Linux ps Command: Inspect Running Processes

ps shows a list of processes at the time of execution. It is used to check the PID, user, status, and command to investigate whether a particular service or task is actually running.

What is the ps command?

ps outputs a snapshot at a single point in time and then ends. If you want to see values that change in real time, use top or htop. The option notation includes both POSIX and BSD styles, so the meaning can differ depending on whether a hyphen is used.

Basic syntax

ps [options]

The installed implementation and options may vary depending on the distribution. Check the description for the current system with man ps.

Examples

Processes of the current terminal

When run without arguments, it usually only shows processes connected to the current terminal.

ps

System-wide processes

It provides a broad view of the user, PID, parent PID, and command.

ps -ef

Specifying fields of a specific PID

If you specify only the necessary columns, it is easier to read scripts and check results.

ps -p 1234 -o pid,ppid,user,stat,cmd

How to Read Process Columns

ps shows a snapshot of processes at the time of execution. By specifying columns, you can clearly distinguish PID (process ID), PPID (parent PID), status, and command name.

ps -o pid,ppid,stat,comm -p $$

Example output (numbers will vary each time you run it):

    PID    PPID STAT COMMAND
   4321    4100 Ss   bash

$$ is the PID of the current shell. The first letter of STAT indicates the state such as running or waiting, and the following letters indicate additional states. Since processes may terminate immediately after being queried, make sure to verify the PID and command again before executing a termination command.

Main Options and Format

Precautions when using

ps | grep name can include the grep process itself in the results. pgrep is convenient for searching PIDs by name. Since processes can be created and terminated quickly, make sure to check again that the PID is still the same task before sending a signal right after the output.

Frequently Asked Questions

What is the difference between ps and top?

ps prints a snapshot of the process list at the moment it is executed and then exits. top periodically refreshes the screen so you can observe changes over time.

Official Documentation

The exact behavior of options and implementation differences can be found in the official ps documentation.