CIS241

System-Level Programming and Utilities

Linux Processes

Erik Fredericks, frederer@gvsu.edu
Fall 2025

Based on material provided by Erin Carrier, Austin Ferguson, and Katherine Bowers

What is a process?

• Execution of a program/command

  • Or, something running in memory!

• User process

  • Process begun by a user on a terminal

• Daemon process

  • System process
  • Not associated with a terminal

Viewing processes

• ps - list active processes
• ps -f - more information
• ps -u - what processes am I running?
• top - interactive
  • If installed, htop even more interactive

Processes and Terms

Process ID (PID)

• Unique identifier assigned to a process

Child process

• Process started by another process (parent process)

Parent process

• Process that has started other processes (child processes)

Parent Process ID (PPID)

• PID of the parent process

The init daemon has a PID of 1 and a PPID of 0
0 refers to the kernel itself

Process tree

pstree

Processes

init daemon

• Starts most other daemons during the system initialization process
• Including those that allow for user logins

The login program starts a bash shell

• bash shell then interprets user commands and starts all user processes
• Each process on the Linux system can be traced back to the init daemon by examining PPIDs

Processes

Process state (ps aux or ps -l)

• Current processor state of process
• Generally sleeping (S) or running (R)
• This column is the most valuable to system administrators

Zombie process

• Process finished, but parent has not released child process’s PID
• Defunct process
• Process state is Z

Foreground and background

Processes typically run in the foreground when you launch them

• Takes over the shell until completed!
• Kind of annoying if you want to do other things...

• To suspend a running process: Ctrl + Z

• To resume suspended process in foreground: fg

• To resume suspended process in background: bg

• jobs to show foreground, background, and suspended processes!

• Can also run in background immediately: command &

Stopping a process

In the foreground (running)

• Ctrl + c

In the background

• kill pid - stop gracefully -- lookup with ps
  • What is gracefully?
• kill -9 pid - force kill - stop immediately!
• killall processname - kills ALL processes with processname (typically requires sudo)

Stopping a process

64 different types of kill signals!

• Why?

Killing processes

Trap: the ability of a process to ignore a kill signal

• The SIGKILL signal cannot be trapped by any process
• Use only as last resort because it prevents a process from closing temporary files and resources properly

When a parent process receives a kill signal

• Terminates all child processes before terminating self
• To kill several related processes send signal to parent process
• Kill parent process in order to kill zombie processes