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

The table below shows some common signals and their corresponding numbers.

To check signal name and number we can use `kill -l` command.

kill -l Option

To display all the available signals, you can use the below command option:

 Syntax:

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

kill PID Option

This option specifies the process ID of the process to be killed.

 Syntax:

kill pid

kill -s Option

This option specifies the signal to be sent to the process.

 Syntax:

kill {-signal | -s signal} pid

kill Command in Linux -FAQs

How to Kill a Process from the Linux Command Line?

To kill a process from the Linux command line, you can use the kill command followed by the process ID (PID) of the target process. For example, to terminate a process with PID 1234, you would use the following command:

kill 1234

By default, the kill command sends the SIGTERM signal, allowing the process to perform cleanup operations before termination. If the process doesn’t respond to SIGTERM or if immediate termination is required, you can use the SIGKILL signal with the -9 option:

kill -9 1234

It’s important to note that forcefully terminating a process with SIGKILL may result in data loss or corruption, so it’s recommended to try SIGTERM first and resort to SIGKILL only if necessary.

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

How can I find the process ID (PID) of a specific program in Linux?

To find the PID of a specific program, you can use the pgrep command followed by the program’s name. For example, to find the PID of the “firefox” process, you would enter:

pgrep firefox

What is the purpose of the pkill command, and how does it differ from the kill command?

The pkill command is used to send signals to processes based on their name, providing a more convenient way to terminate processes. It differs from the kill command by allowing users to specify processes by name rather than PID.

How can I check the resource usage (CPU, memory) of a specific process in Linux?

To check the resource usage of a specific process, you can use the top command along with the process ID (PID). For instance, to monitor the resource usage of a process with PID 1234, you would run:

top -p 1234