How to See PID in Linux

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Introduction

A process ID (PID) is a unique number that identifies a running process in a Linux system. It is used by the operating system to keep track of processes and manage their resources. PIDs are also used by users to monitor and control processes.

In this tutorial, we will show you how to see PID in Linux. We will cover the following topics:

  • What is a PID?
  • How to find the PID of a process
  • How to use PIDs to manage processes
  • Finding the PID of a Specific Process
  • Using PIDs to Control Processes

What is a PID?

A PID is a unique number that identifies a running process in a Linux system. It is a 32-bit integer, and it is typically displayed as a decimal number.

PIDs are assigned by the kernel when a process is created. The kernel keeps track of all running processes and their PIDs in a data structure called the process table.

How to Find the PID of a Process

There are several ways to find the PID of a process. The most common way is to use the ps command. The ps command lists all running processes, and it includes the PID of each process.

To use the ps command, simply type the following command into a terminal window:

ps

The output of the ps command will look something like this:

PID TTY      STAT   TIME COMMAND
1     pts/0    S+    0:00 /sbin/init
2     pts/0    S+    0:00 /usr/lib/systemd/systemd
3     pts/0    S+    0:00 /usr/lib/systemd/systemd-udevd
4     pts/0    S+    0:00 /usr/lib/systemd/systemd-logind
5     pts/0    S+    0:00 /usr/lib/systemd/systemd-journald
6     pts/0    S+    0:00 /usr/lib/systemd/systemd-tmpfiles
7     pts/0    S+    0:00 /usr/lib/systemd/systemd-udevd-kernel
8     pts/0    S+    0:00 /usr/lib/systemd/systemd-udevd-control
9     pts/0    S+    0:00 /usr/lib/systemd/systemd-udevd-rules
10    pts/0    S+    0:00 /usr/lib/systemd/systemd-udevd-trigger

The first column in the output of the ps command is the PID column. The PID of the init process is 1, the PID of the systemd process is 2, and so on.

You can also use the top command to find the PID of a process. The top command displays a real-time list of the running processes, and it includes the PID of each process.

To use the top command, simply type the following command into a terminal window:

top

The output of the top command will look something like this:

top - 16:28:34 up 1 day,  4:18,  1 user,  load average: 0.33, 0.35, 0.37
Tasks: 188 total,   1 running, 187 sleeping,   0 stopped,   0 zombie
%Cpu(s):  0.3 us,  0.3 sy,  0.0 ni, 99.4 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
KiB Mem :  1633940 total,  1548424 used,   85516 free,   15572 buffers
KiB Swap:  2097148 total,        0 used,  2097148 free,   40560 cached

  PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND
13326 root      20   0 132448  11208   908 S   0.0  0.7   0:00.03 Xorg
14768 root      20   0   6004   4604   392 S   0.0  0.3   0:00.

How to Use PIDs to Manage Processes

PIDs can be used to manage processes in a variety of ways. For example, you can use PIDs to:

  • Kill a process: You can use the kill command to kill a process. The kill command takes the PID of the process as an argument. For example, to kill the process with the PID of 1234, you would type the following command into a terminal window:
kill 1234
  • Suspend a process: You can use the suspend command to suspend a process. The suspend command takes the PID of the process as an argument. For example, to suspend the process with the PID of 1234, you would type the following command into a terminal window:
suspend 1234
  • Resume a process: You can use the resume command to resume a suspended process. The resume command takes the PID of the process as an argument. For example, to resume the process with the PID of 1234, you would type the following command into a terminal window:
resume 1234
  • Change the priority of a process: You can use the nice command to change the priority of a process. The nice command takes the PID of the process and a priority level as arguments. The priority level can be a number from -20 to 19. A higher priority level means that the process will be given more CPU time. For example, to change the priority of the process with the PID of 1234 to 10, you would type the following command into a terminal window:
nice 10 1234

Finding the PID of a Specific Process

If you know the name of the process you want to find the PID of, you can use the pgrep command. The pgrep command takes the name of the process as an argument and returns the PID of the first matching process. For example, to find the PID of the firefox process, you would type the following command into a terminal window:

pgrep firefox

You can also use the pidof command to find the PID of a specific process. The pidof command takes the name of the process as an argument and returns the PID of all matching processes. For example, to find the PID of all firefox processes, you would type the following command into a terminal window:

pidof firefox

Using PIDs to Control Processes

Once you have found the PID of a process, you can use it to control the process in a variety of ways. For example, you can use the kill command to terminate a process, the nice command to change the priority of a process, and the renice command to change the scheduling policy of a process.

To kill a process, you can use the kill command followed by the PID of the process. For example, to kill the process with the PID of 1234, you would type the following command into a terminal window:

kill 1234

To change the priority of a process, you can use the nice command followed by the PID of the process and a priority level. The priority level can be a number from -20 to 19. A higher priority level means that the process will be given more CPU time. For example, to change the priority of the process with the PID of 1234 to 10, you would type the following command into a terminal window:

nice 10 1234

To change the scheduling policy of a process, you can use the renice command followed by the PID of the process and a scheduling policy. The scheduling policy can be one of the following:

  • SCHED_OTHER: The default scheduling policy.
  • SCHED_FIFO: The first-in, first-out scheduling policy.
  • SCHED_RR: The round-robin scheduling policy.
  • SCHED_BATCH: The batch scheduling policy.

For example, to change the scheduling policy of the process with the PID of 1234 to SCHED_FIFO, you would type the following command into a terminal window:

renice -p 1234 SCHED_FIFO

Conclusion

PIDs are a powerful tool for managing processes in a Linux system. By understanding how to find and use PIDs, you can gain more control over your system and its processes.

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