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.
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 abc def ghi $ tac -r -s 'x\|[^x]' testfile ihg fed cba% $ 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.
. work just as well as the regex? What does this match that . doesn't? \n? . 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. If you're not concerned about the LF character (see mosvy's comments), then it is much more efficient to use rev rather than tac -r -s 'x\|[^x]', piping it to tac if needed.
$ cat testfile abc def ghi $ rev testfile cba fed ihg $ rev testfile|tac ihg fed cba This solution is way cheaper than tac -r -s 'x\|[^x]' because regular expressions tend to slow things down quite dramatically.
printf "foo\nbar\n" | rev | tac, the output will not start with "\n" and end with "f", but start with "r" and end with "\n" ;-) rev with perl -ne 'print scalar reverse', though.
rev?