TR
Section: POSIX Programmer's Manual (1P)
Updated: 2017
Page Index
PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
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.1-2017,
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.1-2017,
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.1-2017,
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.1-2017, 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
The Base Definitions volume of POSIX.1-2017,
Table 5-1, Escape Sequences and Associated Actions,
Chapter 8, Environment Variables,
Section 12.2, Utility Syntax Guidelines
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .