Processes and Services

Every running program in Linux is a process. Understanding how to view, manage, and control processes — as well as how to work with system services — is essential for system administration and troubleshooting.

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What is a Process?

A process is an instance of a running program. Each process has:

Attribute	Description
PID	Process ID — unique numeric identifier
PPID	Parent Process ID — the process that started this one
UID	User ID — who owns the process
State	Running, sleeping, stopped, zombie, etc.
Nice value	Process priority (-20 highest to 19 lowest)
Memory	Resident and virtual memory usage
CPU	CPU time consumed

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Viewing Processes

ps — Process Status

ps                        # show processes in current terminal
ps aux                    # show all processes (BSD syntax)
ps -ef                    # show all processes (System V syntax)
ps aux --sort=-%mem       # sort by memory usage (descending)
ps aux --sort=-%cpu       # sort by CPU usage (descending)
ps -u alice               # show processes owned by alice
ps -p 1234                # show info about PID 1234
ps aux | grep nginx       # find nginx processes

Understanding ps aux output:

USER    PID  %CPU %MEM   VSZ   RSS TTY   STAT START   TIME COMMAND
root      1   0.0  0.1 16968 10240 ?     Ss   09:00   0:01 /sbin/init
alice  1234   2.5  1.0 98760 40960 pts/0 S+   10:30   0:45 python3 app.py

Process States

State	Symbol	Description
Running	R	Currently executing on CPU
Sleeping	S	Waiting for an event (interruptible)
Uninterruptible sleep	D	Waiting for I/O (cannot be interrupted)
Stopped	T	Stopped by a signal (e.g., Ctrl+Z)
Zombie	Z	Finished but parent hasn't read exit status

top — Real-Time Process Monitor

top                       # interactive process monitor

top keyboard shortcuts:

Key	Action
M	Sort by memory
P	Sort by CPU
k	Kill a process (enter PID)
r	Renice a process
1	Show individual CPU cores
q	Quit

htop — Enhanced Process Monitor

sudo apt install htop     # install htop
htop                      # interactive, colourful process monitor

Tip: htop provides a much better experience than top with mouse support, colour coding, and an intuitive interface.

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Managing Processes

Foreground and Background

./long_task.sh             # run in foreground
./long_task.sh &           # run in background
Ctrl+Z                     # suspend (stop) the foreground process
bg                         # resume suspended process in background
fg                         # bring background process to foreground
jobs                       # list background/stopped jobs
fg %2                      # bring job number 2 to foreground

Signals and kill

Send signals to processes:

kill 1234                  # send SIGTERM (graceful shutdown) to PID 1234
kill -9 1234               # send SIGKILL (force kill) — use as last resort
kill -HUP 1234             # send SIGHUP (reload configuration)
killall nginx              # kill all processes named nginx
pkill -f "python app.py"  # kill processes matching a pattern

Common Signals