CODEWITHSUNDEEP
bash
Amanda Doan
Amanda Doan

Reputation: 81

How to get exit codes for different sections of a command in bash

Let's say I have a line in my bash script with ssh bad@location "find -name 'fruit.txt' | grep "Apple" and I'm trying to retrieve the exit codes of ssh, find . -name 'fruit.txt', and "grep "Apple` to see which command went bad.

So far, I've tried something like echo $? ${PIPESTATUS[0]} ${PIPESTATUS[1]}, but it looks like $? returns the same thing as ${PIPESTATUS[0]} in this case. I only need to return the first non-zero exit code along with dmesg for debugging purposes.

I've also considered using set -o pipefail, which will return a failure exit code if any command errors, but I'd like to somehow know which command failed for debugging.

I'd like either get an exit code of 255 (from ssh) and its corresponding dmesg, or somehow get all of the exit codes.

Upvotes: 1

Views: 545

Answers (1)

Charles Duffy
Charles Duffy

Reputation: 295815

ssh only returns one exit status (per channel) to the calling shell; if you want to get exit status for the individual pipeline components it ran remotely, you need to collect them remotely, put them in with the data, and then parse them back out. One way to do that, if you have a very new version of bash, is like so:

#!/usr/bin/env bash

# note <<'EOF' not just <<EOF; with the former, the local shell does not munge
# heredoc contents.
remote_script=$(cat <<'EOF'
  tempfile=$(mktemp "${TMPDIR:-/tmp}/output.XXXXXX"); mktemp_rc=$?
  find -name 'fruit.txt' | grep Apple >"$tempfile"
  printf '%s\0' "$mktemp_rc" "${PIPESTATUS[@]}"
  cat "$tempfile"
  rm -f -- "$tempfile"
  exit 0  # so a bad exit status will be from ssh itself
EOF
)

# note that collecting a process substitution PID needs bash 4.4!
exec {ssh_fd}< <(ssh bad@location "$remote_script" </dev/null); ssh_pid=$!
IFS= read -r -d '' mktemp_rc <&$ssh_fd   # read $? of mktemp
IFS= read -r -d '' find_rc <&$ssh_fd     # read $? of find
IFS= read -r -d '' grep_rc <&$ssh_fd     # read $? of grep
cat <&$ssh_fd                            # spool output of grep to our own output
wait "$ssh_pid"; ssh_rc=$?               # let ssh finish and read its $?

echo "mktemp exited with status $mktemp_rc" >&2
echo "find exited with status $find_rc" >&2
echo "grep exited with status $grep_rc" >&2
echo "ssh exited with status $ssh_rc" >&2

How does this work?

  • exec {fd_var_name}< <(...) uses the bash 4.1 automatic file descriptor allocation feature to generate a file descriptor number, and associate it with content read from the process substitution running ....
  • In bash 4.4 or newer, process substitutions also set $!, so their PIDs can be captured, to later wait for them and collect their exit status; this is what we're storing in ssh_pid.
  • IFS= read -r -d '' varname reads from stdin up to the next NUL (in read -d '', the first character of '' is treated as the end of input; as an empty string in a C-derived language, the first byte of the string is its NUL terminator).

This could theoretically be made easier by writing the output before the exit status values -- you wouldn't need a temporary file on the remote machine that way -- but the caveat there is that if there were a NUL anywhere in the find | grep output, then some of that output could be picked up by the reads. (Similarly, you could store output in a variable instead of a temporary file, but again, that would destroy any NULs in the stream's output).

Upvotes: 1

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