How do I reverse a for loop?

Question

How do I properly do a for loop in reverse order?

for f in /var/logs/foo*.log; do
    bar "$f"
done

I need a solution that doesn't break for funky characters in the file names.

Answer 1

In bash or ksh, put the file names in an array, and iterate over that array in reverse order.

files=(/var/logs/foo*.log)
for ((i=${#files[@]}-1; i>=0; i--)); do
  bar "${files[$i]}"
done

The code above also works in zsh if the ksh_arrays option is set (it is in ksh emulation mode). There's a simpler method in zsh, which is to reverse the order of the matches through a glob qualifier:

for f in /var/logs/foo*.log(On); do bar $f; done

POSIX doesn't include arrays, so if you want to be portable, your only option to directly store an array of strings is the positional parameters.

set -- /var/logs/foo*.log
i=$#
while [ $i -gt 0 ]; do
  eval "f=\${$i}"
  bar "$f"
  i=$((i-1))
done

Answer 2

Try this, unless you consider line breaks as "funky characters":

ls /var/logs/foo*.log | tac | while read f; do
    bar "$f"
done

Answer 3

find /var/logs/ -name 'foo*.log' -print0 | tail -r | xargs -0 bar

Should operate the way you want (this was tested on Mac OS X and I have a caveat below...).

From the man page for find:

-print0
         This primary always evaluates to true.  It prints the pathname of the current file to standard output, followed by an ASCII NUL character (charac-
         ter code 0).

Basically, you're finding the files that match your string + glob and terminating each with a NUL character. If your filenames contain newlines or other strange characters, find should handle this well.

tail -r

takes the standard input through the pipe and reverses it (note that tail -r prints all of the input to stdout, and not just the last 10 lines, which is the standard default. man tail for more info).

We then pipe that to xargs -0 :

-0      Change xargs to expect NUL (``\0'') characters as separators, instead of spaces and newlines.  This is expected to be used in concert with the
         -print0 function in find(1).

Here, xargs expects to see arguments separated by the NUL character, which you passed from find and reversed with tail.

My caveat: I've read that tail doesn't play well with null-terminated strings. This worked well on Mac OS X, but I can't guarantee that's the case for all *nixes. Tread carefully.

I should also mention that GNU Parallel is often used as an xargs alternative. You may check that out, too.

I may be missing something, so others should chime in.