Process signals

Signals are software interruptions They provide a way for the user (or a process ) to directly communicate with a process. 
Software may be programmed to respond intelligently to a wide array of signals, and certain signals cause processes to behave in a standardized, predefined way at the   kernel  level.

Signals can be sent with the kill command, which is named for its default signal (SIGKILL) that instructs the OS to forcefully terminate a process before doing anything else.

Signal names are commonly abbreviated without their SIG prefix, e.g., "KILL", including in the command arguments of kill.

Linux Signals are:

Signal Name	Signal Number	Description
SIGHUP	1	Hang up detected on controlling terminal or death of controlling process
SIGINT	2	Issued if the user sends an interrupt signal (Ctrl + C)
SIGQUIT	3	Issued if the user sends a quit signal (Ctrl + D)
SIGFPE	8	Issued if an illegal mathematical operation is attempted
SIGKILL	9	If a process gets this signal it must quit immediately and will not perform any clean-up operations
SIGALRM	14	Alarm clock signal (used for timers)
SIGTERM	15	Software termination signal (sent by kill by default)

List of Signals

There is an easy way to list down all the signals supported by your system. Just issue the kill -l command and it would display all the supported signals −

killall - kill processes by name

killall sends a signal to all processes running any of the specified commands . If no signal name is specified, SIGTERM is sent. To terminate all firefox process (child and parent), enter:

killall processName
killall firefox-bin

To send a KILL signal to firefox, enter:

killall -s SIGKILL firefox-bin

Them we are checking the process and the process ID of Firefox for example..

we can use kill 9 (check the number)





First we can check the Firefox process number 


Them we can use the specific  process id (5954 as we can see in the graphic)



let's check another example, let's check the process called cups used for printers but open too many ports on your computer



we will kill that process. And them we will see the explanation of the pkill command 




pkill - kill process


The pkill command is another command with additional options to kill process by its name, user name, group name, terminal, UID, EUID, and GID. It will send the specified signal (by default SIGTERM) to each process instead of listing them on stdout. To send a kill signal to php-cgi process, enter:


pkill -KILL php-cgi




The above example will kill all users php-cgi process. However, -u option will kill only processes whose effective user ID is set to vivek:


pkill -KILL -u vivek php-cgi


With the kill -l we can see a lit of signal process