDENCES the precedences (but no other properties) of the operators
are altered to be the same as those in most other languages that support the relevant
operators:

+ - ! ~ ++ --
unary plus/minus, logical NOT, complement, {pre,post}{in,de}crement
** exponentiation
* / % multiplication, division, modulus (remainder)
+ - addition, subtraction
<< >> bitwise shift left, right
< > <= >=
comparison
== != equality and inequality
& bitwise AND
^ bitwise XOR
| bitwise OR
&& logical AND
^^ logical XOR
|| logical OR
? : ternary operator
= += -= *= /= %= &= ^= |= <<= >>= &&= ||= ^^= **=
assignment
, comma operator

Note the precedence of exponentiation in both cases is below that of unary operators,
hence `-3**2' evaluates as `9', not `-9'. Use parentheses where necessary: `-(3**2)'.
This is for compatibility with other shells.

Mathematical functions can be called with the syntax `func(args)', where the function
decides if the args is used as a string or a comma-separated list of arithmetic
expressions. The shell currently defines no mathematical functions by default, but the
module zsh/mathfunc may be loaded with the zmodload builtin to provide standard floating
point mathematical functions.

An expression of the form `##x' where x is any character sequence such as `a', `^A', or
`\M-\C-x' gives the value of this character and an expression of the form `#name' gives
the value of the first character of the contents of the parameter name. Character values
are according to the character set used in the current locale; for multibyte character
handling the option MULTIBYTE must be set. Note that this form is different from
`$#name', a standard parameter substitution which gives the length of the parameter name.
`#\' is accepted instead of `##', but its use is deprecated.

Named parameters and subscripted arrays can be referenced by name within an arithmetic
expression without using the parameter expansion syntax. For example,

((val2 = val1 * 2))

assigns twice the value of $val1 to the parameter named val2.

An internal integer representation of a named parameter can be specified with the integer
builtin. Arithmetic evaluation is performed on the value of each assignment to a named
parameter declared integer in this manner. Assigning a floating point number to an
integer results in rounding down to the next integer.

Likewise, floating point numbers can be declared with the float builtin; there are two
types, differing only in their output format, as described for the typeset builtin. The
output format can be bypassed by using arithmetic substitution instead of the parameter
substitution, i.e. `${float}' uses the defined format, but `$((float))' uses a generic
floating point format.

Promotion of integer to floating point values is performed where necessary. In addition,
if any operator which requires an integer (`~', `&', `|', `^', `%', `<<', `>>' and their
equivalents with assignment) is given a floating point argument, it will be silently
rounded down to the next integer.

Users should beware that, in common with many other programming languages but not software
designed for calculation, the evaluation of an expression in zsh is taken a term at a time
and promotion of integers to floating point does not occur in terms only containing
integers. A typical result of this is that a division such as 6/8 is truncated, in this
being rounded down to 0. The FORCE_FLOAT shell option can be used in scripts or functions
where floating point evaluation is required throughout.

Scalar variables can hold integer or floating point values at different times; there is no
memory of the numeric type in this case.

If a variable is first assigned in a numeric context without previously being declared, it
will be implicitly typed as integer or float and retain that type either until the type is
explicitly changed or until the end of the scope. This can have unforeseen consequences.
For example, in the loop

for (( f = 0; f < 1; f += 0.1 )); do
# use $f
done

if f has not already been declared, the first assignment will cause it to be created as an
integer, and consequently the operation `f += 0.1' will always cause the result to be
truncated to zero, so that the loop will fail. A simple fix would be to turn the
initialization into `f = 0.0'. It is therefore best to declare numeric variables with
explicit types.

CONDITIONAL EXPRESSIONS

A conditional expression is used with the [[ compound command to test attributes of files
and to compare strings. Each expression can be constructed from one or more of the
following unary or binary expressions:

-a file
true if file exists.

-b file
true if file exists and is a block special file.

-c file
true if file exists and is a character special file.

-d file
true if file exists and is a directory.

-e file
true if file exists.

-f file
true if file exists and is a regular file.

-g file
true if file exists and has its setgid bit set.

-h file
true if file exists and is a symbolic link.

-k file
true if file exists and has its sticky bit set.

-n string
true if length of string is non-zero.

-o option
true if option named option is on. option may be a single character, in which case
it is a single letter option name. (See the section `Specifying Options'.)

-p file
true if file exists and is a FIFO special file (named pipe).

-r file
true if file exists and is readable by current process.

-s file
true if file exists and has size greater than zero.

-t fd true if file descriptor number fd is open and associated with a terminal device.
(note: fd is not optional)

-u file
true if file exists and has its setuid bit set.

-w file
true if file exists and is writable by current process.

-x file
true if file exists and is executable by current process. If file exists and is a
directory, then the current process has permission to search in the directory.

-z string
true if length of string is zero.

-L file
true if file exists and is a symbolic link.

-O file
true if file exists and is owned by the effective user ID of this process.

-G file
true if file exists and its group matches the effective group ID of this process.

-S file
true if file exists and is a socket.

-N file
true if file exists and its access time is not newer than its modification time.

file1 -nt file2
true if file1 exists and is newer than file2.

file1 -ot file2
true if file1 exists and is older than file2.

file1 -ef file2
true if file1 and file2 exist and refer to the same file.

string = pattern
string == pattern
true if string matches pattern. The `==' form is the preferred one. The `=' form
is for backward compatibility and should be considered obsolete.

string != pattern
true if string does not match pattern.

string =~ regexp
true if string matches the regular expression regexp. If the option RE_MATCH_PCRE
is set regexp is tested as a PCRE regular expression using the zsh/pcre module,
else it is tested as a POSIX extended regular expression using the zsh/regex
module. Upon successful match, some variables will be updated; no variables are
changed if the matching fails.

If the option BASH_REMATCH is not set the scalar parameter MATCH is set to the
substring that matched the pattern and the integer parameters MBEGIN and MEND to
the index of the start and end, respectively, of the match in string, such that if
string is contained in variable var the expression `${var[$MBEGIN,$MEND]}' is
identical to `$MATCH'. The setting of the option KSH_ARRAYS is respected.
Likewise, the array match is set to the substrings that matched parenthesised
subexpressions and the arrays mbegin and mend to the indices of the start and end
positions, respectively, of the substrings within string. The arrays are not set
if there were no parenthesised subexpresssions. For example, if the string `a
short string' is matched against the regular expression `s(...)t', then (assuming
the option KSH_ARRAYS is not set) MATCH, MBEGIN and MEND are `short', 3 and 7,
respectively, while match, mbegin and mend are single entry arrays containing the
strings `hor', `4' and `6', respectively.

If the option BASH_REMATCH is set the array BASH_REMATCH is set to the substring
that matched the pattern followed by the substrings that matched parenthesised
subexpressions within the pattern.

string1 < string2
true if string1 comes before string2 based on ASCII value of their characters.

string1 > string2
true if string1 comes after string2 based on ASCII value of their characters.

exp1 -eq exp2
true if exp1 is numerically equal to exp2. Note that for purely numeric
comparisons use of the ((...)) builtin described in the section `ARITHMETIC
EVALUATION' is more convenient than conditional expressions.

exp1 -ne exp2
true if exp1 is numerically not equal to exp2.

exp1 -lt exp2
true if exp1 is numerically less than exp2.

exp1 -gt exp2
true if exp1 is numerically greater than exp2.

exp1 -le exp2
true if exp1 is numerically less than or equal to exp2.

exp1 -ge exp2
true if exp1 is numerically greater than or equal to exp2.

( exp )
true if exp is true.

! exp true if exp is false.

exp1 && exp2
true if exp1 and exp2 are both true.

exp1 || exp2
true if either exp1 or exp2 is true.

For compatibility, if there is a single argument that is not syntactically significant,
typically a variable, the condition is treated as a test for whether the expression
expands as a string of non-zero length. In other words, [[ $var ]] is the same as [[ -n
$var ]]. It is recommended that the second, explicit, form be used where possible.

Normal shell expansion is performed on the file, string and pattern arguments, but the
result of each expansion is constrained to be a single word, similar to the effect of
double quotes.

Filename generation is not performed on any form of argument to conditions. However, it
can be forced in any case where normal shell expansion is valid and when the option
EXTENDED_GLOB is in effect by using an explicit glob qualifier of the form (#q) at the end
of the string. A normal glob qualifier expression may appear between the `q' and the
closing parenthesis; if none appears the expression has no effect beyond causing filename
generation. The results of filename generation are joined together to form a single word,
as with the results of other forms of expansion.

This special use of filename generation is only available with the [[ syntax. If the
condition occurs within the [ or test builtin commands then globbing occurs instead as
part of normal command line expansion before the condition is evaluated. In this case it
may generate multiple words which are likely to confuse the syntax of the test command.

For example,

[[ -n file*(#qN) ]]

produces status zero if and only if there is at least one file in the current directory
beginning with the string `file'. The globbing qualifier N ensures that the expression is
empty if there is no matching file.

Pattern metacharacters are active for the pattern arguments; the patterns are the same as
those used for filename generation, see zshexpn(1), but there is no special behaviour of
`/' nor initial dots, and no glob qualifiers are allowed.

In each of the above expressions, if file is of the form `/dev/fd/n', where n is an
integer, then the test applied to the open file whose descriptor number is n, even if the
underlying system does not support the /dev/fd directory.

In the forms which do numeric comparison, the expressions exp undergo arithmetic expansion
as if they were enclosed in $((...)).

For example, the following:

[[ ( -f foo || -f bar ) && $report = y* ]] && print File exists.

tests if either file foo or file bar exists, and if so, if the value of the parameter
report begins with `y'; if the complete condition is true, the message `File exists.' is
printed.

EXPANSION OF PROMPT SEQUENCES

Prompt sequences undergo a special form of expansion. This type of expansion is also
available using the -P option to the print builtin.

If the PROMPT_SUBST option is set, the prompt string is first subjected to parameter
expansion, command substitution and arithmetic expansion. See zshexpn(1).

Certain escape sequences may be recognised in the prompt string.

If the PROMPT_BANG option is set, a `!' in the prompt is replaced by the current history
event number. A literal `!' may then be represented as `!!'.

If the PROMPT_PERCENT option is set, certain escape sequences that start with `%' are
expanded. Many escapes are followed by a single character, although some of these take an
optional integer argument that should appear between the `%' and the next character of the
sequence. More complicated escape sequences are available to provide conditional
expansion.

SIMPLE PROMPT ESCAPES

Special characters
%% A `%'.

%) A `)'.

Login information
%l The line (tty) the user is logged in on, without `/dev/' prefix. If the name
starts with `/dev/tty', that prefix is stripped.

%M The full machine hostname.

%m The hostname up to the first `.'. An integer may follow the `%' to specify how
many components of the hostname are desired. With a negative integer, trailing
components of the hostname are shown.

%n $USERNAME.

%y The line (tty) the user is logged in on, without `/dev/' prefix. This does not
treat `/dev/tty' names specially.

Shell state
%# A `#' if the shell is running with privileges, a `%' if not. Equivalent to
`%(!.#.%%)'. The definition of `privileged', for these purposes, is that either
the effective user ID is zero, or, if POSIX.1e capabilities are supported, that at
least one capability is raised in either the Effective or Inheritable capability
vectors.

%? The return status of the last command executed just before the prompt.

%_ The status of the parser, i.e. the shell constructs (like `if' and `for') that have
been started on the command line. If given an integer number that many strings will
be printed; zero or negative or no integer means print as many as there are. This
is most useful in prompts PS2 for continuation lines and PS4 for debugging with the
XTRACE option; in the latter case it will also work non-interactively.

%^ The status of the parser in reverse. This is the same as `%_' other than the order
of strings. It is often used in RPS2.

%d
%/ Current working directory. If an integer follows the `%', it specifies a number of
trailing components of the current working directory to show; zero means the whole
path. A negative integer specifies leading components, i.e. %-1d specifies the
first component.

%~ As %d and %/, but if the current working directory starts with $HOME, that part is
replaced by a `~'. Furthermore, if it has a named directory as its prefix, that
part is replaced by a `~' followed by the name of the directory, but only if the
result is shorter than the full path; see Dynamic and Static named directories in
zshexpn(1).

%e Evaluation depth of the current sourced file, shell function, or eval. This is
incremented or decremented every time the value of %N is set or reverted to a
previous value, respectively. This is most useful for debugging as part of $PS4.

%h
%! Current history event number.

%i The line number currently being executed in the script, sourced file, or shell
function given by %N. This is most useful for debugging as part of $PS4.

%I The line number currently being executed in the file %x. This is similar to %i,
but the line number is always a line number in the file where the code was defined,
even if the code is a shell function.

%j The number of jobs.

%L The current value of $SHLVL.

%N The name of the script, sourced file, or shell function that zsh is currently
executing, whichever was started most recently. If there is none, this is
equivalent to the parameter $0. An integer may follow the `%' to specify a number
of trailing path components to show; zero means the full path. A negative integer
specifies leading components.

%x The name of the file containing the source code currently being executed. This
behaves as %N except that function and eval command names are not shown, instead
the file where they were defined.

%c
%.
%C Trailing component of the current working directory. An integer may follow the `%'
to get more than one component. Unless `%C' is used, tilde contraction is
performed first. These are deprecated as %c and %C are equivalent to %1~ and %1/,
respectively, while explicit positive integers have the same effect as for the
latter two sequences.

Date and time
%D The date in yy-mm-dd format.

%T Current time of day, in 24-hour format.

%t
%@ Current time of day, in 12-hour, am/pm format.

%* Current time of day in 24-hour format, with seconds.

%w The date in day-dd format.

%W The date in mm/dd/yy format.

%D{string}
string is formatted using the strftime function. See strftime(3) for more details.
Various zsh extensions provide numbers with no leading zero or space if the number
is a single digit:

%f a day of the month
%K the hour of the day on the 24-hour clock
%L the hour of the day on the 12-hour clock

In addition, if the system supports the POSIX gettimeofday system call, %. provides
decimal fractions of a second since the epoch with leading zeroes. By default
three decimal places are provided, but a number of digits up to 6 may be given
following the %; hence %6. outputs microseconds. A typical example of this is the
format `%D{%H:%M:%S.%.}'.

The GNU extension that a `-' between the % and the format character causes a
leading zero or space to be stripped is handled directly by the shell for the
format characters d, f, H, k, l, m, M, S and y; any other format characters are
provided to the system's strftime(3) with any leading `-' present, so the handling
is system dependent. Further GNU (or other) extensions are also passed to
strftime(3) and may work if the system supports them.

Visual effects
%B (%b)
Start (stop) boldface mode.

%E Clear to end of line.

%U (%u)
Start (stop) underline mode.

%S (%s)
Start (stop) standout mode.

%F (%f)
Start (stop) using a different foreground colour, if supported by the terminal.
The colour may be specified two ways: either as a numeric argument, as normal, or
by a sequence in braces following the %F, for example %F{red}. In the latter case
the values allowed are as described for the fg zle_highlight attribute; see
Character Highlighting in zshzle(1). This means that numeric colours are allowed
in the second format also.

%K (%k)
Start (stop) using a different bacKground colour. The syntax is identical to that
for %F and %f.

%{...%}
Include a string as a literal escape sequence. The string within the braces should
not change the cursor position. Brace pairs can nest.

A positive numeric argument between the % and the { is treated as described for %G
below.

%G Within a %{...%} sequence, include a `glitch': that is, assume that a single
character width will be output. This is useful when outputting characters that
otherwise cannot be correctly handled by the shell, such as the alternate character
set on some terminals. The characters in question can be included within a %{...%}
sequence together with the appropriate number of %G sequences to indicate the
correct width. An integer between the `%' and `G' indicates a character width
other than one. Hence %{seq%2G%} outputs seq and assumes it takes up the width of
two standard characters.

Multiple uses of %G accumulate in the obvious fashion; the position of the %G is
unimportant. Negative integers are not handled.

Note that when prompt truncation is in use it is advisable to divide up output into
single characters within each %{...%} group so that the correct truncation point
can be found.

CONDITIONAL SUBSTRINGS IN PROMPTS

%v The value of the first element of the psvar array parameter. Following the `%'
with an integer gives that element of the array. Negative integers count from the
end of the array.

%(x.true-text.false-text)
Specifies a ternary expression. The character following the x is arbitrary; the
same character is used to separate the text for the `true' result from that for the
`false' result. This separator may not appear in the true-text, except as part of
a %-escape sequence. A `)' may appear in the false-text as `%)'. true-text and
false-text may both contain arbitrarily-nested escape sequences, including further
ternary expressions.

The left parenthesis may be preceded or followed by a positive integer n, which
defaults to zero. A negative integer will be multiplied by -1, except as noted
below for `l'. The test character x may be any of the following:

! True if the shell is running with privileges.
# True if the effective uid of the current process is n.
? True if the exit status of the last command was n.
_ True if at least n shell constructs were started.
C
/ True if the current absolute path has at least n elements relative to the
root directory, hence / is counted as 0 elements.
c
.
~ True if the current path, with prefix replacement, has at least n elements
relative to the root directory, hence / is counted as 0 elements.
D True if the month is equal to n (January = 0).
d True if the day of the month is equal to n.
e True if the evaluation depth is at least n.
g True if the effective gid of the current process is n.
j True if the number of jobs is at least n.
L True if the SHLVL parameter is at least n.
l True if at least n characters have already been printed on the current line.
When n is negative, true if at least abs(n) characters remain before the
opposite margin (thus the left margin for RPROMPT).
S True if the SECONDS parameter is at least n.
T True if the time in hours is equal to n.
t True if the time in minutes is equal to n.
v True if the array psvar has at least n elements.
V True if element n of the array psvar is set and non-empty.
w True if the day of the week is equal to n (Sunday = 0).

%<string<
%>string>
%[xstring]
Specifies truncation behaviour for the remainder of the prompt string. The third,
deprecated, form is equivalent to `%xstringx', i.e. x may be `<' or `>'. The
string will be displayed in place of the truncated portion of any string; note this
does not undergo prompt expansion.

The numeric argument, which in the third form may appear immediately after the `[',
specifies the maximum permitted length of the various strings that can be displayed
in the prompt. In the first two forms, this numeric argument may be negative, in
which case the truncation length is determined by subtracting the absolute value of
the numeric argument from the number of character positions remaining on the
current prompt line. If this results in a zero or negative length, a length of 1
is used. In other words, a negative argument arranges that after truncation at
least n characters remain before the right margin (left margin for RPROMPT).

The forms with `<' truncate at the left of the string, and the forms with `>'
truncate at the right of the string. For example, if the current directory is
`/home/pike', the prompt `%8<..<%/' will expand to `..e/pike'. In this string, the
terminating character (`<', `>' or `]'), or in fact any character, may be quoted by
a preceding `\'; note when using print -P, however, that this must be doubled as
the string is also subject to standard print processing, in addition to any
backslashes removed by a double quoted string: the worst case is therefore `print
-P "%<\\\\<<..."'.

If the string is longer than the specified truncation length, it will appear in
full, completely replacing the truncated string.

The part of the prompt string to be truncated runs to the end of the string, or to
the end of the next enclosing group of the `%(' construct, or to the next
truncation encountered at the same grouping level (i.e. truncations inside a `%('
are separate), which ever comes first. In particular, a truncation with argument
zero (e.g., `%<<') marks the end of the range of the string to be truncated while
turning off truncation from there on. For example, the prompt `%10<...<%~%<<%# '
will print a truncated representation of the current directory, followed by a `%'
or `#', followed by a space. Without the `%<<', those two characters would be
included in the string to be truncated. Note that `%-0<<' is not equivalent to
`%<<' but specifies that the prompt is truncated at the right margin.

Truncation applies only within each individual line of the prompt, as delimited by
embedded newlines (if any). If the total length of any line of the prompt after
truncation is greater than the terminal width, or if the part to be truncated
contains embedded newlines, truncation behavior is undefined and may change in a
future version of the shell. Use `%-n(l.true-text.false-text)' to remove parts of
the prompt when the available space is less than n.

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