Capture stdout/stderr of a command while keeping the exit code

capture-output.sh

#!/bin/bash

syntax() {
	echo "$0 [options] [--] cmd..."
	echo "Options:"
	echo "  --stdout <logfile>: append stdout to logfile"
	echo "  --stderr <logfile>: append stderr to logfile"
	echo "Returns exit code returned by captured command"
	exit 1
}

capture_stdout=
capture_stderr=

if [ $# = 0 ]; then
	syntax
fi

while [ $# -gt 0 ]; do
	case "$1" in
	--stdout=*)
		capture_stdout="${1#--stdout=}"
		shift
		;;
	--stdout)
		capture_stdout="$2"
		shift 2
		;;
	--stderr=*)
		capture_stderr="${1#--stderr=}"
		shift
		;;
	--stderr)
		capture_stderr="$2"
		shift 2
		;;
	-h|--help|"-?")
		syntax
		;;
	--)
		shift
		break
		;;
	-*)
		echo >&2 "Unknow argument: $1"
		syntax
		;;
	*)
		break
		;;
	esac
done

if [ $# = 0 ]; then
	echo >&2 "Missing command to execute and capture output from"
	syntax
fi

if [ -z "${capture_stdout}" -a -z "${capture_stderr}" ]; then
	echo >&2 "Not capturing any output"
	syntax
fi

if [ -n "${capture_stdout}" ]; then
	exec 3>&1 >&-
	coproc stdout {
		exec tee -a "${capture_stdout}" >&3
	}
	fd0=${stdout[0]}
	fd1=${stdout[1]}
	exec {fd0}>&- >&${fd1} {fd1}>&-
fi

if [ -n "${capture_stderr}" ]; then
	exec 4>&2 2>&-
	coproc stderr {
		exec tee -a "${capture_stderr}" >&4 2>&4
	}
	fd0=${stderr[0]}
	fd1=${stderr[1]}
	exec {fd0}>&- 2>&${fd1} {fd1}>&-
fi

"$@"
rc=$? 3>&- 4>&-

if [ -n "${capture_stdout}" ]; then
	exec >&3 3>&-
	wait "${stdout_PID}"
fi
if [ -n "${capture_stderr}" ]; then
	exec 2>&4 4>&-
	wait "${stderr_PID}"
fi

exit "${rc}"