Identifying received signal name in Bash

Question

When a signal is received, I can execute some commands using trap. Example:

trap 'echo hello world' 1 2 

If any of the signals specified is received, the hello world' is displayed.

But how can I print/identify the received signal name?

Answer 1

(If you only have the number of a signal and want the name, kill -l $SIGNAL_NUM prints the name of a signal; you can avoid that by using the signal names instead of numbers in your call to trap as below.)

This answer says that there's no way to access the signal name, but if you have a separate function for each signal that you trap, then you already know the signal name:

trap 'echo trapped the HUP signal' HUP trap 'echo different trap for the INT signal' INT 

In many cases, that may be sufficient, but another answer on that same question uses that fact to provide a workaround to fake the behavior you want. It takes a function and a list of signals and sets a separate trap for each signal on that function called with the signal name, so internally it's actually a separate function for each signal but it looks like a single trap on a single function that gets the signal name as an argument:

Code:

#!/bin/bash  trap_with_arg() {     func="$1" ; shift     for sig ; do         trap "$func $sig" "$sig"     done }  func_trap() {     echo "Trapped: $1" }  trap_with_arg func_trap INT TERM EXIT  echo "Send signals to PID $$ and type [enter] when done." read # Wait so the script doesn't exit. 

If I run that, then I can send signals to the process and I get output like

Trapped: INT Trapped: TERM Trapped: EXIT 

Answer 2

Within the trap (when triggered via a signal), the $? variable is initially set to the signal number plus 128, so you can assign the signal number to a variable by making the first statement of the trap action to something like

sig=$(($? - 128)) 

You can then get the name of the signal using the kill command

kill -l $sig 

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Update: As noted in the comments, this doesn't work for some builtin shell commands. To have these set the trap's $? variable, they can be run in a subshell, Eg

(read) 

instead of

read