Multiple Bash traps for the same signal

Technically you can’t set multiple traps for the same signal, but you can add to an existing trap:

1. Fetch the existing trap code using trap -p
2. Add your command, separated by a semicolon or newline
3. Set the trap to the result of #2

Here is a bash function that does the above:

# note: printf is used instead of echo to avoid backslash
# processing and to properly handle values that begin with a '-'.

log() { printf '%s\n' "$*"; }
error() { log "ERROR: $*" >&2; }
fatal() { error "$@"; exit 1; }

# appends a command to a trap
#
# - 1st arg:  code to add
# - remaining args:  names of traps to modify
#
trap_add() {
    trap_add_cmd=$1; shift || fatal "${FUNCNAME} usage error"
    for trap_add_name in "$@"; do
        trap -- "$(
            # helper fn to get existing trap command from output
            # of trap -p
            extract_trap_cmd() { printf '%s\n' "$3"; }
            # print existing trap command with newline
            eval "extract_trap_cmd $(trap -p "${trap_add_name}")"
            # print the new trap command
            printf '%s\n' "${trap_add_cmd}"
        )" "${trap_add_name}" \
            || fatal "unable to add to trap ${trap_add_name}"
    done
}
# set the trace attribute for the above function.  this is
# required to modify DEBUG or RETURN traps because functions don't
# inherit them unless the trace attribute is set
declare -f -t trap_add

Example usage:

trap_add 'echo "in trap DEBUG"' DEBUG