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PROGRAM:

NAME

tr — translate characters

SYNOPSIS

tr [−c|−C] [−s] string1 string2

tr −s [−c|−C] string1

tr −d [−c|−C] string1

tr −ds [−c|−C] string1 string2

DESCRIPTION

The tr utility shall copy the standard input to the standard output with substitution or
deletion of selected characters. The options specified and the string1 and string2
operands shall control translations that occur while copying characters and single-
character collating elements.

OPTIONS

The tr utility shall conform to the Base Definitions volume of POSIX.1‐2008, Section 12.2,
Utility Syntax Guidelines.

The following options shall be supported:

−c Complement the set of values specified by string1. See the EXTENDED DESCRIPTION
section.

−C Complement the set of characters specified by string1. See the EXTENDED
DESCRIPTION section.

−d Delete all occurrences of input characters that are specified by string1.

−s Replace instances of repeated characters with a single character, as described
in the EXTENDED DESCRIPTION section.

OPERANDS

The following operands shall be supported:

string1, string2
Translation control strings. Each string shall represent a set of characters to
be converted into an array of characters used for the translation. For a
detailed description of how the strings are interpreted, see the EXTENDED
DESCRIPTION section.

STDIN

The standard input can be any type of file.

INPUT FILES

None.

ENVIRONMENT VARIABLES

The following environment variables shall affect the execution of tr:

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_COLLATE
Determine the locale for the behavior of range expressions and equivalence
classes.

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) and the behavior of character classes.

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 tr output shall be identical to the input, with the exception of the specified
transformations.

STDERR

The standard error shall be used only for diagnostic messages.

OUTPUT FILES

None.

EXTENDED DESCRIPTION

The operands string1 and string2 (if specified) define two arrays of characters. The
constructs in the following list can be used to specify characters or single-character
collating elements. If any of the constructs result in multi-character collating elements,
tr shall exclude, without a diagnostic, those multi-character elements from the resulting
array.

character Any character not described by one of the conventions below shall represent
itself.

\octal Octal sequences can be used to represent characters with specific coded values.
An octal sequence shall consist of a <backslash> followed by the longest
sequence of one, two, or three-octal-digit characters (01234567). The sequence
shall cause the value whose encoding is represented by the one, two, or three-
digit octal integer to be placed into the array. Multi-byte characters require
multiple, concatenated escape sequences of this type, including the leading
<backslash> for each byte.

\character
The <backslash>-escape sequences in the Base Definitions volume of POSIX.1‐2008,
Table 5-1, Escape Sequences and Associated Actions ('\\', '\a', '\b', '\f',
'\n', '\r', '\t', '\v') shall be supported. The results of using any other
character, other than an octal digit, following the <backslash> are unspecified.
Also, if there is no character following the <backslash>, the results are
unspecified.

c−c In the POSIX locale, this construct shall represent the range of collating
elements between the range endpoints (as long as neither endpoint is an octal
sequence of the form \octal), inclusive, as defined by the collation sequence.
The characters or collating elements in the range shall be placed in the array
in ascending collation sequence. If the second endpoint precedes the starting
endpoint in the collation sequence, it is unspecified whether the range of
collating elements is empty, or this construct is treated as invalid. In locales
other than the POSIX locale, this construct has unspecified behavior.

If either or both of the range endpoints are octal sequences of the form \octal,
this shall represent the range of specific coded values between the two range
endpoints, inclusive.

[:class:] Represents all characters belonging to the defined character class, as defined
by the current setting of the LC_CTYPE locale category. The following character
class names shall be accepted when specified in string1:

alnum blank digit lower punct upper
alpha cntrl graph print space xdigit

In addition, character class expressions of the form [:name:] shall be
recognized in those locales where the name keyword has been given a charclass
definition in the LC_CTYPE category.

When both the −d and −s options are specified, any of the character class names
shall be accepted in string2. Otherwise, only character class names lower or
upper are valid in string2 and then only if the corresponding character class
(upper and lower, respectively) is specified in the same relative position in
string1. Such a specification shall be interpreted as a request for case
conversion. When [:lower:] appears in string1 and [:upper:] appears in string2,
the arrays shall contain the characters from the toupper mapping in the LC_CTYPE
category of the current locale. When [:upper:] appears in string1 and [:lower:]
appears in string2, the arrays shall contain the characters from the tolower
mapping in the LC_CTYPE category of the current locale. The first character from
each mapping pair shall be in the array for string1 and the second character
from each mapping pair shall be in the array for string2 in the same relative
position.

Except for case conversion, the characters specified by a character class
expression shall be placed in the array in an unspecified order.

If the name specified for class does not define a valid character class in the
current locale, the behavior is undefined.

[=equiv=] Represents all characters or collating elements belonging to the same
equivalence class as equiv, as defined by the current setting of the LC_COLLATE
locale category. An equivalence class expression shall be allowed only in
string1, or in string2 when it is being used by the combined −d and −s options.
The characters belonging to the equivalence class shall be placed in the array
in an unspecified order.

[x*n] Represents n repeated occurrences of the character x. Because this expression
is used to map multiple characters to one, it is only valid when it occurs in
string2. If n is omitted or is zero, it shall be interpreted as large enough to
extend the string2-based sequence to the length of the string1-based sequence.
If n has a leading zero, it shall be interpreted as an octal value. Otherwise,
it shall be interpreted as a decimal value.

When the −d option is not specified:

* If string2 is present, each input character found in the array specified by string1
shall be replaced by the character in the same relative position in the array
specified by string2. If the array specified by string2 is shorter that the one
specified by string1, or if a character occurs more than once in string1, the results
are unspecified.

* If the −C option is specified, the complements of the characters specified by string1
(the set of all characters in the current character set, as defined by the current
setting of LC_CTYPE, except for those actually specified in the string1 operand) shall
be placed in the array in ascending collation sequence, as defined by the current
setting of LC_COLLATE.

* If the −c option is specified, the complement of the values specified by string1 shall
be placed in the array in ascending order by binary value.

* Because the order in which characters specified by character class expressions or
equivalence class expressions is undefined, such expressions should only be used if
the intent is to map several characters into one. An exception is case conversion, as
described previously.

When the −d option is specified:

* Input characters found in the array specified by string1 shall be deleted.

* When the −C option is specified with −d, all characters except those specified by
string1 shall be deleted. The contents of string2 are ignored, unless the −s option is
also specified.

* When the −c option is specified with −d, all values except those specified by string1
shall be deleted. The contents of string2 shall be ignored, unless the −s option is
also specified.

* The same string cannot be used for both the −d and the −s option; when both options
are specified, both string1 (used for deletion) and string2 (used for squeezing) shall
be required.

When the −s option is specified, after any deletions or translations have taken place,
repeated sequences of the same character shall be replaced by one occurrence of the same
character, if the character is found in the array specified by the last operand. If the
last operand contains a character class, such as the following example:

tr −s '[:space:]'

the last operand's array shall contain all of the characters in that character class.
However, in a case conversion, as described previously, such as:

tr −s '[:upper:]' '[:lower:]'

the last operand's array shall contain only those characters defined as the second
characters in each of the toupper or tolower character pairs, as appropriate.

An empty string used for string1 or string2 produces undefined results.

EXIT STATUS

The following exit values shall be returned:

0 All input was processed successfully.

>0 An error occurred.

CONSEQUENCES OF ERRORS

Default.

The following sections are informative.

APPLICATION USAGE

If necessary, string1 and string2 can be quoted to avoid pattern matching by the shell.

If an ordinary digit (representing itself) is to follow an octal sequence, the octal
sequence must use the full three digits to avoid ambiguity.

When string2 is shorter than string1, a difference results between historical System V and
BSD systems. A BSD system pads string2 with the last character found in string2. Thus, it
is possible to do the following:

tr 0123456789 d

which would translate all digits to the letter 'd'. Since this area is specifically
unspecified in this volume of POSIX.1‐2008, both the BSD and System V behaviors are
allowed, but a conforming application cannot rely on the BSD behavior. It would have to
code the example in the following way:

tr 0123456789 '[d*]'

It should be noted that, despite similarities in appearance, the string operands used by
tr are not regular expressions.

Unlike some historical implementations, this definition of the tr utility correctly
processes NUL characters in its input stream. NUL characters can be stripped by using:

tr −d '\000'

EXAMPLES

1. The following example creates a list of all words in file1 one per line in file2,
where a word is taken to be a maximal string of letters.

tr −cs "[:alpha:]" "[\n*]" <file1 >file2

2. The next example translates all lowercase characters in file1 to uppercase and writes
the results to standard output.

tr "[:lower:]" "[:upper:]" <file1

3. This example uses an equivalence class to identify accented variants of the base
character 'e' in file1, which are stripped of diacritical marks and written to file2.

tr "[=e=]" "[e*]" <file1 >file2

RATIONALE

In some early proposals, an explicit option −n was added to disable the historical
behavior of stripping NUL characters from the input. It was considered that automatically
stripping NUL characters from the input was not correct functionality. However, the
removal of −n in a later proposal does not remove the requirement that tr correctly
process NUL characters in its input stream. NUL characters can be stripped by using tr −d
'\000'.

Historical implementations of tr differ widely in syntax and behavior. For example, the
BSD version has not needed the bracket characters for the repetition sequence. The tr
utility syntax is based more closely on the System V and XPG3 model while attempting to
accommodate historical BSD implementations. In the case of the short string2 padding, the
decision was to unspecify the behavior and preserve System V and XPG3 scripts, which might
find difficulty with the BSD method. The assumption was made that BSD users of tr have to
make accommodations to meet the syntax defined here. Since it is possible to use the
repetition sequence to duplicate the desired behavior, whereas there is no simple way to
achieve the System V method, this was the correct, if not desirable, approach.

The use of octal values to specify control characters, while having historical precedents,
is not portable. The introduction of escape sequences for control characters should
provide the necessary portability. It is recognized that this may cause some historical
scripts to break.

An early proposal included support for multi-character collating elements. It was pointed
out that, while tr does employ some syntactical elements from REs, the aim of tr is quite
different; ranges, for example, do not have a similar meaning (``any of the chars in the
range matches'', versus ``translate each character in the range to the output
counterpart''). As a result, the previously included support for multi-character collating
elements has been removed. What remains are ranges in current collation order (to support,
for example, accented characters), character classes, and equivalence classes.

In XPG3 the [:class:] and [=equiv=] conventions are shown with double brackets, as in RE
syntax. However, tr does not implement RE principles; it just borrows part of the syntax.
Consequently, [:class:] and [=equiv=] should be regarded as syntactical elements on a par
with [x*n], which is not an RE bracket expression.

The standard developers will consider changes to tr that allow it to translate characters
between different character encodings, or they will consider providing a new utility to
accomplish this.

On historical System V systems, a range expression requires enclosing square-brackets,
such as:

tr '[a-z]' '[A-Z]'

However, BSD-based systems did not require the brackets, and this convention is used here
to avoid breaking large numbers of BSD scripts:

tr a-z A-Z

The preceding System V script will continue to work because the brackets, treated as
regular characters, are translated to themselves. However, any System V script that
relied on "a‐z" representing the three characters 'a', '−', and 'z' have to be rewritten
as "az−".

The ISO POSIX‐2:1993 standard had a −c option that behaved similarly to the −C option, but
did not supply functionality equivalent to the −c option specified in POSIX.1‐2008. This
meant that historical practice of being able to specify tr −cd\000−\177 (which would
delete all bytes with the top bit set) would have no effect because, in the C locale,
bytes with the values octal 200 to octal 377 are not characters.

The earlier version also said that octal sequences referred to collating elements and
could be placed adjacent to each other to specify multi-byte characters. However, it was
noted that this caused ambiguities because tr would not be able to tell whether adjacent
octal sequences were intending to specify multi-byte characters or multiple single byte
characters. POSIX.1‐2008 specifies that octal sequences always refer to single byte binary
values when used to specify an endpoint of a range of collating elements.

Earlier versions of this standard allowed for implementations with bytes other than eight
bits, but this has been modified in this version.

FUTURE DIRECTIONS

None.

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