tr performs translation when set1 and set2 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:]'
But note that using ranges like
a-z above is not portable.
tr is performing translation, set1 and set2
typically 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 says that the result is undefined. In this situation,
tr pads set2 to the length of set1 by repeating
the last character of set2 as many times as necessary. System V
tr truncates set1 to the length of set2.
By default, GNU
tr handles this case like BSD
When the --truncate-set1 (-t) option is given,
tr handles this case like the System V
instead. This option is ignored for operations other than translation.
Acting like System V
tr in this case breaks the relatively common
tr -cs A-Za-z0-9 '\012'
because it converts only zero bytes (the first element in the complement of set1), 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.
Assuming a POSIX compliant
tr, here is a better
way to write it:
tr -cs '[:alnum:]' '[\n*]'