9.1.2 Translating

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 array1 to the corresponding character in array2. Characters not in array1 are passed through unchanged.

As a GNU extension to POSIX, when a character appears more than once in array1, only the final instance 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:]'

However, ranges like a-z are not portable outside the C locale.

When tr is performing translation, array1 and array2 typically have the same length. If array1 is shorter than array2, the extra characters at the end of array2 are ignored.

On the other hand, making array1 longer than array2 is not portable; POSIX says that the result is undefined. In this situation, BSD tr pads array2 to the length of array1 by repeating the last character of array2 as many times as necessary. System V tr truncates array1 to the length of array2.

By default, GNU tr handles this case like BSD tr. 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 System V tr in this case breaks the relatively common BSD idiom:

tr -cs A-Za-z0-9 '\012'

because it converts only zero bytes (the first element in the complement of array1), rather than all non-alphanumerics, to newlines.

By the way, the above idiom is not portable because it uses ranges, and it assumes that the octal code for newline is 012. Here is a better way to write it:

tr -cs '[:alnum:]' '[\n*]'