Today I learnt a bit about tr command.But I was stuck at understanding the difference between -c and -C.On the manual, it said: -C      Complement the set of characters in string1, that is ``-C ab'' includes every character except for `a' and `b'. -c      Same as -C but complement the set of values in string1.I'm not quite understand what does set of values in string1 of -c option mean.I thought it may treat string 1 ab as a whole and will escape single a and b.So I did an experiment:  echo ab_a_b | tr -C 'ba' 'c'abcacbc%                                                                                                                                                                               echo ab_a_b | tr -c 'ba' 'c'abcacbc%Things didn't match my expectation!So, what's the difference between -C and -c in tr command?Software Version: BSD 2004 on OSX10.10
What's the difference between `-C` and `-c` in `tr` command?
command line;tr
The POSIX manual says this:If the -C option is specified, the complements of the characters specified by string1 (the set of all characters in the current character set, as defined by the current setting of LC_CTYPE, except for those actually specified in the string1 operand) shall be placed in the array in ascending collation sequence, as defined by the current setting of LC_COLLATE.If the -c option is specified, the complement of the values specified by string1 shall be placed in the array in ascending order by binary value.and contains the following noteThe ISO POSIX-2:1993 standard had a -c option that behaved similarly to the -C option, but did not supply functionality equivalent to the -c option specified in POSIX.1-2008. This meant that historical practice of being able to specify tr -cd\000-\177 (which would delete all bytes with the top bit set) would have no effect because, in the C locale, bytes with the values octal 200 to octal 377 are not characters.From this it appears that the -c option let you specify numeric values representing ASCII character instead of using the characters themselves.  