tr : translate or delete characters
[SYNOPSIS] tr OPTION SET1 [SET2]
OPTION
-d delete characters in SET1, do not translate
-s replace each sequence of a repeated character that is listed in the last specified SET, with a single occurrence of that character
-t first truncate SET1 to length of SET2 
usage example :
[root@localhost ~]# cat test.txt
AAAAA aaaaa
BBBBB bbbbb
CCCCC ccccc
11111 22222
33333 44444
[root@localhost ~]# tr [:lower:] [:upper:] < test.txt
AAAAA AAAAA
BBBBB BBBBB
CCCCC CCCCC
11111 22222
33333 44444
[root@localhost ~]# tr [:blank:] '+' < test.txt
AAAAA+aaaaa
BBBBB+bbbbb
CCCCC+ccccc
11111+22222
33333+44444
[root@localhost ~]# tr '1' '5' < test.txt
AAAAA aaaaa
BBBBB bbbbb
CCCCC ccccc
55555 22222
33333 44444