Best way to prefix output of a command and check its return value

Question

is there a better way than the following code to prefix the output of a command but still being able to check its return value?

command > "$tmpfile" 2>&1
ret=$?
sed 's/^/  | /' "$tmpfile"
if [[ ! "$ret" -eq 0 ]]; then
    rm "$tmpfile"
    exit 1
fi

Does this help? Get exit status of process that's piped to another — steeldriver, Jun 11 '20 at 07:22
don't know what you mean with prefix but your solution to save exit code is fine. for one-liners another common method is cmd && sed || rm (but this won't work for exit 1) — alecxs, Jun 11 '20 at 07:38
the existing answer using ret=${PIPESTATUS[0]} is perfectly complete and obviates the need for a temp file altogether, but I'll just add that if you do use a temp file, the better way to remove it is via a trap, e.g., traprm -f "$tmpfile"EXIT https://unix.stackexchange.com/questions/480330/temporary-folder-that-automatically-destroyed-after-process-exit — michael, Jun 11 '20 at 08:17
Does this answer your question? Get exit status of process that's piped to another — Chris Davies, Jun 14 '20 at 08:36

Stéphane Chazelas · Accepted Answer · 2020-06-11T10:38:50.400

With bash and a few other shells, you can use the pipefail option, which when enabled causes the exit status of a pipeline to be that of the rightmost component that failed as opposed to the rightmost component:

(set -o pipefail; cmd | sed 's/^/  | /') || exit

Another option is to redirect stdout to a process substitution instead of using a pipe:

cmd > >(sed 's/^/  | /') || exit

Though beware that sed which in that case is started in background and is not waited for, may not have finished writing by the time the next command is run.

Specific to bash, there's the $PIPESTATUS special array (zsh has a $pipestatus equivalent) which records the exit status of every component in a pipeline:

cmd | sed 's/^/  | /'; ret=${PIPESTATUS[0]}
[ "$ret" -eq 0 ] || exit "$ret"

POSIXly, you can do:

{
  ret=$(
    exec 4>&1
    {
      cmd 3>&- 4>&-
      echo "$?" >&4
    } | sed 's/^/  | /' >&3 3>&- 4>&-
  )
} 3>&1
[ "$ret" -eq 0 ] || exit "$ret"

You could use a helper function:

prefix() (
  prefix=$1; shift
  exit "$(
    exec 4>&1
    { 
      "$@" 3>&- 4>&-
      echo "$?" >&4 3>&- 4>&-
    } | PREFIX=$prefix awk '{print ENVIRON["PREFIX"] $0}' >&3 3>&- 4>&-
  )"
) 3>&1

prefix '  | ' cmd

(here switching to awk so as to be able to use arbitrary prefixes.

The pipefail option is exaclty what I needed, thanks! Now I can do if command | sed... — gcomte, Jun 11 '20 at 12:47

Best way to prefix output of a command and check its return value

1 Answers1