Linux Basics – the ‘kill’ command and process signals
The kill command
The kill command is used in Linux to terminate a running process. Various signals may be passed via the kill command to either terminate the process gracefully, tell the process to restart or to shut it down forcefully. We will first cover the process table and then some commonly used signals. Don’t forget to download our Linux commands cheat sheet guide and we offer Linux server services.
Process Table
The process table describes all the processes that are currently loaded. The ps command shows the processes. By default, it shows only processes that maintain a connection with a terminal, a console, a serial line, or a pseudo terminal. Other processes that can run without communication with a user on a terminal are system processes that Linux manages shared resources. To see all processes, we use -e option and -f to get full information (ps -ef). Below is some sample output from ps -ef.
UID PID PPID C STIME TTY TIME CMD root 1 0 0 2010 ? 00:01:48 init root 21033 1 0 Apr04 ? 00:00:39 crond root 24765 1 0 Apr08 ? 00:00:01 /usr/sbin/httpd
Note that the init process is always PID (Process Identifier) one. Killing this process is not recommended unless you plan on crashing your system. Below is an image depicting the logic behind the process control and state. The PID for other processes is an integer between 2 and 32,768.
Process Signals
The entries in the “Action” column of the tables below specify the default disposition for each signal, as follows:
- Term* Default action is to terminate the process.
- Ign Default action is to ignore the signal.
- Core Default action is to terminate the process and dump core.
- Stop Default action is to stop the process.
- Cont Default action is to continue the process if it is currently stopped.
99% of the time we will be using a Terminate switch such as SIGHUP,SIGTERM or SIGKILL.
The signals SIGKILL and SIGSTOP cannot be caught, blocked or ignored.
Kill command to forcefully kill a process
kill -9 is used to forcefully terminate a process in Linux ( 9 = SIGKILL ). The syntax of kill command is:
ps -ef| grep process_identifier // will give you PID kill -9 PID
Killing Multiple Processes
With kill command, you can specify multiple PID’s at the same time and all the processes will be signaled as per the example below:
kill -9 pid1 pid 2
You can also script a kill if there are many of the same processes as follows:
for ID in $(ps -ef |grep process_identifier |grep -v grep |awk '{print $2}'); do kill -9 $ID ; done ---- OR ------ for ID in $(pgrep process_identifier); do kill -9 $ID ; done
Hopefully, these tips will help you to your path to a top Linux Administrator.
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