kill command in Linux (located in /bin/kill), is a built-in command which is used to terminate processes manually. kill command sends a signal to a process that terminates the process. If the user doesn’t specify any signal which is to be sent along with the kill command, then a default TERM signal is sent that terminates the process.
Basic Syntax of `kill` command in Linux
The basic syntax of the `kill` command is as follows:
Syntax:
kill [signal] PID
Here,
- PID = The `kill` command requires the process ID (PID) of the process we want to terminate.
- [signal] = We have to specify the signal and if we don’t specify the signal, the default signal `TERM` is sent to terminate the process
Signals can be specified in three ways:
Signals can be specified in three ways; they are as follows:
1) By number:
We can specify a signal using a number. For example, we have a PID `1212` and want to send a `SIGKILL` signal to kill this PID. SIGKILL has a signal number of `9` (To find signal numbers run `kill -l` command).
Syntax:
kill -9 1212
2) With SIG prefix (e.g/ -SIGkill)
We can also specify signal using SIG prefix. For example, we need to send a signal `SIGTERM` and PID is `1432`. To just check signal number of `SIGTERM` signal we can use `kill -l` command.
Syntax:
kill -SIGTERM 1432
3) Without SIG prefix:
We can also specify signals without using SIG prefix. For example, if want to send signal `TERM` and PID `1234`. To just check signal number of `TERM` signal we can use `kill -l` command.
Syntax:
kill -TERM 1234
Some Common Signals in `kill` command in Linux
The table below shows some common signals and their corresponding numbers.
| Signal Name | Signal Number | Description |
|---|
| SIGHUP | 1 | It hangup detected on controlling terminals or death of controlling process. |
| SIGINT | 2 | It interrupts from keyboard. |
| SIGKILL | 9 | It kills signal. |
| SIGTERM | 15 | It terminates signal. |
To check signal name and number we can use `kill -l` command.
Options and examples of `kill` command in Linux
1. `kill -l `
To display all the available signals, you can use the below command option:
Syntax:
kill -l
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kill -l
Note:
- Negative PID values are used to indicate the process group ID. If you pass a process group ID then all the process within that group will receive the signal.
- A PID of -1 is very special as it indicates all the processes except kill and init, which is the parent process of all processes on the system.
- To display a list of running processes use the command ps and this will show you running processes with their PID number. To specify which process should receive the kill signal we need to provide the PID.
Syntax:
ps
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ps
2. `kill PID`
This option specifies the process ID of the process to be killed.
Syntax:
kill pid
3. `kill -s`
This option specifies the signal to be sent to the process.
Syntax:
kill {-signal | -s signal} pid Frequently Asked Questions on `kill` command in Linux
How to kill multiple processes at once?
We can use kill command to kill multiple processes at once. We just need to specify multiple PIDs separated by spaces, or we can also use `killall` command to kill all the processes with a specific name.
For Example:
If we want to kill processes PIDs like 1234, 4321, and 2342, we can we use the following command:
kill 1234 4321 2342
If we want to kill all the processes with the name “firefox”. We can use the following command.
killall firefox
Is it safe to kill the system processes?
No, it is important to note that only kill processes that we know are safe to kill and killing system processes can cause system instability. It is always better to use other methods like restarting the system or identifying the root cause of the issue and fixing it.
What is the difference between SIGTERM and SIGKILL in `kill` command in Linux?
SIGTERM, it sends a termination signal to the process which helps in exit gracefully. Whereas SIGKILL sends a kill signal to the process, which terminate the processes forcefully and immediately.
For Example:
To send signal `SIGTERM` and PID `4321`, we can use the following command.
kill -15 4321
To send signal `SIGKILL` and PID `4321`, we can use the following command.
kill -9 4321
Conclusion
The `kill` command in Linux is a very powerful utility for managing processes. We have understood the different ways to specify the signals and available options in the kill command which can help us to manage our system resources efficiently and resolve issues quickly and effectively.