skill Command in Unix and Linux

skill Command in Unix and Linux is used to send an signals to users and process. Using skill command we can halt a user terminal or can logout him. But keep in mind that you should be root to stop other users.

There is a package called procps It includes various useful (nifty) utilities. One of such utility is skill which is responsible to send a signal to users and process such as:

•   Halt user terminal
•   Kill user and logout

Usage:   skill [signal to send] [options] process selection criteria
Example: skill -KILL -v pts/*

1. To halt/stop a user called ravi

Send STOP signal to user ravi, type the following command:

[root@TechTutorial ~]# skill -STOP -u ravi

2. To resume already halted user called ravi

Send CONT signal to user ravi, type the following command:

[root@TechTutorial ~]# skill -CONT -u ravi

3. To kill and logout user called ravi

You can send KILL signal, type the following command:

[root@TechTutorial ~]# skill -KILL -u ravi

4. To kill and logout all users

type the following command:

Note: Do not run this command on any production boxes, this command just for demonstration purpose only.

[root@TechTutorial ~]# skill -KILL -v /dev/pts/*

PROCESS SELECTION OPTIONS

Selection criteria can be: terminal, user, pid, command. The options below may be used to ensure correct interpretation.
-t    The next argument is a terminal (tty or pty).
-u    The next argument is a username.
-p    The next argument is a process ID number.
-c    The next argument is a command name.

SIGNALS

The signals listed below may be available for use with skill. When known, numbers and default behavior are shown.

Name	Num	Action	Description
0	0	n/a	exit code indicates if a signal may be sent
ALRM	14	exit
HUP	1	exit
INT	2	exit
KILL	9	exit	this signal may not be blocked
PIPE	13	exit
POLL		exit
PROF		exit
TERM	15	exit
USR1		exit
USR2		exit
VTALRM		exit
STKFLT		exit	may not be implemented
PWR		ignore	may exit on some systems
WINCH		ignore
CHLD		ignore
URG		ignore
TSTP		stop	may interact with the shell
TTIN		stop	may interact with the shell
TTOU		stop	may interact with the shell
STOP		stop	this signal may not be blocked
CONT		restart	continue if stopped, otherwise ignore
ABRT	6	core
FPE	8	core
ILL	4	core
QUIT	3	core
SEGV	11	core
TRAP	5	core
SYS		core	may not be implemented
EMT		core	may not be implemented
BUS		core	core dump may fail
XCPU		core	core dump may fail
XFSZ		core	core dump may fail

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