The Single UNIX ® Specification, Version 2
Copyright © 1997 The Open Group


tr - translate characters


tr [-cs] string1 string2

tr -s[-c] string1

tr -d[-c] string1

tr -ds[-c] string1 string2


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


The tr utility supports the XBD specification, Utility Syntax Guidelines  .

The following options are supported:

Complement the set of characters specified by string1. See the EXTENDED DESCRIPTION section.
Delete all occurrences of input characters that are specified by string1.
Replace instances of repeated characters with a single character, as described in the EXTENDED DESCRIPTION section.


The following operands are supported:
Translation control strings. Each string represents 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.


The standard input can be any type of file.




The following environment variables affect the execution of tr:
Provide a default value for the internationalisation variables that are unset or null. If LANG is unset or null, the corresponding value from the implementation-dependent default locale will be used. If any of the internationalisation variables contains an invalid setting, the utility will behave as if none of the variables had been defined.
If set to a non-empty string value, override the values of all the other internationalisation variables.
Determine the locale for the behaviour of range expressions and equivalence classes.
Determine the locale for the interpretation of sequences of bytes of text data as characters (for example, single- versus multi-byte characters in arguments) and the behaviour of character classes.
Determine the locale that should be used to affect the format and contents of diagnostic messages written to standard error.
Determine the location of message catalogues for the processing of LC_MESSAGES .




The tr output is identical to the input, with the exception of the specified transformations.


Used only for diagnostic messages.




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 will exclude, without a diagnostic, those multi-character elements from the resulting array.
Any character not described by one of the conventions below represents itself.
Octal sequences can be used to represent characters with specific coded values. An octal sequence consists of a backslash followed by the longest sequence of one-, two- or three-octal-digit characters (01234567). The sequence causes the character 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-dependent. Multi-byte characters require multiple, concatenated escape sequences of this type, including the leading \ for each byte.
The backslash-escape sequences in the table in the XBD specification, File Format Notation  (\\, \a, \b, \f, \n, \r, \t, \v) are supported. The results of using any other character, other than an octal digit, following the backslash are unspecified.
Represents the range of collating elements between the range endpoints, inclusive, as defined by the current setting of the LC_COLLATE locale category. The starting endpoint must precede the second endpoint in the current collation order. The characters or collating elements in the range are placed in the array in ascending collation sequence.
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 will 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:] are recognised 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 will 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 is interpreted as a request for case conversion. When [:lower:] appears in string1 and [:upper:] appears in string2, the arrays will 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 will contain the characters from the tolower mapping in the LC_CTYPE category of the current locale. The first character from each mapping pair will be in the array for string1 and the second character from each mapping pair will be in the array for string2 in the same relative position.

Except for case conversion, the characters specified by a character class expression are 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 behaviour is undefined.

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 is 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 are placed in the array in an unspecified order.

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 is 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 is interpreted as an octal value. Otherwise, it is 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 will 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 will 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 will 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.


The following exit values are returned:
All input was processed successfully.
An error occurred.




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 will pad 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 the document, both the BSD and System V behaviours are allowed, but a portable application cannot rely on the BSD behaviour. 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 previous versions, the Issue 4 tr correctly processes NUL characters in its input stream. NUL characters can be stripped by using:

tr -d '\000'


  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 lower-case characters in file1 to upper-case and writes the results to standard output.
    tr "[:lower:]" "[:upper:]" <file1

    Note that the caveat expressed in the corresponding Issue 3 example is no longer in effect. This case conversion is now a special case that employs the tolower and toupper classifications, ensuring that proper mapping is accomplished (when the locale is correctly defined).

  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





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Copyright © 1997 The Open Group
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