Signal handling with trap in Bash Scripting - Step-by-Step Execution

Concept Flow - Signal handling with trap

Start script

↓

Set trap for signal

↓

Run main loop or commands

↓

Signal received?

No→Continue running

Yes↓

Execute trap command

↓

Continue or exit script

The script sets a trap to catch signals, runs commands, and when a signal arrives, it runs the trap command before continuing or exiting.

Execution Sample

Bash Scripting

trap "echo 'Signal received, exiting'; exit" SIGINT
while true; do
  echo "Running..."
  sleep 1
done

This script prints 'Running...' every second until you press Ctrl+C, which triggers the trap to print a message and exit.

Execution Table

Step	Action	Signal Received	Trap Command Executed	Output	Script State
1	Start script and set trap	No	No		Running
2	Print 'Running...'	No	No	Running...	Running
3	Sleep 1 second	No	No		Running
4	Print 'Running...'	No	No	Running...	Running
5	Sleep 1 second	No	No		Running
6	User presses Ctrl+C (SIGINT)	Yes	Yes	Signal received, exiting	Exiting
7	Exit script	No	No		Exited

💡 User pressed Ctrl+C, SIGINT caught, trap executed, script exited

Variable Tracker

Variable	Start	After 1	After 2	After SIGINT	Final
Script State	Not started	Running	Running	Exiting	Exited

Key Moments - 3 Insights

Why does the script not exit immediately when started?

Because the trap only runs when the specified signal (SIGINT) is received. Until then, the script keeps running the loop as shown in rows 2-5 of the execution_table.

What happens when Ctrl+C is pressed during the sleep?

The signal SIGINT is sent, the trap command runs (row 6), printing the message and exiting the script immediately.

Can the trap command run multiple times if multiple signals are sent?

Yes, each time the signal is received, the trap command runs. But in this example, the trap exits the script, so it runs only once.

Visual Quiz - 3 Questions

Look at the execution_table, what is the script state after step 4?

A Exiting

B Exited

C Running

D Not started

💡 Hint

Check the 'Script State' column at step 4 in the execution_table

At which step does the trap command execute?

A Step 6

B Step 4

C Step 2

D Step 7

💡 Hint

Look for 'Trap Command Executed' marked 'Yes' in the execution_table

If the trap command did not call exit, what would happen after step 6?

A Script would exit immediately

B Script would continue running the loop

C Script would pause indefinitely

D Script would restart

💡 Hint

Refer to the key_moments explanation about trap behavior without exit

Concept Snapshot

trap "command" SIGNAL
- Sets a command to run when SIGNAL is received
- Commonly used to handle Ctrl+C (SIGINT)
- Allows cleanup or graceful exit
- Script continues unless trap command exits
- Example: trap "echo Exiting; exit" SIGINT

Full Transcript

This visual trace shows how a bash script uses trap to handle signals. The script sets a trap for SIGINT (Ctrl+C). It runs a loop printing 'Running...' every second. When the user presses Ctrl+C, the signal is caught, the trap command runs printing 'Signal received, exiting', then the script exits. Variables like script state change from running to exiting to exited. Key moments include understanding that the trap only runs on signal, and that exit in the trap stops the script. The quiz tests understanding of script state and trap execution steps.