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 --delete and --squeeze-repeats, 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:
tr -d '\0'
tr -cs '[:alnum:]' '[\n*]'
tr -s '\n'
#!/bin/sh cat -- "$@" \ | tr -s '[:punct:][:blank:]' '[\n*]' \ | tr '[:upper:]' '[:lower:]' \ | uniq -d
tr -d axM
However, when ‘-’ is one of those characters, it can be tricky because
‘-’ has special meanings. Performing the same task as above but also
removing all ‘-’ characters, we might try
tr -d -axM, but
that would fail because tr would try to interpret -a as
a command-line option. Alternatively, we could try putting the hyphen
inside the string,
tr -d a-xM, but that wouldn't work either because
it would make tr interpret
a-x as the range of characters
‘a’...‘x’ rather than the three.
One way to solve the problem is to put the hyphen at the end of the list
tr -d axM-
Or you can use ‘--’ to terminate option processing:
tr -d -- -axM
More generally, use the character class notation
with ‘-’ (or any other character) in place of the ‘c’:
tr -d '[=-=]axM'
Note how single quotes are used in the above example to protect the square brackets from interpretation by a shell.