Signal handling with trap in Bash Scripting - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to catch the SIGINT signal and run the cleanup function.

Bash Scripting

trap cleanup [1]

Drag options to blanks, or click blank then click option'

ASIGKILL

BSIGINT

CSIGTERM

DSIGSTOP

Attempts:

3 left

💡 Hint

Common Mistakes

Using SIGKILL or SIGSTOP which cannot be trapped.

Confusing SIGTERM with SIGINT.

The trap command catches signals. SIGINT is the interrupt signal sent by Ctrl+C.

2fill in blank

medium

Complete the code to ignore the SIGTERM signal.

Bash Scripting

trap '' [1]

Drag options to blanks, or click blank then click option'

ASIGTERM

BSIGHUP

CSIGQUIT

DSIGINT

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to ignore SIGINT instead of SIGTERM.

Using an invalid signal name.

Using trap '' SIGTERM tells the script to ignore the termination signal.

3fill in blank

hard

Fix the error in the trap command to correctly catch SIGQUIT and run the quit_handler function.

Bash Scripting

trap [1] SIGQUIT

Drag options to blanks, or click blank then click option'

Aquit_handler;

Bquit_handler &

Cquit_handler()

Dquit_handler

Attempts:

3 left

💡 Hint

Common Mistakes

Adding parentheses after the function name.

Adding extra characters like '&' or ';' after the function name.

The trap command expects the function name without parentheses or extra characters.

4fill in blank

hard

Fill both blanks to run cleanup on SIGINT and SIGTERM signals.

Bash Scripting

trap cleanup [1] [2]

Drag options to blanks, or click blank then click option'

ASIGINT

BSIGTERM

CSIGKILL

DSIGSTOP

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to trap SIGKILL or SIGSTOP which cannot be caught.

Using wrong signal names.

Both SIGINT and SIGTERM are common signals to stop a script. Trap them to run cleanup.

5fill in blank

hard

Fill all three blanks to create a trap that runs handle_exit on EXIT and SIGINT.

Bash Scripting

trap [1] [2] [3]

Drag options to blanks, or click blank then click option'

Ahandle_exit

BEXIT

CSIGINT

DSIGTERM

Attempts:

3 left

💡 Hint

Common Mistakes

Putting signals before the function name.

Using parentheses with the function name.

The trap command syntax is: trap function signal1 signal2 .... Here, handle_exit runs on EXIT and SIGINT.

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