Terminate Processes

Kill Command#

It’s actually a tool for sending signals to processes, not necessarily killing them.

man kill

kill - send a signal to a process

Default Signal#

The default signal for the kill command is SIGTERM, corresponding to number 15, for a graceful termination.

Available Signals#

kill -l

 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
......

kill -l 9 15

KILL
TERM

Specify Signal Type#

kill -s SIGKILL <PID>

kill -9 <PID>

Multiple Processes#

kill -15 <PID> <PID> <PID>

Special Process Number#

The process number -1 represents all processes, and the following command will terminate all processes that you (the current user) can terminate.

kill -9 -1

If I want to execute as the kuga user, I can first view the processes of that user as the root user.

ps -fu kuga

UID          PID    PPID  C STIME TTY          TIME CMD
kuga       42994       1  0 14:13 ?        00:00:00 /lib/systemd/systemd --user
kuga       42995   42994  0 14:13 ?        00:00:00 (sd-pam)
kuga       43056   42991  0 14:13 ?        00:00:00 sshd: kuga@pts/0
kuga       43057   43056  0 14:13 pts/0    00:00:00 -bash

Then, execute the command kill -9 -1 in the kuga terminal.

Connection to x.x.x.x closed by remote host.
Connection to x.x.x.x closed.

This will immediately disconnect, and viewing the kuga user processes on root will find that they are all gone.

Pkill Command#

It sends signals to processes based on their names.

man pkill

signal for processes based on name and other attributes

Default Signal#

The default signal for the pkill command is SIGTERM, corresponding to number 15, for a graceful termination.

Specify Signal Type#

pkill -SIGKILL bash

pkill -9 bash

Specify User Processes#

pkill -u kuga

pkill -9 -u kuga bash

Precision Process Name#

pkill -x sshd

Pgrep#

It looks up process IDs based on process names.

man pgrep

look up for processes based on name and other attributes 

Ignore Case#

pgrep -i BASH

Specify User Processes#

pgrep -u kuga

44087
44088
44149
44150

Process IDs and Names#

Without -l, it will only display the process ID.

pgrep -l bash

42977 bash
44150 bash

Number of Matching Processes#

pgrep -c bash

2

Specify Parent PID#

pgrep -P 1729

42912
44084

Recently Started Processes#

pgrep -n -l

pgrep -n bash -l

Earliest Started Processes#

pgrep -o -l

1 systemd

Precision Process Name#

pgrep -x sshd

Common Signals#

SIGTERM - 15#

Requests the process to terminate. This signal is “friendly” and allows the process to perform cleanup before exiting. It is the default signal for kill and pkill commands. Usually used for a graceful termination of processes, giving them time to handle unfinished tasks.

SIGKILL - 9#

Forces the process to terminate. This signal cannot be caught, blocked, or ignored, and the process will be immediately terminated. Sending SIGKILL will directly stop the process without allowing it to perform any cleanup operations. Used for processes that cannot be normally terminated, when SIGTERM is ineffective.

SIGINT - 2#

Interrupt signal, usually sent by the user through Ctrl+C, used to interrupt foreground running processes. Used for manually interrupting processes, especially interactive processes.

SIGQUIT - 3#

Quit signal, usually sent by the user through Ctrl+\, indicating that the process should generate a core dump and exit. Used for debugging, when you want the process to generate a core dump for analysis.

SIGHUP - 1#

Hangup signal, usually indicating that the terminal or console connection has been disconnected. Many daemon processes will reload their configuration files when they receive the SIGHUP signal. Used for reloading the configuration of daemon processes or causing them to restart.