kill command in Linux

The kill command is commonly used to terminate running processes, it takes the process ID or name as its arguments and sends a signal to the process and the process will act according the the sent signal.

Table of contents.

1. Introduction.
2. Syntax.
3. Commands.
4. Summary.
5. References.

Introduction.

The kill command is commonly used to terminate processes in Linux, give a processes id PID it sends the default signal SIGTERM which terminates the process.

Keep in mind that processes can only be killed by the owner(user), a user cannot kill processes owned by other users, a user cannot kill processes used by the system and finally a root user can kill any process either owned by the system or any other user.

Syntax.

The syntax is as follows,

kill [signal or option] PID(s)

Various options for kill command are,

• -l, to list all signal names.
• -s -signal, to specify the signal name.
• -n sig, to specify the signal number.

One can also use killall which kills multiple processes at the same time.
Various options for killall are,

• -e, --exact, to check for exact matches in case of very long names.
• -g, --process-group, to kill a process group.
• -y, --younger-than, to kill processes younger than specified time.
• -o, --older-than, to kill processes older than specified time.
• -i, --interactive, to ask for confirmation before killing a process.
• -l, --list, to list all signal names.
• -q, --quiet, to kill processes without printing any output.
• -s, --signal SIGNAL, to send a signal instead of a SIGTERM.
• -u, --user USER to kill processes by a specific user.
• -n , --ns PID, kill processes in the same name-space as the PID.
• -r, --regexp, kills processes matched by a regular expression.
• -v, --verbose, to print out debugging information.
• -I, --ignore-case, matches a processes while ignoring case sensitivity.

The pkill command kills a process based on it's name.
Commonly used options include,

• -n, to kill the newly discovered processes.
• -o, to kill the older processes.
• -u, to kill processes owned by a specified user.
• -x, to kill processes matching a specified pattern.
• -signal to kill processes with a specified signal.

Commands.

To view currently running processes we can either use top, ps command or htop commands.

An example
To list all running processes we write,

ps -a

or

ps -aux

Process IDs

A PID can be,

• > 0, (greater than 0) in such a case, the signal is sent to a process with an ID equal to the PID.
• == 0, the signal is sent to processes in the current process group, i.e processes in that GID of the shell that sent the signal. To get ids execute the id command.
• == -1, the signal is sent to all processes with the same UID as the user who executes the command.
• < -1, the signal is sent to all processes in the process group where GID is equal to the absolute value of the PID.

To get a process' id we can write,

pidof bash

The above command returns the PID os bash process.

Terminating processes

The kill command sends a signal to a specified process or group of processes. The signal specifies how a process will be affected. It can be killed, reloaded.

To list all signal names we write,

kill -l

Commonly used signals are 1 which reloads a process, 2 which sends an interrupt from the keyboard signal, 9 which sends a kill signal to kill the process, 15 which kills a process gracefully and 17, 18, 23 which sends a signal to stop a process.

An example

To send a SIGKILL signal to a process with a PID 2342, we write

kill -9 2342

We can also write,

kill -SIGKILL 2342

We can also kill multiple processes by passing multiple process ids as follows,

kill -9 7888 2313 6719 6671

We can also kill a process by its name as follows,

kill firefox

The command kills a firefox process currently running.

Assuming there are multiple instances of firefox running we can use the killall command,

killall -9 firefox

or by using pkill command,

pkill firefox

To reload a process's settings we can send the SIGHUP signal,

kill -1 2278

Where 2278 is the PID of the process we want to reload.

Summary.

We use ps, top or htop command to get PIDs which we then use to kill a process. We can also kill a process using the pkill command using its name.
The killall kills all instances of a process.

You can also kill processes while in the htop or top windows.

References.

1. Linux processes: signals, termination, zombies, cleanup.
2. Execute man kill or kill --help.
3. For process signals execute kill -l.
4. htop