I want to use the tacto reverse a text file character by character. On the info page for coreutils I found an example saying: #Reverse a file character by character tac -r -s 'x\|[^x]'

However running tac -r -s seems to open standard input instead of printing the file. What does 'x\|[^x]' mean and what should I be doing?

I also noted that the output for tac [file] and tac -r [file] are same and they're the same as cat [file]. Still can't figure out char by char reverse.

  • So did you try the command it told you to or not? – Michael Homer May 28 '18 at 6:35
  • Are you talking about reversing the order of the lines and then reversing the order of the characters of the lines as well? Do you have rev? – Kusalananda May 28 '18 at 6:39
  • Reversing characters not lines. – Weezy May 28 '18 at 8:57

To reverse a file character-by-character using tac, use:

tac -r -s 'x\|[^x]'

This is documented in info tac:

# Reverse a file character by character.
tac -r -s 'x\|[^x]'

-r causes the separator to be treated as a regular expression. -s SEP uses SEP as the separator. x\|[^x] is a regular expression that matches every character (those that are x, and those that are not x).

$ cat testfile
$ tac -r -s 'x\|[^x]' testfile


tac file is not the same as cat file unless file has only one line. tac -r file is the same as tac file because the default separator is \n, which is the same when treated as a regular expression and not.

  • 1
    Wouldn't . work just as well as the regex? What does this match that . doesn't? \n? – muru May 28 '18 at 9:06
  • 2
    @muru Yes, if you use . it swaps bytes over the \n: printf 'ab\ncd\n'|tac -r -s . |hexdump -c -> \n d \n c b a, and tac -r -s . | tac -r -s . isn't idempotent. – Michael Homer May 28 '18 at 9:13

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