Bash trap command

Sometimes you want to catch a special signal, interruption, or user input in your script to prevent the unpredictable.

trap is your command to try:

• trap <arg/function> <signal>

When a signal is received, the function or command listed as <arg> runs. Some of the common signal types you can trap:

• SIGINT - user sends an interrupt signal (Ctrl + C)
• SIGQUIT - user sends a quit signal (Ctrl + D)
• SIGFPE - attempted an illegal mathematical operation

You can check out all signal types by entering the following command:

kill -l

Notice the numbers before each signal name. You can use that number to avoid typing long strings in trap:

# 2 corresponds to SIGINT and 15 corresponds to SIGTERM
trap booh 2 15

One common use of trap is to do cleanup of temporary files:

trap "rm -f folder; exit" 2

Here is a complete runnable example. The script sets a trap for SIGINT, sends itself the signal, and the handler runs:

#!/bin/bash
trap 'echo "Caught SIGINT"' SIGINT
echo "before kill"
kill -SIGINT $$
echo "after kill"

This prints:

before kill
Caught SIGINT
after kill

Exercise

Fix the trap handler below so that instead of printing TODO: replace me, it prints Bye! when SIGINT is received.