Signal handling with trap in Bash Scripting

Introduction

Signal handling lets your script respond to events like pressing Ctrl+C. Using trap helps your script clean up or stop safely.

You want to stop a script safely when a user presses Ctrl+C.

You need to delete temporary files if the script is interrupted.

You want to run a command when the script exits for any reason.

You want to ignore certain signals to keep the script running.

You want to log a message when the script receives a specific signal.

Syntax

Bash Scripting

trap 'commands' SIGNALS

Replace 'commands' with the commands to run when the signal is caught.

Replace SIGNALS with one or more signals like SIGINT, SIGTERM, EXIT.

Examples

Runs a message and exits when you press Ctrl+C (SIGINT).

Bash Scripting

trap 'echo "Script interrupted!"; exit' SIGINT

Deletes a temp file when the script ends for any reason.

Bash Scripting

trap 'rm -f /tmp/tempfile' EXIT

Ignores the SIGTERM signal so the script won't stop from it.

Bash Scripting

trap '' SIGTERM

Sample Program

This script runs an endless loop. When you press Ctrl+C, it prints a message and exits safely.

Bash Scripting

#!/bin/bash

trap 'echo "You pressed Ctrl+C! Cleaning up..."; exit' SIGINT

echo "Script is running. Press Ctrl+C to stop."

while true; do
  sleep 1
done

Important Notes

Use EXIT signal to run commands when the script finishes normally or is interrupted.

You can trap multiple signals by listing them after the commands, like: trap 'commands' SIGINT SIGTERM.

Always test your trap commands to make sure they do what you want.

Summary

trap lets your script catch signals like Ctrl+C and respond.

Use trap to clean up or stop safely when interrupted.

You can run commands on exit or ignore signals with trap.

Practice

1. What does the trap command do in a bash script?

easy

A. It creates a loop that repeats commands indefinitely.

B. It catches signals like Ctrl+C and runs specified commands.

C. It defines a function to be used later in the script.

D. It pauses the script until a key is pressed.

Solution

1. Step 1: Understand the purpose of trap

   The trap command is used to catch signals sent to the script, such as Ctrl+C (SIGINT).

2. Step 2: What trap does with signals

   When a signal is caught, trap runs the commands specified by the user to handle that signal.

3. Final Answer:

   It catches signals like Ctrl+C and runs specified commands. -> Option B

4. Quick Check:

   trap catches signals = B [OK]

Hint: Trap runs commands on signals like Ctrl+C [OK]

Common Mistakes:

• Thinking trap creates loops
• Confusing trap with function definition
• Assuming trap pauses script

2. Which of the following is the correct syntax to run echo "Bye" when the script receives a SIGINT signal?

easy

A. trap 'echo "Bye"' SIGINT

B. trap echo "Bye" SIGINT

C. trap (echo "Bye") SIGINT

D. trap {echo "Bye"} SIGINT

Solution

1. Step 1: Check trap syntax for commands

   The commands to run must be quoted as a single string, usually with single quotes to preserve special characters.

2. Step 2: Identify correct quoting style

   trap 'echo "Bye"' SIGINT uses single quotes around the entire command string, which is the correct syntax.

3. Final Answer:

   trap 'echo "Bye"' SIGINT -> Option A

4. Quick Check:

   trap 'commands' SIGNAL = A [OK]

Hint: Use single quotes around commands in trap [OK]

Common Mistakes:

• Not quoting commands properly
• Using parentheses or braces incorrectly
• Omitting quotes causing syntax errors

3. What will be the output of this script when you press Ctrl+C during its execution?

#!/bin/bash
trap 'echo "Interrupted! Cleaning up..."; exit' SIGINT
while true; do
  echo "Running..."
  sleep 2
done

medium

A. The script ignores Ctrl+C and keeps running.

B. It crashes with an error message.

C. It prints "Running..." once and then exits.

D. It prints "Interrupted! Cleaning up..." and exits.

Solution

1. Step 1: Understand trap on SIGINT

   The trap command catches the SIGINT signal (Ctrl+C) and runs the echo and exit commands.

2. Step 2: Effect of trap during infinite loop

   When Ctrl+C is pressed, the script prints the message and exits immediately, stopping the loop.

3. Final Answer:

   It prints "Interrupted! Cleaning up..." and exits. -> Option D

4. Quick Check:

   trap on SIGINT prints message and exits = D [OK]

Hint: trap runs commands on Ctrl+C, then script exits [OK]

Common Mistakes:

• Thinking script ignores Ctrl+C
• Assuming loop stops without message
• Expecting error instead of clean exit

4. Identify the error in this script snippet:

trap echo "Exiting" SIGTERM
sleep 10

medium

A. sleep command cannot be used with trap.

B. SIGTERM is not a valid signal name.

C. Missing quotes around the command to run in trap.

D. trap must be placed after sleep command.

Solution

1. Step 1: Check trap command syntax

   The command to run on signal must be quoted as a single string to avoid syntax errors.

2. Step 2: Identify missing quotes

   Here, echo "Exiting" is not quoted, so the shell misinterprets the command.

3. Final Answer:

   Missing quotes around the command to run in trap. -> Option C

4. Quick Check:

   trap commands need quotes = C [OK]

Hint: Always quote trap commands to avoid syntax errors [OK]

Common Mistakes:

• Not quoting trap commands
• Using invalid signal names
• Placing trap after commands needing it

5. You want a bash script to delete a temporary file /tmp/mytempfile when it exits, whether normally or due to Ctrl+C. Which trap command correctly achieves this?

hard

A. trap 'rm -f /tmp/mytempfile' EXIT

B. trap 'rm -f /tmp/mytempfile' SIGINT

C. trap 'rm -f /tmp/mytempfile' SIGTERM

D. trap 'rm -f /tmp/mytempfile' SIGINT SIGTERM

Solution

1. Step 1: Understand EXIT signal

   EXIT is a special trap signal that runs commands when the script ends for any reason, including normal exit or Ctrl+C.

2. Step 2: Why EXIT is better than SIGINT or SIGTERM alone

   Using EXIT ensures cleanup runs on all script exits, not just specific signals.

3. Final Answer:

   trap 'rm -f /tmp/mytempfile' EXIT -> Option A

4. Quick Check:

   Use EXIT to clean up on all exits = A [OK]

Hint: Use trap on EXIT to clean up on any script exit [OK]

Common Mistakes:

• Using only SIGINT or SIGTERM misses normal exit
• Not quoting the command
• Expecting multiple signals needed for all exits