Cheatsheets:LinuxCLI/SysOps

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Quick reference for smart people — part of our Linux cheatsheets collection.

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 Linux command line? Every command here has a built-in manual — try man <command> or tldr for short examples.

Many commands in this cheatsheet may require root priveledges. Use sudo. Don't log in as root.

Who is logged in

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

Processes & resource

On Linux systems, each process has three identifiants:

ID	Identifies
PID	The process
PGID	The process group: normally regroups all processes created by one shell command
SID	The controlling terminal

The hierarchy is therefore:

Start processes

cmd &                             # run in background (new job)
nohup cmd &                       # ignore SIGHUP; survives terminal exit
nohup cmd >out.log 2>&1 &         # ...and redirect both stdout and stderr
setsid cmd                        # run in a new session (detached from TTY)
setsid -f cmd                     # ...and fork if already a process group leader
disown %1                         # remove job from shell's job table
disown -h %1                      # keep job but mark to ignore SIGHUP
set -m                            # enable job control in a non-interactive shell
nice -n 10 cmd                    # start with lower priority (higher niceness)
renice -n 5 -p "$pid"             # change niceness of a running process
timeout 30s cmd                   # kill cmd if it runs longer than 30s
timeout -s KILL 30s cmd           # ...send SIGKILL instead of SIGTERM
start-stop-daemon --start --background --exec /usr/bin/cmd

where cmd is any arbitrary command.

Monitor processes

ps "$pid"                         # show info on a particular process
ps -o pid,pgid,sid,comm -p 3995   # show PID, PGID, SID for the process
ps aux                            # show all processes of all users
ps aux --sort=-%cpu | head -10    # top CPU consumers
ps aux --sort=-%mem | head -10    # top memory consumers
free -h                           # RAM and swap usage
uptime                            # load average and uptime
pgrep -af "$name"                 # Search by process name: show PID and command line

htop                              # CLI interactive process manager (may require manual installation: older systems only pre-install `top`)

Column	Meaning
`USER`	owner of the process
`%CPU` / `%MEM`	CPU / memory share
`VSZ` / `RSS`	virtual / resident memory
`STAT`	state: R running, S sleeping, Z zombie
`COMMAND`	the program and its arguments

Killing processes

kill PID               # kill by PID
kill -9 PID            # force-kill by PID
kill -PGID             # kill by PGID
kill -s SID            # kill by SID

pkill -u username      # kill every process of a user
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 "$port/tcp"      # Search by TCP port: show PID(s) bound to port
fuser -k "$port/tcp"   # Kill by TCP port: terminate process bound to port

kill -l                # list all available signal names

Signal	Meaning
1 `HUP`	reload configuration
15 `TERM`	graceful shutdown (the default)
9 `KILL`	force kill — may cause data loss

kill -9 is a last resort — prefer plain kill (SIGTERM) so the process can save state and close files cleanly.

Disk usage

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

"Disk full" with df -h showing free space? Check inodes with df -i — millions of tiny files can exhaust the inode table first.

Users & groups

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

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

Services & logs (systemd)

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

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

Emergencies quick reference

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