Signal handling with trap in Bash Scripting - Mini Project: Build & Apply

Signal handling with trap

📖 Scenario: You are writing a simple bash script that runs a loop and waits for user interruption. You want to handle the interrupt signal gracefully to clean up before exiting.

🎯 Goal: Build a bash script that uses trap to catch the SIGINT signal (Ctrl+C) and print a friendly message before exiting.

📋 What You'll Learn

Create a variable called count and set it to 0

Create a trap command to catch SIGINT and run a function called handle_interrupt

Define a function called handle_interrupt that prints "Interrupt caught! Exiting..." and exits the script

Create a while loop that increments count every second and prints the current count

Print the final message when the script is interrupted

💡 Why This Matters

🌍 Real World

Signal handling is important in scripts that run continuously or wait for user input. It helps to clean up resources or save state before the script stops.

💼 Career

Many system administration and automation tasks require writing scripts that handle signals to avoid abrupt termination and data loss.

1

Create a counter variable

Create a variable called count and set it to 0.

Bash Scripting

# Create a variable called count and set it to 0
# Your code here

Hint

Use count=0 to create the variable.

2

Set up the trap for SIGINT

Create a trap command to catch the SIGINT signal and run a function called handle_interrupt.

Bash Scripting

count=0
# Create a trap for SIGINT to call handle_interrupt
# Your code here

Hint

Use trap handle_interrupt SIGINT to catch Ctrl+C.

3

Define the interrupt handler function

Define a function called handle_interrupt that prints "Interrupt caught! Exiting..." and exits the script.

Bash Scripting

count=0
trap handle_interrupt SIGINT

# Define the handle_interrupt function
# Your code here

Hint

Define the function with handle_interrupt() { ... } and use echo and exit 0.

4

Create the counting loop and print output

Create a while loop that increments count every second and prints the current count using echo. The script should run until interrupted. The output should show the count numbers starting from 1.

Bash Scripting

count=0
trap handle_interrupt SIGINT

handle_interrupt() {
    echo "Interrupt caught! Exiting..."
    exit 0
}

# Create a while loop to increment count and print it every second
# Your code here

Hint

Use while true; do ... done, increment count with count=$((count + 1)), print with echo, and pause with sleep 1.

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