Process

A process is a program that has been loaded into memory and is executing. The operating system gives each process a process ID (PID), its own address space, and a share of CPU time.

Why it matters

Looking at processes explains most performance and outage problems: which program uses all the memory, which one is stuck, and whether a service is actually running. A single program can start several processes, and each process can contain multiple threads.

When a process uses too much memory, Linux may terminate it with the out-of-memory killer.

Example

$ ps aux --sort=-%mem | head -3
USER   PID %CPU %MEM COMMAND
mysql  941  2.1 38.4 /usr/sbin/mysqld

The database process uses about 38 percent of memory.

Common mistakes

• Killing a process with SIGKILL first, which gives it no chance to save data; try SIGTERM.
• Confusing a process with a thread.
• Leaving zombie or runaway processes running unnoticed.

Related

• Daemon (Background Service)
• Kernel
• Bash
• Operating System (OS)

This page is general information written by our editorial team and updated when documentation changes. Servers and providers differ, so test changes safely first. See our editorial policy and disclaimer.