Bash trap Command - Handle Signals

Using the trap Command

The trap command runs a command when your script receives a signal, or when it exits.

It is most often used to clean up, for example to delete temporary files, even if the script is stopped early.

Syntax

trap 'commands' SIGNAL

Common signals:

• EXIT: The script ends, for any reason
• INT: The user pressed Ctrl+C
• TERM: The script was asked to stop (for example by kill)
• ERR: A command failed

Run Code When the Script Ends

A trap on EXIT always runs when the script ends:

#!/bin/bash
trap 'echo "Goodbye!"' EXIT
echo "Working..."

./bye.sh
Working...
Goodbye!

Clean Up Temporary Files

Put the cleanup code in a function, and call it from the trap. The temporary file is removed even if the script fails or is stopped:

#!/bin/bash
tmpfile=$(mktemp)
cleanup() {
  rm -f "$tmpfile"
  echo "Removed the temporary file"
}
trap cleanup EXIT
echo "data" > "$tmpfile"
echo "Using the temporary file"

./cleanup.sh
Using the temporary file
Removed the temporary file

Catch Ctrl+C

A trap on INT runs when the user presses Ctrl+C. Here, the script prints a message and exits:

#!/bin/bash
trap 'echo " Stopped by user"; exit 130' INT
echo "Press Ctrl+C to stop"
sleep 60

./wait.sh
Press Ctrl+C to stop
^C Stopped by user

The Try it Yourself editor has no Ctrl+C, so its versions of the INT examples send the signal with kill -INT $$ ($$ is the script's own process ID).

Ignore a Signal

Use an empty string to ignore a signal. This script cannot be stopped with Ctrl+C while the loop runs:

#!/bin/bash
trap '' INT
for i in 1 2 3 4 5; do
  echo "Step $i"
  sleep 1
done

Use trap - INT to go back to the normal behavior.

Report Errors

A trap on ERR runs every time a command fails. $LINENO holds the line number:

#!/bin/bash
trap 'echo "Error on line $LINENO"' ERR
echo "Start"
ls missing.txt 2> /dev/null
echo "End"

./errors.sh
Start
Error on line 4
End