How to Safely Kill Linux Processes: kill, killall, and pkill Explained

Understanding Linux termination signals

When a process is stopped by the operating system or a user, a termination signal is sent. The most common signals are:

SIGHUP (1) : Hangup – sent when the controlling terminal closes.

SIGINT (2) : Interrupt – generated by Ctrl+C in the terminal.

SIGKILL (9) : Kill – forces immediate termination, cannot be caught.

SIGTERM (15) : Termination – default signal for kill, allows graceful shutdown and can be handled by the process.

SIGSTOP (19/17/23/24) : Stop – pauses a process; it can be resumed later.

Using kill to terminate a process

The kill command requires the target process ID (PID) and optionally a signal. If no signal is specified, SIGTERM (15) is used. kill [signal] <PID> Example: start a background sleep job and then terminate it.

$ sleep 120 &
[1] 125686
$ kill 125686
[1]+  terminated  sleep 120

You can also specify the signal by name or number:

$ kill -SIGKILL 125746
$ kill -9 125759

Finding a process ID

Before using kill, you need the PID. The pidof command returns the PID(s) of a given executable name. pidof exact_process_name Example for Java processes:

pidof java
8075 1032

Using killall to terminate by name

If you do not know the PID or want to kill all instances of a program (including child processes), use killall. The syntax is similar to kill and defaults to SIGTERM (15) when no signal is given. killall [signal] <process-name> Example: kill all running sleep commands.

$ sleep 120 &
[1] 112351
$ sleep 2000 &
[2] 112362
$ killall sleep
[1]-  Terminated  sleep 120
[2]+  Terminated  sleep 2000

Using pkill for pattern‑based termination

The pkill command combines pgrep pattern matching with kill. It can match processes by name, owner, or other criteria. pkill [options] pattern Useful options include: -u: match processes owned by a specific user. -x: require an exact name match. -signal: specify the termination signal (default SIGTERM).

Example: terminate all nginx processes.

pkill nginx