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TR(P) TR(P)
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 substitu-
tion 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
IEEE Std 1003.1-2001, Section 12.2, Utility Syntax Guidelines.
The following options shall be supported:
-c Complement the set of values specified by string1. See the EXTENDED DESCRIP-
TION 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 IEEE Std 1003.1-2001,
Section 8.2, Internationalization Variables for the precedence of interna-
tionalization 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 char-
acters 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 val-
ues. 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. If the size
of a byte on the system is greater than nine bits, the valid escape sequence
used to represent a byte is implementation-defined. Multi-byte characters
require multiple, concatenated escape sequences of this type, including the
leading ββ\ββ for each byte.
\character
The backslash-escape sequences in the Base Definitions volume of
IEEE Std 1003.1-2001, Table 5-1, Escape Sequences and Associated Actions (
ββ\\ββ , ββ\aββ , ββ\bββ , ββ\fββ , ββ\nββ , ββ\rββ , ββ\tββ , ββ\vββ ) shall be supported.
The results of using any other character, other than an octal digit, follow-
ing the backslash 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 pre-
cedes 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 end-
points, inclusive.
:class:
Represents all characters belonging to the defined character class, as
defined by the current setting of the LC_CTYPE locale category. The follow-
ing 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 con-
tain the characters from the toupper mapping in the LC_CTYPE category of the cur-
rent 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 expres-
sion 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 cur-
rent locale, the behavior is undefined.
=equiv=
Represents all characters or collating elements belonging to the same equiv-
alence 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 expres-
sion 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:
* 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. When the array specified by string2 is shorter that the one specified
by 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 ββ[:space:]ββ
the last operandβs array shall contain all of the characters in that character
class. However, in a case conversion, as described previously, such as:
tr -s ββ[:upper:]ββ ββ[:lower:]ββ
the last operandβs 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 Sys-
tem 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 ββdββ . Since this area is specifi-
cally unspecified in this volume of IEEE Std 1003.1-2001, both the BSD and System V
behaviors are allowed, but a conforming application cannot rely on the BSD behav-
ior. It would have to code the example in the following way:
tr 0123456789 ββ[d*]ββ
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 ββ\000ββ
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 ββeββ in file1, which are stripped of diacritical marks and writ-
ten 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 auto-
matically 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 β\000β.
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 char-
acters 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 (ββany
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 syn-
tactical 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-brack-
ets, such as:
tr ββ[a-z]ββ ββ[A-Z]ββ
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 ββaββ , ββ-ββ , and ββzββ 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
IEEE Std 1003.1-2001. This meant that historical practice of being able to specify
tr -d\200-\377 (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. How-
ever, 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 charac-
ters or multiple single byte characters. IEEE Std 1003.1-2001 specifies that octal
sequences always refer to single byte binary values.
FUTURE DIRECTIONS
None.
SEE ALSO
sed
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating
System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C)
2001-2003 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 .
POSIX 2003 TR(P)
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