kill and signal types in Linux CLI - Commands & Configuration

Introduction

Sometimes a program or process on your computer stops responding or needs to be stopped. The kill command lets you send signals to these processes to control or stop them safely or forcefully.

When a program freezes and you want to stop it without restarting your computer

When you want to politely ask a program to close and save its work

When a program ignores polite requests and you need to force it to stop immediately

When you want to send a custom signal to a process to trigger a specific action

When managing background tasks and you need to control their behavior

Commands

This command sends the SIGTERM signal to the process with ID 1234. SIGTERM politely asks the process to stop, allowing it to clean up before exiting.

Terminal

kill -SIGTERM 1234

Expected Output

No output (command runs silently)

→

-SIGTERM - Specifies the polite termination signal

This command sends the SIGKILL signal (signal number 9) to process 1234. It forcefully stops the process immediately without cleanup.

Terminal

kill -9 1234

Expected Output

No output (command runs silently)

→

-9 - Force kill signal that cannot be ignored

Lists all available signal names and their numbers so you can choose which signal to send.

Terminal

kill -l

Expected Output

 1) SIGHUP  2) SIGINT  3) SIGQUIT  4) SIGILL  5) SIGTRAP  6) SIGABRT  7) SIGBUS  8) SIGFPE  9) SIGKILL  10) SIGUSR1

11) SIGSEGV  12) SIGUSR2  13) SIGPIPE  14) SIGALRM  15) SIGTERM  16) SIGSTKFLT  17) SIGCHLD  18) SIGCONT  19) SIGSTOP  20) SIGTSTP

21) SIGTTIN  22) SIGTTOU  23) SIGURG  24) SIGXCPU  25) SIGXFSZ  26) SIGVTALRM  27) SIGPROF  28) SIGWINCH  29) SIGIO  30) SIGPWR

31) SIGSYS

Sends the SIGINT signal to process 1234, which usually interrupts the process as if you pressed Ctrl+C.

Terminal

kill -SIGINT 1234

Expected Output

No output (command runs silently)

→

-SIGINT - Interrupt signal to stop a process gracefully

Key Concept

If you remember nothing else from this pattern, remember: use SIGTERM to politely stop a process and SIGKILL to force it to stop immediately.

Common Mistakes

Using kill without specifying a signal and expecting it to force stop the process

By default, kill sends SIGTERM which the process can ignore or delay, so it may not stop immediately.

Use kill -9 to send SIGKILL if you need to force stop a process immediately.

Trying to kill a process with the wrong process ID

The kill command will fail silently or affect the wrong process if the ID is incorrect.

Verify the process ID using commands like ps or pidof before using kill.

Using kill -9 as the first option without trying polite signals

SIGKILL does not allow the process to clean up, which can cause data loss or corruption.

Try kill with SIGTERM first to allow graceful shutdown before using SIGKILL.

Summary

kill sends signals to processes to control or stop them.

SIGTERM (default) politely asks a process to stop, allowing cleanup.

SIGKILL (signal 9) forcefully stops a process immediately without cleanup.