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 fire for the same signal?

Answer 1

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 2

Edit:

It appears that I misread the question. The answer is simple:

handler1 () { do_something; } handler2 () { do_something_else; } handler3 () { handler1; handler2; }  trap handler3 SIGNAL1 SIGNAL2 ... 

---

Original:

Just list multiple signals at the end of the command:

trap function-name SIGNAL1 SIGNAL2 SIGNAL3 ... 

You can find the function associated with a particular signal using trap -p:

trap -p SIGINT 

Note that it lists each signal separately even if they're handled by the same function.

You can add an additional signal given a known one by doing this:

eval "$(trap -p SIGUSR1) SIGUSR2" 

This works even if there are other additional signals being processed by the same function. In other words, let's say a function was already handling three signals - you could add two more just by referring to one existing one and appending two more (where only one is shown above just inside the closing quotes).

If you're using Bash >= 3.2, you can do something like this to extract the function given a signal. Note that it's not completely robust because other single quotes could appear.

[[ $(trap -p SIGUSR1) =~ trap\ --\ \'([^\047]*)\'.* ]] function_name=${BASH_REMATCH[1]} 

Then you could rebuild your trap command from scratch if you needed to using the function name, etc.