GNU tr also accepts the --help and --version options.
This manual page documents the GNU version of tr. tr copies the standard input to the standard output, performing one of the following operations:
The string1 and (if given) string2 arguments define ordered sets of characters, referred to below as set1 and set2. These sets are the characters of the input that tr operates on. The --complement (-c) option replaces set1 with its complement (all of the characters that are not in set1).
The format of the string1 and string2 arguments resembles the format of regular expressions; however, they are not regular expressions, only lists of characters. Most characters simply represent themselves in these strings, but the strings can contain the shorthands listed below, for convenience. Some of them can be used only in string1 or string2, as noted below.
Backslash escapes. A backslash followed by a character not listed below causes an error message.
Ranges. The notation `m-n' expands to all of the characters from m through n, in ascending order. m should collate before n; if it doesn't, an error results. As an example, `0-9' is the same as `0123456789'. Although GNU tr does not support the System V syntax that uses square brackets to enclose ranges, translations specified in that format will still work as long as the brackets in string1 correspond to identical brackets in string2.
Repeated characters. The notation `[c*n]' in string2 expands to n copies of character c. Thus, `[y*6]' is the same as `yyyyyy'. The notation `[c*]' in string2 expands to as many copies of c as are needed to make set2 as long as set1. If n begins with a 0, it is interpreted in octal, otherwise in decimal.
Character classes. The notation `[:class-name:]' expands to all of the characters in the (predefined) class named class-name. The characters expand in no particular order, except for the `upper' and `lower' classes, which expand in ascending order. When the --delete (-d) and --squeeze-repeats (-s) options are both given, any character class can be used in string2. Otherwise, only the character classes `lower' and `upper' are accepted in string2, and then only if the corresponding character class (`upper' and `lower', respectively) is specified in the same relative position in string1. Doing this specifies case conversion. The class names are given below; an error results when an invalid class name is given.
Equivalence classes. The syntax `[=c=]' expands to all of the characters that are equivalent to c, in no particular order. Equivalence classes are a recent invention intended to support non-English alphabets. But there seems to be no standard way to define them or determine their contents. Therefore, they are not fully implemented in GNU tr; each character's equivalence class consists only of that character, which makes this a useless construction currently.
tr performs translation when string1 and string2 are both given and the --delete (-d) option is not given. tr translates each character of its input that is in set1 to the corresponding character in set2. Characters not in set1 are passed through unchanged. When a character appears more than once in set1 and the corresponding characters in set2 are not all the same, only the final one is used. For example, these two commands are equivalent:
tr aaa xyz tr a z
A common use of tr is to convert lowercase characters to uppercase. This can be done in many ways. Here are three of them:
tr abcdefghijklmnopqrstuvwxyz ABCDEFGHIJKLMNOPQRSTUVWXYZ tr a-z A-Z tr '[:lower:]' '[:upper:]'
When tr is performing translation, set1 and set2 should normally have the same length. If set1 is shorter than set2, the extra characters at the end of set2 are ignored.
On the other hand, making set1 longer than set2 is not portable; POSIX.2 says that the result is undefined. In this situation, the BSD tr pads set2 to the length of set1 by repeating the last character of set2 as many times as necessary. The System V tr truncates set1 to the length of set2.
By default, GNU tr handles this case like the BSD tr does. When the --truncate-set1 (-t) option is given, GNU tr handles this case like the System V tr instead. This option is ignored for operations other than translation.
Acting like the System V tr in this case breaks the relatively common BSD idiom:
tr -cs A-Za-z0-9 '\012'
When given just the --delete (-d) option, tr removes any input characters that are in set1.
When given just the --squeeze-repeats (-s) option, tr replaces each input sequence of a repeated character that is in set1 with a single occurrence of that character.
When given both the --delete and the --squeeze-repeats options, tr first performs any deletions using set1, then squeezes repeats from any remaining characters using set2.
The --squeeze-repeats option may also be used when translating, in which case tr first performs translation, then squeezes repeats from any remaining characters using set2.
Here are some examples to illustrate various combinations of options:
Remove all zero bytes:
Put all words on lines by themselves. This converts all non-alphanumeric characters to newlines, then squeezes each string of repeated newlines into a single newline:
Convert each sequence of repeated newlines to a single newline:
GNU tr also accepts the following options in any combination with the others.
Setting the environment variable POSIXLY_CORRECT turns off several warning and error messages, for strict compliance with POSIX.2. The messages normally occur in the following circumstances:
1. When the --delete option is given but --squeeze-repeats is not, and string2 is given, GNU tr by default prints a usage message and exits, because string2 would not be used. The POSIX specification says that string2 must be ignored in this case. Silently ignoring arguments is a bad idea.
2. When an ambiguous octal escape is given. For example, \400 is actually \40 followed by the digit 0, because the value 400 octal does not fit into a single byte.
Note that GNU tr does not provide complete BSD or System V compatibility. For example, there is no option to disable interpretation of the POSIX constructs [:alpha:], [=c=], and [c*10]. Also, GNU tr does not delete zero bytes automatically, unlike traditional UNIX versions, which provide no way to preserve zero bytes.