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Linux — Process Management

Short & clear revision notes · 7 topics

01 · What is a Process?

When you run any command in Linux, the system creates a process — an instance of a running program with its own special environment.

Every process is tracked by a unique PID (Process ID) — a five-digit number. No two processes share the same PID at the same time. PIDs eventually roll over and repeat once all numbers are used up.

💡 Every time you type a command, you start a process. Even ls is a process.

02 · Foreground vs Background Processes

Foreground

  • Runs connected to your terminal — takes input from keyboard, outputs to screen
  • You cannot run anything else until it finishes
  • Default behaviour for all commands
$ ls ch*.doc        # runs in foreground — prompt returns only after it finishes

Background

  • Runs disconnected from the keyboard
  • You can keep using the terminal while it runs
  • Add & at the end of any command to run it in the background
$ ls ch*.doc &      # runs in background
[1] 1234            # [job number] PID

When the background job finishes:

[1] + Done    ls ch*.doc &
$              # prompt available immediately

💡 If a background process needs keyboard input, it pauses and waits until you bring it to the foreground.

Foreground Background
Terminal blocked? Yes No
How to start Just run the command Add & at the end
Good for Quick commands Long-running tasks

03 · Listing Processes — ps

The ps (process status) command shows running processes.

$ ps
  PID TTY      TIME CMD
18358 ttyp3 00:00:00 sh
18361 ttyp3 00:01:31 abiword
18789 ttyp3 00:00:00 ps

ps -f — full listing

$ ps -f
UID     PID  PPID C STIME TTY  TIME CMD
amrood 6738  3662 0 10:23 pts/6 0:00 first_one
amrood 6739  3662 0 10:22 pts/6 0:00 second_one
amrood 3662  3657 0 08:10 pts/6 0:00 -ksh
amrood 6892  3662 4 10:51 pts/6 0:00 ps -f

Columns in ps -f

Column Meaning
UID User ID — who is running this process
PID Process ID
PPID Parent Process ID — who started this process
C CPU utilisation
STIME Process start time
TTY Terminal associated with the process
TIME Total CPU time used
CMD The command that started the process

Other ps options

Option What it shows
-a All users' processes
-x Processes with no terminal
-u Additional info (like -f)
-e Extended information

04 · Stopping Processes — kill

Foreground process

Press Ctrl+C to interrupt and stop it immediately.

Background process

Find the PID with ps, then use kill:

$ kill 6738        # send termination signal to PID 6738
Terminated

If the process ignores a normal kill:

$ kill -9 6738     # force kill — cannot be ignored
Terminated

⚠️ kill -9 is the nuclear option — the process gets no chance to clean up. Use it only when a normal kill doesn't work.

05 · Parent, Child, Zombie & Orphan Processes

Parent & Child

Every process has a parent. Most commands you run have the shell as their parent. The PPID column in ps -f shows the parent's PID.

shell (PPID)
  └── command you ran (PID)  ← child process

Orphan Process

When a parent process is killed before its child, the child becomes an orphan. Linux reassigns it to the init process (PID 1), which becomes its new parent.

Zombie Process

When a child process finishes but the parent hasn't acknowledged it yet, the child becomes a zombie. It shows up in ps with state Z.

  • It is dead and not using resources
  • It still has an entry in the process table
  • Eventually cleaned up when the parent reads the exit status
Type Situation State in ps
Normal Running as expected S, R
Orphan Parent died first Adopted by init
Zombie Finished but not cleaned up Z

06 · Daemon Processes

A daemon is a background system process that:

  • Usually runs with root permissions
  • Has no controlling terminal (shows ? in the TTY column of ps -ef)
  • Waits silently for something to happen (e.g. a print command, a network request)
$ ps -ef | grep "?"    # daemons show ? in TTY column

💡 Examples of daemons: print daemon (waits for print jobs), web server daemon (waits for HTTP requests), cron daemon (waits for scheduled tasks).

07 · Monitoring & Job Management

top — live process monitor

$ top
  • Updates frequently in real time
  • Shows CPU usage, memory, load averages, and busy processes
  • Interactive — press q to quit

Job ID vs Process ID

Job ID Process ID (PID)
What it is Short number assigned by the shell Unique system-wide number
Scope Only within your current shell session System-wide
Used for Managing background/suspended jobs Killing or tracking any process
Example [1] 6738

💡 A single job can contain multiple processes. Using the job ID is easier than tracking each process individually.

Quick Reference

Run in background:
  command &

List processes:
  ps              # your processes
  ps -f           # full details
  ps -a           # all users
  top             # live monitor

Stop a process:
  Ctrl+C          # stop foreground process
  kill PID        # stop background process by PID
  kill -9 PID     # force kill (last resort)

Process states in ps:
  R  = Running
  S  = Sleeping (waiting)
  Z  = Zombie (dead but not cleaned up)
  ?  = No terminal (daemon)

Linux Process Management Notes — for revision use