<<< Previous Home Next >>>
The Open Group Base Specifications Issue 7
IEEE Std 1003.1-2008, 2016 Edition
Copyright © 2001-2016 The IEEE and The Open Group

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 XBD 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 XBD 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
[XSI] [Option Start] Determine the location of message catalogs for the processing of LC_MESSAGES. [Option End]

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 XBD 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

[XSI] [Option Start] 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. [Option End]

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:

When the -d option is specified:

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.

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.

SEE ALSO

sed

XBD Escape Sequences and Associated Actions, Environment Variables, Utility Syntax Guidelines

CHANGE HISTORY

First released in Issue 2.

Issue 6

The -C operand is added, and the description of the -c operand is changed to align with the IEEE P1003.2b draft standard.

The normative text is reworded to avoid use of the term "must" for application requirements.

IEEE Std 1003.1-2001/Cor 2-2004, item XCU/TC2/D6/31 is applied, removing text describing behavior on systems with bytes consisting of more than eight bits.

IEEE Std 1003.1-2001/Cor 2-2004, item XCU/TC2/D6/32 is applied, updating an example in the EXAMPLES section to avoid using unspecified behavior.

IEEE Std 1003.1-2001/Cor 2-2004, item XCU/TC2/D6/33 is applied, making a correction to the RATIONALE.

Issue 7

SD5-XCU-ERN-30 is applied.

SD5-XCU-ERN-97 is applied, updating the SYNOPSIS.

Austin Group Interpretation 1003.1-2001 #132 is applied, adding rationale to the \ character construct.

POSIX.1-2008, Technical Corrigendum 1, XCU/TC1-2008/0145 [325] is applied.

POSIX.1-2008, Technical Corrigendum 2, XCU/TC2-2008/0196 [663] is applied.

End of informative text.

 

return to top of page
UNIX ® is a registered Trademark of The Open Group.
POSIX ® is a registered Trademark of The IEEE.
Copyright © 2001-2016 The IEEE and The Open Group, All Rights Reserved
[ Main Index | XBD | XSH | XCU | XRAT ]
<<< Previous Home Next >>>