06. Bash trap Command

The trap command in Bash lets you catch signals sent to a script and run custom code when those signals occur.

It is commonly used to:

• Handle interruptions (like Ctrl + C)

• Perform cleanup before exit

• Prevent accidental termination

• Handle script errors safely

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1. What is a Signal?

A signal is a notification sent to a process to inform it that an event occurred.

Common signals:

Signal	Name	Trigger
INT	Interrupt	Ctrl + C from keyboard
TERM	Terminate	kill <pid> default signal
KILL	Kill	kill -9 <pid> (cannot trap)
EXIT	Exit	Script finishes execution
ERR	Error	Command fails (non-zero status)

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2. Basic Syntax

trap 'commands' SIGNAL

or using a function:

trap function_name SIGNAL

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3. Example: Detect Ctrl + C (INT Signal)

Script

#!/bin/bash
# 06. Bash trap Command
trap bashtrap INT

# Function executed on CTRL+C
bashtrap() {
    echo "CTRL+C Detected!... executing bash trap!"
}

# Loop from 1 to 10
for a in $(seq 1 10); do
    echo "$a/10 to Exit."
    sleep 1
done

echo "Exit Bash Trap Example!!!"

What Happens

• When Ctrl + C is pressed:

  • Bash sends the INT signal

  • trap catches it

  • Function bashtrap() runs

  • Script continues execution

Output (Sample)

1/10 to Exit.
2/10 to Exit.
^CCTRL+C Detected!... executing bash trap!
3/10 to Exit.
...
10/10 to Exit.
Exit Bash Trap Example!!!

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4. Trap Multiple Signals

You can trap multiple signals at once.

trap cleanup INT TERM EXIT

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5. Example: Cleanup Before Exit

Useful for deleting temp files, stopping services, restoring settings.

#!/bin/bash

cleanup() {
    echo "Cleaning up resources..."
    rm -f temp.txt
}

trap cleanup EXIT

echo "Temporary data" > temp.txt
echo "Script running..."
sleep 3

What Happens

• When script exits normally or due to interruption

• cleanup() runs automatically

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6. Prevent Script From Being Killed Easily

You can ignore signals.

trap '' INT

Now Ctrl + C will not stop the script.

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7. Restore Default Behavior

trap - INT

Removes the custom trap and restores default signal handling.

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8. Trap Errors

Run a function when any command fails.

#!/bin/bash

error_handler() {
    echo "An error occurred!"
}

trap error_handler ERR

ls /nonexistent
echo "This will still run"

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9. Common Use Cases

Graceful Shutdown

trap "echo Shutting down...; exit" INT TERM

Temporary File Cleanup

trap "rm -f /tmp/script.$$" EXIT

Debugging

trap 'echo "Line $LINENO failed"' ERR

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10. Important Notes

• SIGKILL cannot be trapped

• Use quotes to avoid early variable expansion

• Traps apply to the current shell and subshells differently

• EXIT trap always runs when script finishes

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11. Best Practices

• Always use traps for cleanup in production scripts

• Combine EXIT + INT for reliability

• Keep trap handlers lightweight

• Avoid complex logic inside trap functions

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12. Quick Reference

Task	Command
Catch Ctrl+C	trap handler INT
Cleanup on exit	trap handler EXIT
Ignore Ctrl+C	trap '' INT
Restore default	trap - INT
Catch errors	trap handler ERR

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If you're writing automation, pentesting scripts, or long-running recon tools, trap is essential for safe interruption and cleanup.