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PROGRAM:
NAME
echo — write arguments to standard output
SYNOPSIS
echo [string...]
DESCRIPTION
The echo utility writes its arguments to standard output, followed by a <newline>. If
there are no arguments, only the <newline> is written.
OPTIONS
The echo utility shall not recognize the "−−" argument in the manner specified by
Guideline 10 of the Base Definitions volume of POSIX.1‐2008, Section 12.2, Utility Syntax
Guidelines; "−−" shall be recognized as a string operand.
Implementations shall not support any options.
OPERANDS
The following operands shall be supported:
string A string to be written to standard output. If the first operand is −n, or if any
of the operands contain a <backslash> character, the results are implementation-
defined.
On XSI-conformant systems, if the first operand is −n, it shall be treated as a
string, not an option. The following character sequences shall be recognized on
XSI-conformant systems within any of the arguments:
\a Write an <alert>.
\b Write a <backspace>.
\c Suppress the <newline> that otherwise follows the final argument in the
output. All characters following the '\c' in the arguments shall be
ignored.
\f Write a <form-feed>.
\n Write a <newline>.
\r Write a <carriage-return>.
\t Write a <tab>.
\v Write a <vertical-tab>.
\\ Write a <backslash> character.
\0num Write an 8-bit value that is the zero, one, two, or three-digit octal
number num.
STDIN
Not used.
INPUT FILES
None.
ENVIRONMENT VARIABLES
The following environment variables shall affect the execution of echo:
LANG Provide a default value for the internationalization variables that are unset or
null. (See the Base Definitions volume of POSIX.1‐2008, Section 8.2,
Internationalization Variables for the precedence of internationalization
variables used to determine the values of locale categories.)
LC_ALL If set to a non-empty string value, override the values of all the other
internationalization variables.
LC_CTYPE Determine the locale for the interpretation of sequences of bytes of text data
as characters (for example, single-byte as opposed to multi-byte characters in
arguments).
LC_MESSAGES
Determine the locale that should be used to affect the format and contents of
diagnostic messages written to standard error.
NLSPATH Determine the location of message catalogs for the processing of LC_MESSAGES.
ASYNCHRONOUS EVENTS
Default.
STDOUT
The echo utility arguments shall be separated by single <space> characters and a <newline>
character shall follow the last argument. Output transformations shall occur based on the
escape sequences in the input. See the OPERANDS section.
STDERR
The standard error shall be used only for diagnostic messages.
OUTPUT FILES
None.
EXTENDED DESCRIPTION
None.
EXIT STATUS
The following exit values shall be returned:
0 Successful completion.
>0 An error occurred.
CONSEQUENCES OF ERRORS
Default.
The following sections are informative.
APPLICATION USAGE
It is not possible to use echo portably across all POSIX systems unless both −n (as the
first argument) and escape sequences are omitted.
The printf utility can be used portably to emulate any of the traditional behaviors of the
echo utility as follows (assuming that IFS has its standard value or is unset):
* The historic System V echo and the requirements on XSI implementations in this volume
of POSIX.1‐2008 are equivalent to:
printf "%b\n$*"
* The BSD echo is equivalent to:
if [ "X$1" = "X−n" ]
then
shift
printf "%s$*"
else
printf "%s\n$*"
fi
New applications are encouraged to use printf instead of echo.
EXAMPLES
None.
RATIONALE
The echo utility has not been made obsolescent because of its extremely widespread use in
historical applications. Conforming applications that wish to do prompting without
<newline> characters or that could possibly be expecting to echo a −n, should use the
printf utility derived from the Ninth Edition system.
As specified, echo writes its arguments in the simplest of ways. The two different
historical versions of echo vary in fatally incompatible ways.
The BSD echo checks the first argument for the string −n which causes it to suppress the
<newline> that would otherwise follow the final argument in the output.
The System V echo does not support any options, but allows escape sequences within its
operands, as described for XSI implementations in the OPERANDS section.
The echo utility does not support Utility Syntax Guideline 10 because historical
applications depend on echo to echo all of its arguments, except for the −n option in the
BSD version.
FUTURE DIRECTIONS
None.
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