Kill process in Linux or terminate a process in UNIX or Linux systems

Kill command works under both Linux and UNIX/BSD like operating systems.

1.   First, you need to find out process PID (process id)

Use ps command or pidof command to find out process ID (PID). Syntax:
ps aux | grep processname
pidof processname

For example if process name is lighttpd, you can use any one of the following command to obtain process ID:
# ps aux | grep lighttpd

Output:

lighttpd 3486 0.0 0.1 4248 1432 ? S Jul31 0:00 /usr/sbin/lighttpd -f /etc/lighttpd/lighttpd.conf
lighttpd 3492 0.0 0.5 13752 3936 ? Ss Jul31 0:00 /usr/bin/php5-cg

OR use pidof command which is use to find the process ID of a running program:
# pidof lighttpd

Output:

3486

2.   Kill process using PID (process id)

Above command tell you PID (3486) of lighttpd process. Now kill process using this PID:
# kill 3486
OR
# kill -9 3486

Where, -9 is special Kill signal, which will kill the process.

killall command examples

DO NOT USE killall command on UNIX system (Linux only command). You can also use killall command. The killall command kill processes by name (no need to find PID):
# killall -9 lighttpd
Kill Firefox process:
# killall -9 firefox-bin
As I said earlier killall on UNIX system does something else. It kills all process and not just specific process. Do not use killall on UNIX system (use kill -9).