multiple bash traps for the same signal

Question

When I use the trap command in bash, the previous trap for the given signal is replaced.

Is there a way of making more than one trap<

Answer 1

I didn't like having to play with these string manipulations which are confusing at the best of times, so I came up with something like this:

(obviously you can modify it for other signals)

exit_trap_command=""
function cleanup {
    eval "$exit_trap_command"
}
trap cleanup EXIT

function add_exit_trap {
    local to_add=$1
    if [[ -z "$exit_trap_command" ]]
    then
        exit_trap_command="$to_add"
    else
        exit_trap_command="$exit_trap_command; $to_add"
    fi
}

Answer 2

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

Answer 3

I liked Richard Hansen's answer, but I don't care for embedded functions so an alternate is:

#===================================================================
# FUNCTION trap_add ()
#
# Purpose:  appends a command to a trap
#
# - 1st arg:  code to add
# - remaining args:  names of traps to modify
#
# Example:  trap_add 'echo "in trap DEBUG"' DEBUG
#
# See: http://stackoverflow.com/questions/3338030/multiple-bash-traps-for-the-same-signal
#===================================================================
trap_add() {
    trap_add_cmd=$1; shift || fatal "${FUNCNAME} usage error"
    new_cmd=
    for trap_add_name in "$@"; do
        # Grab the currently defined trap commands for this trap
        existing_cmd=`trap -p "${trap_add_name}" |  awk -F"'" '{print $2}'`

        # Define default command
        [ -z "${existing_cmd}" ] && existing_cmd="echo exiting @ `date`"

        # Generate the new command
        new_cmd="${existing_cmd};${trap_add_cmd}"

        # Assign the test
         trap   "${new_cmd}" "${trap_add_name}" || \
                fatal "unable to add to trap ${trap_add_name}"
    done
}

Answer 4

Here's another option:

on_exit_acc () {
    local next="$1"
    eval "on_exit () {
        local oldcmd='$(echo "$next" | sed -e s/\'/\'\\\\\'\'/g)'
        local newcmd=\"\$oldcmd; \$1\"
        trap -- \"\$newcmd\" 0
        on_exit_acc \"\$newcmd\"
    }"
}
on_exit_acc true

Usage:

$ on_exit date
$ on_exit 'echo "Goodbye from '\''`uname`'\''!"'
$ exit
exit
Sat Jan 18 18:31:49 PST 2014
Goodbye from 'FreeBSD'!
tap# 

Answer 5

No

About the best you could do is run multiple commands from a single trap for a given signal, but you cannot have multiple concurrent traps for a single signal. For example:

$ trap "rm -f /tmp/xyz; exit 1" 2
$ trap
trap -- 'rm -f /tmp/xyz; exit 1' INT
$ trap 2
$ trap
$

The first line sets a trap on signal 2 (SIGINT). The second line prints the current traps — you would have to capture the standard output from this and parse it for the signal you want. Then, you can add your code to what was already there — noting that the prior code will most probably include an 'exit' operation. The third invocation of trap clears the trap on 2/INT. The last one shows that there are no traps outstanding.

You can also use trap -p INT or trap -p 2 to print the trap for a specific signal.

Answer 6

I would like to propose my solution of multiple trap functions for simple scripts

# Executes cleanup functions on exit
function on_exit {
    for FUNCTION in $(declare -F); do
        if [[ ${FUNCTION} == *"_on_exit" ]]; then
            >&2 echo ${FUNCTION}
            eval ${FUNCTION}
        fi
    done
}
trap on_exit EXIT

function remove_fifo_on_exit {
    >&2 echo Removing FIFO...
}

function stop_daemon_on_exit {
    >&2 echo Stopping daemon...
}