Trap a command in bash without removing the previous trap command

add_trap.bash

#!/bin/bash

# This function adds an additional command to the trap stack. Each command
# added in this way will be called in reverse order when the signal is received.
#
# This function works with a simple concatenation with the currently existing
# trap function. Popping something from the trap stack without running it is
# left as an exercise to the reader; it requires at least a parser capable of
# matching { } correctly.
#
# Usage: `add_trap <COMMAND> <SIGNAL>`
#
# THIS FUNCTION DOES NOT SUPPORT MUTLILINE COMMANDS OR COMMANDS WITH ANY
# SINGLE QUOTES, EVEN IF THEY'RE ESCAPED, SO DON'T TRY IT
add_trap() {
	# Check for --help
	for arg in "$@"
	do
		case "$arg" in
		'-h' | '--help' )
			echo "Usage: add_trap <COMMAND> <REASON>"
			echo
			echo "Push a COMMAND on to the trap stack; this COMMAND will be run before and in addition to any other current trap commands for the given REASON"

			return 0
		esac
	done

	# Check number of arguments
	if test "$#" -gt 2
	then
		{
			echo "Error: more than two argument given. Did you forget to enclose your COMMAND in quotes?"
			echo
			echo "Usage: add_trap <COMMAND> <REASON>"
		} >&2
		return 1
	fi

	# Strip trailing whitespace and semicolons from the command
	COMMAND="$(echo "$1" | sed -e 's/^\(.*[^;\s]\)[; ]*$/\1/g')"
	TRAP="$2"

	# Test that we have a command
	if test -z "$COMMAND"
	then
		{
			echo "Error: need a command to trap with"
			echo
			echo "Usage: add_trap <COMMAND> <REASON>"
		} >&2
		return 1
	fi

	# Test that we have a signal or reason to trap on
	if test -z "$TRAP"
	then
		{
			echo "Error: need a signal or other reason to trap on (such as EXIT or SIGINT)"
			echo
			echo "Usage: add_trap <COMMAND> <REASON>"
		} >&2
		return 1
	fi

	# Check no single quotes in $COMMAND
	if echo "$COMMAND" | grep -q "'"
	then
		echo "can't add commands with single quote characters"
		return 1
	fi

	# TODO: detect and reject multiline commands

	CURRENT_TRAP="$(trap -p "$TRAP" | grep -o -P "(?<=-- ').*(?=' EXIT)")"
	trap "{ $COMMAND ; } ; $CURRENT_TRAP" "$TRAP"
}

# Same as above, but without correctness checking
add_trap_unchecked() {
	COMMAND="$1"
	TRAP="$2"

	CURRENT_TRAP="$(trap -p "$TRAP" | grep -o -P "(?<=-- ').*(?=' EXIT)")"
	trap "{ $COMMAND ; } ; $CURRENT_TRAP" "$TRAP"
}