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Linux and Unix kill command tutorial with examples

Tutorial on using kill, a UNIX and Linux command for terminating a process. Examples of killing a process, sending a SIGTERM, listing signal names and numbers, and handling 'operation not permitted' errors.

Estimated reading time: 4 minutes

Table of contents

Terminal showing kill man page

What is the kill command in UNIX?

The kill command is a command line utility to for terminating processes. It is normally a shell builtin meaning the command is called from a users shell rather than an external executable program. By default the kill command will send a TERM signal to a process allowing the process to perform any cleanup operations before shutting down. The kill command also supports sending any other signal to a process. The kill command is used primarily to terminate or restart processes.

How to kill a process

To kill, or terminate a process first find out the process identifier number or PID of the process to be killed, then pass the PID number to the kill command. In the following example suppose that we are running the mutt terminal email program and that we wish to terminate it. To find the process identifier the ps command is used along with grep to find the PID.

ps -e | grep mutt
17146 pts/1    00:00:00 mutt

The same may also be achieved by running the pgrep command. This will return the process identifier or identifiers for the search pattern.

pgrep mutt
17146

Once the PID for the mutt program is known it may be used with the kill command.

kill 17146

This sends a TERM signal to the process indicating it should be terminated. When a process receives a TERM it acts as a request to terminate the running process. A UNIX process may catch a TERM and handle termination gracefully such as releasing resources or saving state.

It is also possible to use pkill to achieve the same result.

pkill mutt

How to (really) kill a process

If a signal does not respond to a TERM signal the KILL signal may be used. The KILL signal cannot be ignored by UNIX processes and the process is killed immediately. Note that this does not allow the process to perform any cleanup when shutting down the process. To send a process a signal other than TERM use the -s option followed by the name of the signal.

kill -s KILL 17146

Using the signal number is more commonly used and is equivalent.

kill -s 9 17146

Some shell built-ins also support the following syntax for even more brevity.

kill -9 17146

How to list signal names and numbers

To view signal names and numbers pass the -L option to the kill command.

kill -L
 1 HUP      2 INT      3 QUIT     4 ILL      5 TRAP     6 ABRT     6 IOT      7 BUS
 8 FPE      9 KILL    10 USR1    11 SEGV    12 USR2    13 PIPE    14 ALRM    15 TERM
16 STKFLT  17 CHLD    17 CLD     18 CONT    19 STOP    20 TSTP    21 TTIN    22 TTOU
23 URG     24 XCPU    25 XFSZ    26 VTALRM  27 PROF    28 WINCH   29 IO      29 POLL
30 PWR     31 UNUSED  31 SYS     34 RTMIN   64 RTMAX

To just list the names of signals use the -l option.

kill -l
HUP INT QUIT ILL TRAP ABRT BUS FPE KILL USR1 SEGV USR2 PIPE ALRM TERM STKFLT CHLD CONT STOP TSTP TTIN TTOU URG XCPU XFSZ VTALRM PROF WINCH POLL PWR SYS

Any of these signals may be sent to a process using either the number or name of a signal. The following are equivalent ways to send a process a HUP signal.

kill -s HUP 17146
kill -s 1 17146
kill -HUP 17146
kill -1 17146

How to handle ‘operation not permitted’ errors

To kill a process a user account must have sufficient permissions. If, for example, a normal user account tries to kill a process owned by root an error will be shown. In the following example the process 6425 is owned by root.

kill 6245
kill: sending signal to 6245 failed: Operation not permitted

If the user account has sudo permissions the account can simultaneously be elevated and run the command with the following.

sudo !!

If a user account does not have sudo permssions a systems administrator will need to assign higher permissions to achieve this.

How to show signals that are sent to a process

To enable verbose logging pass the --verbose flag to the kill command. Note that this is not supported by all shell built-ins so may not be available on your system.

kill --verbose 17146
sending signal 15 to pid 17146

The number relates to the signal mapped in the table of available signals available via kill -L. In this case it is the TERM signal.

Further reading

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