The Linux operating system has many important concepts that help developers and administrators ensure a smooth functioning of the applications developed for Linux. One of such important concepts of operating systems is concept of processes. A process in Linux represents an instance of a running program. The Linux operating system assigns a unique identifier, process id, to each running process. In order to know the details related to the processes running under Linux, you can use the ps command.

The ps command has the following basic syntax:
ps [options]

When you run the ps command on the command prompt, it displays the information about at least two processes, shell and ps. Shell is the process that runs in order to provide users an environment to interact with the operating system. The ps process is displayed as a result of executing the ps command, which terminates immediately after running the ps command.

The ps command displays four pieces of information related to the running processes by default. These pieces of information are PID, TTY, TIME and CMD. PID is the process id of the process, TTY indicates the name of logged in terminal (console), TIME is an indicative of the processing time the CPU has spent executing the process, and CMD is the name of command result in the process execution.
There are various common usages of ps command with varying values of the options used while running the ps command. For example, in order to get a list of processes containing the word, app, you can issue the ps command with the grep command as follows:
ps -ef| grep app
Similarly, the ps command can issue the list of processes based on various criteria specified by various options, such as aux, -u, -ef, -x, etc. These options can be used for various purposes, such as killing a non-responsive process. You can easily find the process id of a particular process and issue the kill command with the process id thus found.