kill and signal types in Linux CLI - Step-by-Step Execution

Concept Flow - kill and signal types

Start: Identify process

↓

Choose signal type

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Send signal with kill command

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Process receives signal

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Process reacts: terminate, pause, ignore, etc.

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End: Signal effect observed

The flow shows how you pick a process, choose a signal, send it with kill, and the process reacts accordingly.

Execution Sample

Linux CLI

kill -SIGTERM 1234
kill -9 1234
kill -SIGSTOP 1234

Send different signals (terminate, force kill, stop) to process with ID 1234.

Execution Table

Step	Command	Signal Sent	Process Reaction	Output/Result
1	kill -SIGTERM 1234	SIGTERM (15)	Process requests graceful shutdown	No output if successful
2	kill -9 1234	SIGKILL (9)	Process immediately terminates	No output if successful
3	kill -SIGSTOP 1234	SIGSTOP (19)	Process pauses (stops execution)	No output if successful
4	kill -SIGCONT 1234	SIGCONT (18)	Process resumes execution	No output if successful
5	kill -SIGINT 1234	SIGINT (2)	Process interrupts (like Ctrl+C)	No output if successful
6	kill -SIGUSR1 1234	SIGUSR1 (10)	Process handles user-defined signal	No output if successful
7	kill -SIGTERM 999999	SIGTERM (15)	No such process	kill: (999999) - No such process

💡 Execution stops after all signals sent or if process does not exist.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	After Step 7
Process State	Running	Graceful shutdown requested	Terminated	Paused	Resumed	Interrupted	User signal handled	No process found

Key Moments - 3 Insights

Why does kill -9 immediately stop the process while kill -SIGTERM allows it to clean up?

kill -9 sends SIGKILL which cannot be caught or ignored, forcing immediate termination. kill -SIGTERM sends SIGTERM which the process can catch and handle to clean up before exiting, as shown in steps 1 and 2 of the execution_table.

What happens if you send a signal to a process ID that does not exist?

The kill command returns an error message like 'No such process' as shown in step 7 of the execution_table, because the system cannot find the process to signal.

How does SIGSTOP differ from SIGTERM in process control?

SIGSTOP pauses the process without terminating it, allowing it to be resumed later with SIGCONT, as shown in steps 3 and 4. SIGTERM requests termination.

Visual Quiz - 3 Questions

Look at the execution_table, what signal does 'kill -9 1234' send?

A SIGKILL (9)

B SIGTERM (15)

C SIGSTOP (19)

D SIGINT (2)

💡 Hint

Check step 2 in the execution_table under 'Signal Sent' column.

At which step does the process get paused?

A Step 1

B Step 3

C Step 5

D Step 7

💡 Hint

Look for 'Paused' in the 'Process Reaction' column in the execution_table.

If you send SIGTERM to a non-existing process, what output do you expect?

A Process terminates silently

B No output

C kill: (999999) - No such process

D Process pauses

💡 Hint

See step 7 in the execution_table for the error message.

Concept Snapshot

kill command sends signals to processes by PID
Common signals: SIGTERM(15) graceful stop, SIGKILL(9) force stop, SIGSTOP(19) pause
Process reacts based on signal: terminate, pause, ignore
Use kill -SIGNAL PID or kill -number PID
SIGKILL cannot be caught or ignored
SIGSTOP pauses process until SIGCONT resumes it

Full Transcript

This lesson shows how the kill command sends different signals to processes by their ID. First, you identify the process to signal. Then you choose a signal type like SIGTERM to ask the process to stop nicely, or SIGKILL to force it to stop immediately. The process reacts depending on the signal: it may shut down, pause, or ignore it. The execution table traces sending signals like SIGTERM, SIGKILL, SIGSTOP, and SIGCONT to process 1234, showing how the process state changes from running to terminated or paused. If you send a signal to a non-existing process, kill reports an error. Key points include that SIGKILL cannot be caught or ignored, while SIGTERM can be handled by the process. SIGSTOP pauses the process until SIGCONT resumes it. This helps you control processes safely and effectively in Linux.