Cheatsheets:LinuxCLI/SysOps - Revision history

Rongzhou: /* Killing processes */

2026-09-22T14:57:14Z

Killing processes

← Older revision		Revision as of 14:57, 22 September 2026
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	pkill -u username # kill every process of a user		pkill -u username # kill every process of a user
	pkill -f "pattern" # kill processes whose command line matches		pkill -f "pattern" # kill processes whose command line matches
			pkill -f "pattern" -s 9 # force kill processes whose command line matches (signal 9 = FORCE KILL)

	fuser "$port/tcp" # Search by TCP port: show PID(s) bound to port		fuser "$port/tcp" # Search by TCP port: show PID(s) bound to port
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	\| 15 <syntaxhighlight lang="text" inline>TERM</syntaxhighlight> \|\| graceful shutdown (the default)		\| 15 <syntaxhighlight lang="text" inline>TERM</syntaxhighlight> \|\| graceful shutdown (the default)
	\|-		\|-
	\| 9 <syntaxhighlight lang="text" inline>KILL</syntaxhighlight> \|\| force kill — '''may cause ~~memory leaks~~'''		\| 9 <syntaxhighlight lang="text" inline>KILL</syntaxhighlight> \|\| force kill — '''may cause data loss'''
	\|}		\|}

Rongzhou: /* Processes & resource hogs */

2026-09-22T09:20:03Z

Processes & resource hogs

@@ Line 18: / Line 18: @@
 </syntaxhighlight>
-== Processes & resource hogs ==
+== Processes & resource ==
 On Linux systems, each process has three identifiants:
@@ Line 56: / Line 56: @@
 @enduml
 </uml>
 <syntaxhighlight lang="bash" copy>
@@ Line 86: / Line 108: @@
 |}
-Killing processes:
+=== Killing processes ===
 <syntaxhighlight lang="bash" copy>

Rongzhou: /* Processes & resource hogs */

2026-09-19T09:30:53Z

Processes & resource hogs

@@ Line 19: / Line 19: @@
 == Processes & resource hogs ==
 <syntaxhighlight lang="bash" copy>
 ps "$pid"                         # show info on a particular process
-ps aux                            # all processes, all users
+ps -o pid,pgid,sid,comm -p 3995   # show PID, PGID, SID for the process
 ps aux --sort=-%cpu | head -10    # top CPU consumers
 ps aux --sort=-%mem | head -10    # top memory consumers
 free -h                           # RAM and swap usage
 uptime                            # load average and uptime
 pgrep -af "$name"                 # Search by process name: show PID and command line
 </syntaxhighlight>
@@ Line 48: / Line 89: @@
 <syntaxhighlight lang="bash" copy>
-kill -9 PID            # force-kill by PID (find it in ps)
+kill PID               # kill by PID
 pkill -u username      # kill every process of a user
@@ Line 56: / Line 100: @@
 fuser -k "$port/tcp"   # Kill by TCP port: terminate process bound to port
-kill -l                # list all signal names
+kill -l                # list all available signal names
 </syntaxhighlight>

Rongzhou: /* Processes & resource hogs */

2026-09-19T08:52:07Z

Processes & resource hogs

← Older revision		Revision as of 08:52, 19 September 2026
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	<syntaxhighlight lang="bash" copy>		<syntaxhighlight lang="bash" copy>
			ps "$pid" # show info on a particular process
	ps aux # all processes, all users		ps aux # all processes, all users
	ps aux --sort=-%cpu \| head -10 # top CPU consumers		ps aux --sort=-%cpu \| head -10 # top CPU consumers

Rongzhou: /* Processes & resource hogs */

2026-09-19T08:48:27Z

Processes & resource hogs

← Older revision		Revision as of 08:48, 19 September 2026
Line 48:		Line 48:
	<syntaxhighlight lang="bash" copy>		<syntaxhighlight lang="bash" copy>
	kill -9 PID # force-kill by PID (find it in ps)		kill -9 PID # force-kill by PID (find it in ps)

	pkill -u username # kill every process of a user		pkill -u username # kill every process of a user
	pkill -f "pattern" # kill processes whose command line matches		pkill -f "pattern" # kill processes whose command line matches

			fuser "$port/tcp" # Search by TCP port: show PID(s) bound to port
			fuser -k "$port/tcp" # Kill by TCP port: terminate process bound to port

	kill -l # list all signal names		kill -l # list all signal names
	</syntaxhighlight>		</syntaxhighlight>

Rongzhou: /* Processes & resource hogs */

2026-09-19T08:46:41Z

Processes & resource hogs

← Older revision		Revision as of 08:46, 19 September 2026
Line 27:		Line 27:
	free -h # RAM and swap usage		free -h # RAM and swap usage
	uptime # load average and uptime		uptime # load average and uptime
			pgrep -af "$name" # Search by process name: show PID and command line
	</syntaxhighlight>		</syntaxhighlight>

Rongzhou at 08:21, 24 August 2026

2026-08-24T08:21:33Z

← Older revision		Revision as of 08:21, 24 August 2026
Line 1:		Line 1:
	{{DesktopLinux}}		{{DesktopLinux}}

	The everyday Linux system-administration commands: see who is on the box, spot CPU/RAM/disk hogs, manage users and groups, and control services with systemd~~. Most of these need <syntaxhighlight lang="bash" inline>sudo</syntaxhighlight>~~.		The everyday Linux system-administration commands: see who is on the box, spot CPU/RAM/disk hogs, manage users and groups, and control services with systemd.

	New to the command line? Every command here has a built-in manual — try <syntaxhighlight lang="bash" inline>man <command></syntaxhighlight> or [https://tldr.sh tldr] for short examples.		New to the Linux command line? Every command here has a built-in manual — try <syntaxhighlight lang="bash" inline>man <command></syntaxhighlight> or [https://tldr.sh tldr] for short examples.

			<blockquote>
			Many commands in this cheatsheet may require root priveledges. Use <syntaxhighlight lang="bash" inline>sudo</syntaxhighlight>. Don't log in as <code>root</code>.
			</blockquote>

	== Who is logged in ==		== Who is logged in ==
Line 20:		Line 24:
	ps aux --sort=-%cpu \| head -10 # top CPU consumers		ps aux --sort=-%cpu \| head -10 # top CPU consumers
	ps aux --sort=-%mem \| head -10 # top memory consumers		ps aux --sort=-%mem \| head -10 # top memory consumers
	~~top # live process view, q to quit~~		htop # CLI interactive process manager (may require manual installation: older systems only pre-install `top`)
	htop # ~~friendlier live view~~ (~~if installed~~)
	free -h # RAM and swap usage		free -h # RAM and swap usage
	uptime # load average and uptime		uptime # load average and uptime
Line 54:		Line 57:
	\| 1 <syntaxhighlight lang="text" inline>HUP</syntaxhighlight> \|\| reload configuration		\| 1 <syntaxhighlight lang="text" inline>HUP</syntaxhighlight> \|\| reload configuration
	\|-		\|-
	\| 9 <syntaxhighlight lang="text" inline>~~KILL~~</syntaxhighlight> \|\| ~~force kill — cannot be caught~~		\| 15 <syntaxhighlight lang="text" inline>TERM</syntaxhighlight> \|\| graceful shutdown (the default)
	\|-		\|-
	\| 15 <syntaxhighlight lang="text" inline>~~TERM~~</syntaxhighlight> \|\| ~~graceful shutdown (the default)~~		\| 9 <syntaxhighlight lang="text" inline>KILL</syntaxhighlight> \|\| force kill — '''may cause memory leaks'''
	\|}		\|}

RonzzWikiCowriterAI: AI-assisted (RonzzWikiCowriter): create SysOps cheatsheet from ops quick-reference notes (via create-page on MediaWiki MCP Server)

2026-08-24T08:07:07Z

AI-assisted (RonzzWikiCowriter): create SysOps cheatsheet from ops quick-reference notes (via create-page on MediaWiki MCP Server)

New page

{{DesktopLinux}}

The everyday Linux system-administration commands: see who is on the box, spot CPU/RAM/disk hogs, manage users and groups, and control services with systemd. Most of these need <syntaxhighlight lang="bash" inline>sudo</syntaxhighlight>.

New to the command line? Every command here has a built-in manual — try <syntaxhighlight lang="bash" inline>man <command></syntaxhighlight> or [https://tldr.sh tldr] for short examples.

== Who is logged in ==

<syntaxhighlight lang="bash" copy>
who # who is currently logged in
w # who is logged in, and what they are running
last # history of all logins (reads /var/log/wtmp)
id username # a user's UID, GID and group memberships
</syntaxhighlight>

== Processes & resource hogs ==

<syntaxhighlight lang="bash" copy>
ps aux # all processes, all users
ps aux --sort=-%cpu | head -10 # top CPU consumers
ps aux --sort=-%mem | head -10 # top memory consumers
top # live process view, q to quit
htop # friendlier live view (if installed)
free -h # RAM and swap usage
uptime # load average and uptime
</syntaxhighlight>

{| class="wikitable"
! Column !! Meaning
|-
| <syntaxhighlight lang="text" inline>USER</syntaxhighlight> || owner of the process
|-
| <syntaxhighlight lang="text" inline>%CPU</syntaxhighlight> / <syntaxhighlight lang="text" inline>%MEM</syntaxhighlight> || CPU / memory share
|-
| <syntaxhighlight lang="text" inline>VSZ</syntaxhighlight> / <syntaxhighlight lang="text" inline>RSS</syntaxhighlight> || virtual / resident memory
|-
| <syntaxhighlight lang="text" inline>STAT</syntaxhighlight> || state: R running, S sleeping, Z zombie
|-
| <syntaxhighlight lang="text" inline>COMMAND</syntaxhighlight> || the program and its arguments
|}

Killing processes:

<syntaxhighlight lang="bash" copy>
kill -9 PID # force-kill by PID (find it in ps)
pkill -u username # kill every process of a user
pkill -f "pattern" # kill processes whose command line matches
kill -l # list all signal names
</syntaxhighlight>

{| class="wikitable"
! Signal !! Meaning
|-
| 1 <syntaxhighlight lang="text" inline>HUP</syntaxhighlight> || reload configuration
|-
| 9 <syntaxhighlight lang="text" inline>KILL</syntaxhighlight> || force kill — cannot be caught
|-
| 15 <syntaxhighlight lang="text" inline>TERM</syntaxhighlight> || graceful shutdown (the default)
|}

<blockquote>
'''<syntaxhighlight lang="bash" inline>kill -9</syntaxhighlight> is a last resort''' — prefer plain <syntaxhighlight lang="bash" inline>kill</syntaxhighlight> (SIGTERM) so the process can save state and close files cleanly.
</blockquote>

== Disk usage ==

<syntaxhighlight lang="bash" copy>
df -h # free space per mounted filesystem
df -h / # free space on one filesystem
df -i # inode usage
du -sh dir/ # total size of a directory
du -sh * # size of each item in the current directory
du -h --max-depth=1 dir/ # per subdirectory
du -sh /* 2>/dev/null | sort -h # largest top-level folders
</syntaxhighlight>

<blockquote>
'''"Disk full" with <syntaxhighlight lang="bash" inline>df -h</syntaxhighlight> showing free space?''' Check inodes with <syntaxhighlight lang="bash" inline>df -i</syntaxhighlight> — millions of tiny files can exhaust the inode table first.
</blockquote>

== Users & groups ==

<syntaxhighlight lang="bash" copy>
useradd -m -s /bin/bash john # create john: home dir (-m) and bash shell (-s)
passwd john # set or change john's password
usermod -aG sudo john # add john to the sudo (admin) group
usermod -d /new/home john # change john's home directory
usermod -L john # lock john's account
usermod -U john # unlock it again
userdel -r john # delete john and their home dir
</syntaxhighlight>

<syntaxhighlight lang="bash" copy>
groups username # groups a user belongs to
groupadd deploy # create a group
groupdel deploy # delete a group
usermod -aG deploy john # add john to the deploy group
</syntaxhighlight>

== Services & logs (systemd) ==

<syntaxhighlight lang="bash" copy>
systemctl status nginx # is it running? why not?
systemctl start nginx # start now
systemctl stop nginx # stop now
systemctl restart nginx # stop, then start again
systemctl reload nginx # reload config without a full restart
systemctl enable nginx # start at boot
systemctl disable nginx # don't start at boot
systemctl --failed # units that failed to start
</syntaxhighlight>

<syntaxhighlight lang="bash" copy>
journalctl -u nginx # logs for one service
journalctl -u nginx -f # follow new log lines live
journalctl -u nginx -n 100 # last 100 lines
journalctl -u nginx --since "1 hour ago"
journalctl -xe # recent log + hints about errors
</syntaxhighlight>

== Emergencies quick reference ==

{| class="wikitable"
! Scenario !! Command
|-
| Server slow — find the CPU hog || <syntaxhighlight lang="bash" inline>ps aux --sort=-%cpu | head -10</syntaxhighlight>
|-
| Out of RAM — find the memory hog || <syntaxhighlight lang="bash" inline>ps aux --sort=-%mem | head -10</syntaxhighlight>
|-
| Disk 100% full — find the biggest folder || <syntaxhighlight lang="bash" inline>du -sh /* 2>/dev/null | sort -h</syntaxhighlight>
|-
| A user is abusing the system — kick them out || <syntaxhighlight lang="bash" inline>pkill -u username</syntaxhighlight> or <syntaxhighlight lang="bash" inline>sudo kill -9 PID</syntaxhighlight>
|-
| Change a password immediately || <syntaxhighlight lang="bash" inline>passwd username</syntaxhighlight>
|-
| Lock / unlock an account || <syntaxhighlight lang="bash" inline>usermod -L username</syntaxhighlight> / <syntaxhighlight lang="bash" inline>usermod -U username</syntaxhighlight>
|-
| Who changed a file || <syntaxhighlight lang="bash" inline>stat file.txt</syntaxhighlight> (access / change / modification times)
|}

== Further reading ==

* <syntaxhighlight lang="bash" inline>man <command></syntaxhighlight> — the built-in manual, always installed
* [https://www.man7.org/linux/man-pages/ man pages online]
* [https://www.freedesktop.org/software/systemd/man/ systemd documentation] — for systemctl and journalctl
* [https://tldr.sh tldr] — community-maintained short examples
* [https://linuxjourney.com Linux Journey] — beginner-friendly tutorial