Cheat sheet

Where am I & what’s here

Task	Rocky / RHEL	Ubuntu / Debian
Print current folder	pwd same on both
List files	ls same on both
List with details / hidden files	ls -l ls -a ls -la same on both
Shortcut “ll”	ls -l	ls -alF
Which distro is this?	cat /etc/os-release same on both
Who am I? / my groups	whoami id same on both
Machine name / IP address	hostname hostname -I same on both

Moving around

Task	Rocky / RHEL	Ubuntu / Debian
Go into a folder	cd Documents same on both
Up one level / home / back	cd .. cd cd - same on both
Absolute path	cd /var/log same on both

Files & folders

Task	Rocky / RHEL	Ubuntu / Debian
Make folder / nested folders	mkdir NAME mkdir -p a/b/c same on both
Make empty file	touch FILE same on both
Write / append text	echo "hi" > FILE echo "hi" >> FILE same on both
Read a file	cat FILE less FILE head FILE tail FILE same on both
Copy / copy folder	cp A B cp -r DIR1 DIR2 same on both
Move / rename	mv OLD NEW same on both
Delete file / folder	rm FILE rm -r DIR rmdir EMPTYDIR same on both
Friendly text editor	sudo dnf install nano → nano FILE	nano FILE (pre-installed)
Write to a root-owned file	echo "hi" | sudo tee /etc/FILE same on both
Search inside text	grep WORD FILE COMMAND | grep WORD same on both
File details / type	stat FILE file FILE same on both

The filesystem

Task	Rocky / RHEL	Ubuntu / Debian
Settings / logs & data / programs	/etc /var /usr/bin same on both
Your own scripts (everyone)	/usr/local/bin (never /usr/bin) same on both
Old folders merged into /usr	/bin → usr/bin /sbin → usr/sbin /lib → usr/lib same on both
Kernel & bootloader	/boot/vmlinuz-* /boot/grub2/	/boot/vmlinuz-* /boot/grub/
Extra service settings	/etc/sysconfig/	/etc/default/
Installed-package database	/var/lib/rpm/	/var/lib/dpkg/
Repo list	/etc/yum.repos.d/	/etc/apt/sources.list.d/
Kernel info (virtual)	cat /proc/cpuinfo /proc/meminfo /proc/1/comm /sys/class/net/ same on both
Devices	ls -l /dev (b = block/disk, c = character) /dev/null same on both
sudo finds /usr/local/bin?	No: use the full path	Yes
The official map	man hier same on both

Finding files & links

Task	Rocky / RHEL	Ubuntu / Debian
Find by name / type / size	find ~ -name "*.txt" find / -type d -name NAME find ~ -size +100M same on both
Changed recently	find ~ -mtime -1 (last day) find ~ -mmin -30 same on both
Act on results	find … -exec COMMAND {} \; find … -delete (look first!) same on both
Instant search	sudo dnf install mlocate → sudo updatedb → locate NAME	sudo apt install plocate → locate NAME
Refresh locate	sudo updatedb same on both
Where is a command?	which CMD whereis CMD type CMD same on both
Shortcut (symlink)	ln -s TARGET LINKNAME readlink LINK same on both
Hard link / inode numbers	ln FILE NEWNAME ls -li same on both
Find broken links	find ~ -xtype l same on both

Installing software

Task	Rocky / RHEL	Ubuntu / Debian
Refresh package list	(automatic) dnf check-update	sudo apt update
Install	sudo dnf install NAME	sudo apt install NAME
Remove	sudo dnf remove NAME	sudo apt remove NAME
Update everything	sudo dnf upgrade	sudo apt update && sudo apt upgrade
Search	dnf search WORD	apt search WORD
Package details	dnf info NAME	apt show NAME
List installed	dnf list installed rpm -qa	apt list --installed dpkg -l
Extra repository	sudo dnf install epel-release	(universe is on by default)
Which package gives a command?	dnf provides '*/bin/NAME'	apt-file search bin/NAME
Which package owns a file?	rpm -qf /usr/bin/ls	dpkg -S /usr/bin/ls
Version & repo / dependencies	dnf info NAME dnf deplist NAME	apt-cache policy NAME apt-cache depends NAME
Package file type	.rpm	.deb
Old command name	yum	apt-get

Users & sudo

Task	Rocky / RHEL	Ubuntu / Debian
Run one command as root	sudo COMMAND same on both
Admin group	wheel	sudo
Add a user	sudo useradd NAME → sudo passwd NAME	sudo adduser NAME
Make a user an admin	sudo usermod -aG wheel NAME	sudo usermod -aG sudo NAME
Change your password	passwd same on both
Root shell (then exit!)	sudo -i sudo su - same on both
Switch user	su - NAME (the dash = full login) same on both
Is root locked?	sudo passwd -S root → LK	sudo passwd -S root → L (always)
Who is logged in?	who w last same on both

vim

Task	Rocky / RHEL	Ubuntu / Debian
Get it	sudo dnf install vim-enhanced (vi is always there)	vim is pre-installed
Modes	i a o A → insert Esc → normal v V → visual : → command line same on both
Save & quit	:w :q :wq (ZZ) :q! (discard) same on both
Move	h j k l w b e 0 ^ $ gg G :42 Ctrl+D / Ctrl+U same on both
Edit	x dd dw cw ciw D yy p P u Ctrl+R . 3dd same on both
Search & replace	/word n N * :%s/old/new/g :g/DEBUG/d :v/ERROR/d same on both
Settings	:set number ~/.vimrc vimtutor same on both
Default editor	echo 'export EDITOR=vim' >> ~/.bashrc sudoedit /etc/FILE same on both

Wildcards & braces

Task	Rocky / RHEL	Ubuntu / Debian
Match	* anything ? one character [abc] [0-9] [!x] one of / not same on both
Make lists	mkdir -p proj/{src,docs} touch f{1..5}.txt cp a.conf{,.bak} same on both
Look first	echo rm *.log → rm *.log same on both
Quote to stop it	find . -name '*.png' echo '*' same on both

Jobs

Task	Rocky / RHEL	Ubuntu / Debian
Stop / pause	Ctrl+C stop Ctrl+Z pause same on both
Background	cmd & bg jobs [-l] same on both
Foreground	fg fg %2 same on both
End a job	kill %1 kill PID same on both
Survive logout	nohup cmd & disown %1 (forever? use a systemd service) same on both

Your hardware

Task	Rocky / RHEL	Ubuntu / Debian
Linux & kernel	cat /etc/os-release uname -r hostnamectl same on both
CPU & memory	lscpu nproc free -h /proc/cpuinfo /proc/meminfo same on both
Disks	lsblk (devices) df -h (space) same on both
Devices	sudo dnf install pciutils usbutils → lspci lsusb	lspci lspci -k lsusb
Model & VM?	sudo dmidecode -s system-product-name systemd-detect-virt same on both

Archives & compression

Task	Rocky / RHEL	Ubuntu / Debian
Compress one file	gzip -k F xz -k F bzip2 -k F → gunzip / unxz / bunzip2 same on both
Read without unpacking	zcat xzcat bzcat same on both
tar	tar -czf a.tar.gz DIR tar -cJf a.tar.xz DIR tar -tf a.tar.gz tar -xf a.tar.gz -C DIR same on both
zip	sudo dnf install zip unzip → zip -r a.zip DIR unzip -l a.zip unzip a.zip -d DIR	sudo apt install zip unzip → zip -r a.zip DIR unzip -l a.zip unzip a.zip -d DIR
What is it?	file NAME same on both

Updates

Task	Rocky / RHEL	Ubuntu / Debian
What’s waiting	dnf check-update dnf updateinfo list --security	sudo apt update apt list --upgradable
Security only	sudo dnf upgrade --security	sudo unattended-upgrade -v
Reboot needed?	dnf needs-restarting -r sudo dnf needs-restarting -s	ls /var/run/reboot-required cat /var/run/reboot-required.pkgs
Kernels	rpm -q kernel	dpkg -l 'linux-image*'
Automatic	sudo dnf install dnf-automatic /etc/dnf/automatic.conf sudo systemctl enable --now dnf-automatic.timer	/etc/apt/apt.conf.d/20auto-upgrades 50unattended-upgrades

More ways to install

Task	Rocky / RHEL	Ubuntu / Debian
Vendor repo	sudo dnf config-manager --add-repo URL.repo → sudo dnf install PKG	curl -fsSL KEY | sudo gpg --dearmor -o /usr/share/keyrings/X.gpg deb [signed-by=…] URL noble main → sudo apt update
Snap	EPEL: sudo dnf install snapd sudo systemctl enable --now snapd.socket	snap find X sudo snap install X snap list sudo snap remove X
Flatpak	sudo flatpak remote-add --if-not-exists flathub https://dl.flathub.org/repo/flathub.flatpakrepo flatpak install flathub ID flatpak list same on both
AppImage	chmod u+x X.AppImage ./X.AppImage (needs fuse-libs)	chmod u+x X.AppImage ./X.AppImage (needs libfuse2t64)

SELinux & AppArmor

Task	Rocky / RHEL	Ubuntu / Debian
Is it on?	getenforce sestatus	sudo aa-status
Labels / profiles	ls -Z ps -eZ id -Z	ls /etc/apparmor.d ps auxZ
What was blocked	sudo ausearch -m avc -ts recent	sudo journalctl -k | grep DENIED
Test briefly	sudo setenforce 0 … sudo setenforce 1	sudo aa-complain PROG … sudo aa-enforce PROG
Fix for real	sudo semanage fcontext -a -t TYPE "/path(/.*)?" sudo restorecon -Rv /path semanage port -a setsebool -P	edit /etc/apparmor.d/PROFILE sudo apparmor_parser -r PROFILE

Networking

Task	Rocky / RHEL	Ubuntu / Debian
My IP addresses	ip a ip -br a (old: ifconfig · Windows: ipconfig) same on both
Gateway / routes	ip route (old: route -n) same on both
Neighbours on the LAN	ip neigh (old: arp -a) same on both
Listening ports	ss -tlnp (old: netstat -tlnp) same on both
Get ifconfig/route/netstat	sudo dnf install net-tools	sudo apt install net-tools
Test the path	ping -c 3 GATEWAY ping -c 3 8.8.8.8 ping -c 3 google.com tracepath 8.8.8.8 same on both
DNS servers in use	cat /etc/resolv.conf	resolvectl status (resolv.conf says 127.0.0.53)
DNS lookup tools	sudo dnf install bind-utils → dig NAME	dig NAME (pre-installed)
Local name overrides	/etc/hosts getent hosts NAME same on both
Permanent config	NetworkManager: nmcli con mod … → nmcli con up … (or nmtui)	Netplan: /etc/netplan/*.yaml → sudo netplan try
Config files	/etc/NetworkManager/system-connections/*.nmconnection	/etc/netplan/*.yaml (spaces, not tabs; chmod 600)
Temporary changes	sudo ip addr add IP/24 dev CARD sudo ip route add default via GW same on both
Rename the machine	sudo hostnamectl set-hostname NAME same on both

Remote access (SSH)

Task	Rocky / RHEL	Ubuntu / Debian
Connect / disconnect	ssh USER@HOST exit same on both
Make a key pair	ssh-keygen -t ed25519 same on both
Copy key to server	ssh-copy-id USER@HOST same on both
Copy key without ssh-copy-id	cat ~/.ssh/id_ed25519.pub | ssh USER@HOST "mkdir -p ~/.ssh && cat >> ~/.ssh/authorized_keys" same on both
Key permissions (server)	chmod 700 ~/.ssh chmod 600 ~/.ssh/authorized_keys same on both
Loose key permissions	refused (see /var/log/secure)	group-write allowed if the group is yours
Nickname a server	~/.ssh/config: Host NAME / HostName IP / User USER → ssh NAME same on both
Apps	Windows Terminal · PuTTY (+PuTTYgen, .ppk keys) · MobaXterm · Mac Terminal · iTerm2 · Termius same on both
PuTTY copy / paste	select = copy right-click = paste same on both
Web console	Cockpit: sudo systemctl enable --now cockpit.socket → https://IP:9090	sudo apt install cockpit → https://IP:9090
Install SSH server	sudo dnf install openssh-server	sudo apt install openssh-server
SSH service name	sshd	ssh

Services

Task	Rocky / RHEL	Ubuntu / Debian
Status / start / stop / restart	systemctl status NAME sudo systemctl start|stop|restart NAME same on both
Start now + at every boot	sudo systemctl enable --now NAME same on both
Auto-starts after install?	No	Yes
Everything in the logs	journalctl -u NAME journalctl -f same on both
PID 1 (the init system)	systemd on both ps -p 1 -o comm same on both
Old-style commands	service NAME restart (“Redirecting to /bin/systemctl…”)	service NAME restart /etc/init.d/NAME restart
/etc/init.d	just a README	compatibility scripts (“via systemctl”)
Old boot on/off tool	chkconfig NAME on	update-rc.d NAME enable
Read / edit a unit	systemctl cat NAME sudo systemctl edit NAME (drop-in) same on both
Unit file folders	/usr/lib/systemd/system (packages) /etc/systemd/system (yours, wins) same on both
After changing a unit file	sudo systemctl daemon-reload same on both
Block / unblock a service	sudo systemctl mask NAME sudo systemctl unmask NAME same on both
Installed units & state	systemctl list-unit-files systemctl --failed same on both
Runlevel → target	runlevel systemctl get-default 3 = multi-user.target 5 = graphical.target same on both
Custom service script location	/usr/local/bin (SELinux blocks /home)	/usr/local/bin
Kernel settings (not systemctl!)	sysctl KEY sudo sysctl -w KEY=VAL /etc/sysctl.d/*.conf sudo sysctl --system same on both

Web server (Apache)

Task	Rocky / RHEL	Ubuntu / Debian
Package & service	httpd	apache2
Install	sudo dnf install httpd	sudo apt install apache2
Config folder	/etc/httpd/	/etc/apache2/
Logs	/var/log/httpd/	/var/log/apache2/
Runs as user	apache	www-data
Web pages go in	/var/www/html/ same on both
Test it	curl localhost same on both

Firewall

Task	Rocky / RHEL	Ubuntu / Debian
Tool	firewalld (firewall-cmd)	ufw
On by default?	Yes (SSH allowed)	No
Show rules	sudo firewall-cmd --list-services	sudo ufw status
Allow web traffic	sudo firewall-cmd --permanent --add-service=http → sudo firewall-cmd --reload	sudo ufw allow 'Apache'
Allow SSH	(already allowed)	sudo ufw allow OpenSSH ← before enabling!
Turn on	sudo systemctl enable --now firewalld	sudo ufw enable
Security guard	SELinux	AppArmor
Low-level rules	sudo nft list ruleset	sudo iptables -L -n
iptables command	sudo dnf install iptables-nft	(pre-installed)

Network & security tools

Task	Rocky / RHEL	Ubuntu / Debian
Fetch a page / headers	curl URL curl -I URL curl -O URL same on both
Download a file	sudo dnf install wget → wget URL	wget URL
Is a port open?	sudo dnf install nmap-ncat → nc -zv HOST PORT	nc -zv HOST PORT
What is listening?	ss -tlnp sudo lsof -i :PORT same on both
Watch packets	sudo dnf install tcpdump → sudo tcpdump -i enp0s3 -c 5 icmp	sudo tcpdump -i enp0s3 -c 5 icmp
Random secret	openssl rand -base64 24 same on both
Self-signed certificate	openssl req -x509 -newkey rsa:2048 -nodes -keyout key.pem -out cert.pem -days 30 -subj "/CN=NAME" same on both
Check a certificate	openssl x509 -in cert.pem -noout -dates openssl s_client -connect HOST:443 same on both
Checksum	sha256sum FILE sha256sum -c SHA256SUMS same on both
My public IP	curl ifconfig.me same on both

Logs

Task	Rocky / RHEL	Ubuntu / Debian
Read the journal	journalctl -u NAME -n 50 journalctl -p err journalctl -xeu NAME same on both
Who can read log files	root (use sudo)	root + adm group
System messages	/var/log/messages	/var/log/syslog
Logins & sudo	/var/log/secure	/var/log/auth.log
Package history	/var/log/dnf.log	/var/log/apt/history.log
Watch a log live	sudo tail -f FILE (Ctrl+C to stop) same on both
Kernel messages	dmesg -T journalctl -k	sudo dmesg -T journalctl -k

Text power tools

Task	Rocky / RHEL	Ubuntu / Debian
Find lines	grep -i -n -c -v -r -E "a|b" same on both
Count	wc -l FILE same on both
Sort / unique / count	sort sort -n sort -rn sort | uniq -c same on both
Columns	cut -d, -f1,3 FILE awk -F, '{print $1}' FILE same on both
Filter by column	awk -F, '$3 > 85 {print $1}' FILE same on both
awk flavor	gawk (awk --version)	mawk (awk -W version)
Find & replace	sed 's/old/new/g' FILE (then -i to save) same on both
Swap characters	tr a-z A-Z tr , '\n' tr -d CHARS same on both
Show and save	COMMAND | tee FILE same on both
List → arguments	find … | xargs COMMAND same on both
Compare files	diff OLD NEW same on both

Permissions

Task	Rocky / RHEL	Ubuntu / Debian
Read permissions	ls -l stat FILE same on both
Make a script runnable	chmod +x script.sh same on both
Private file / normal file / folder	chmod 600 FILE chmod 644 FILE chmod 755 DIR same on both
Change owner / group	sudo chown USER:GROUP FILE sudo chgrp GROUP FILE same on both
Default permissions	umask same on both
Home folder default	700 (drwx------)	750 (drwxr-x---)
/etc/shadow	000, root only	640, root:shadow
Extra security layer	SELinux (ls -Z)	AppArmor

Users & groups

Task	Rocky / RHEL	Ubuntu / Debian
Create a user	sudo useradd NAME → sudo passwd NAME	sudo adduser NAME
useradd makes a home folder?	Yes	Only with -m
Add to a group (keep others!)	sudo usermod -aG GROUP USER sudo gpasswd -a USER GROUP same on both
New group	sudo groupadd NAME same on both
Lock / delete user	sudo usermod -L USER sudo userdel -r USER same on both
Look up	id USER getent passwd USER who last same on both

Cron (scheduling)

Task	Rocky / RHEL	Ubuntu / Debian
Package / service	cronie / crond	cron / cron
Edit / list / remove your jobs	crontab -e crontab -l crontab -r (no undo!) same on both
Time fields	minute hour day-of-month month day-of-week command same on both
Every 5 min / 2:30am weekdays	*/5 * * * * 30 2 * * 1-5 same on both
System jobs (with a user field)	/etc/crontab /etc/cron.d/NAME same on both
Drop-in folders	/etc/cron.hourly cron.daily cron.weekly cron.monthly same on both
Default crontab editor	vi (EDITOR=nano crontab -e)	asks: 1 = nano
cron.d file names with a dot	work	IGNORED
Cron log	/var/log/cron	grep CRON /var/log/syslog
In a crontab, write % as	\% (or use a script) same on both

Processes & disk

Task	Rocky / RHEL	Ubuntu / Debian
List processes	ps aux pgrep -l NAME top htop same on both
htop	EPEL: sudo dnf install htop	pre-installed
Stop a process	kill PID kill -9 PID pkill NAME same on both
Load / cores / memory	uptime nproc free -h same on both
Disk full?	df -h sudo du -h -d 1 / | sort -h sudo find / -size +1G same on both
Filesystem type	xfs	ext4
Empty a big log safely	sudo truncate -s 0 FILE same on both
Deleted but still full?	sudo lsof +L1 → restart / stop that program same on both
Stop by exact name	killall NAME same on both

Disks & mounting

Task	Rocky / RHEL	Ubuntu / Debian
See disks	lsblk sudo fdisk -l df -hT same on both
Partition a disk	sudo fdisk /dev/sdb (n … w) same on both
Format	sudo mkfs.xfs /dev/sdb1	sudo mkfs.ext4 /dev/sdb1
Mount / unmount	sudo mount /dev/sdb1 /data sudo umount /data same on both
“target is busy”	cd ~ lsof /data same on both
Disk ID for fstab	sudo blkid /dev/sdb1 same on both
Mount at boot	UUID=… /data TYPE defaults 0 0 in /etc/fstab same on both
Test fstab (always!)	sudo systemctl daemon-reload sudo mount -a findmnt /data same on both

Backups

Task	Rocky / RHEL	Ubuntu / Debian
Create / list / extract archive	tar -czf A.tar.gz DIR tar -tzf A.tar.gz tar -xzf A.tar.gz -C DEST same on both
Date in a file name	backup-$(date +%F).tar.gz same on both
Mirror a folder	rsync -av SRC/ DEST/ (dry run: -n) same on both
Copy to a server	rsync -av DIR user@host:path/ scp FILE user@host: same on both
Install rsync	sudo dnf install rsync (if missing)	(pre-installed)
Compress one file	gzip FILE gunzip FILE.gz zcat FILE.gz same on both
Zip files	sudo dnf install zip unzip	sudo apt install zip unzip
Make / list / extract zip	zip -r A.zip DIR unzip -l A.zip unzip A.zip -d DEST same on both
Verify backups	sha256sum *.tar.gz > SHA256SUMS sha256sum -c SHA256SUMS same on both

Your company’s app

Task	Rocky / RHEL	Ubuntu / Debian
Program / settings / data	/opt/cht/ourapp /etc/opt/cht/ourapp /var/opt/cht/ourapp same on both
Service account	sudo useradd --system --shell /sbin/nologin ourapp	sudo adduser --system --group ourapp
Owners	program root:root 755 config root:ourapp 640 data ourapp:ourapp 750 same on both
Switch versions	sudo ln -sfn /opt/cht/ourapp/releases/ourapp-X /opt/cht/ourapp/current → sudo systemctl restart ourapp same on both
Test as the app user	sudo -u ourapp /opt/cht/ourapp/current/bin/ourapp --check-config	sudo -u ourapp ourapp --check-config
SELinux labels	ls -Z sudo ausearch -m avc -ts recent sudo restorecon -Rv /opt/cht	(AppArmor: no file labels, ls -Z shows ?)
Open the port	sudo firewall-cmd --permanent --add-port=8080/tcp → --reload	sudo ufw allow 8080/tcp

Shell skills

Task	Rocky / RHEL	Ubuntu / Debian
Repeat last command (as root)	!! sudo !! same on both
Search history	history | grep WORD Ctrl+R same on both
Variables	NAME=value echo "$NAME" export NAME env same on both
Shortcuts	alias NAME='COMMAND' (keep it: add to ~/.bashrc) same on both
Rerun every 2 s	watch -n 2 COMMAND same on both
Session that survives disconnects	sudo dnf install tmux → tmux new -s NAME	tmux new -s NAME
Detach / reattach	Ctrl+B then D tmux ls tmux attach -t NAME same on both
screen	EPEL only: sudo dnf install screen	screen -S NAME Ctrl+A then D screen -r
Time zone	timedatectl sudo timedatectl set-timezone America/Chicago same on both
Clock sync service	chronyd	systemd-timesyncd
Rename the server	sudo hostnamectl set-hostname NAME	sudo hostnamectl set-hostname NAME (+ fix 127.0.1.1 in /etc/hosts)

Capturing output

Task	Rocky / RHEL	Ubuntu / Debian
Save results / add to a file	cmd > file cmd >> file same on both
Errors only / both / silence	cmd 2> err.txt cmd > all.txt 2>&1 cmd >/dev/null 2>&1 same on both
Order matters	> file 2>&1 ✔ 2>&1 > file ✘ (errors still on screen) same on both
See it and save it	cmd | tee file cmd 2>&1 | tee -a log same on both
Write a whole file	cat > file <<EOF … EOF <<'EOF' = no $ expansion same on both
Error message from a script	echo "oops" >&2 same on both
Record a session	script session.log … exit same on both
To the system log	logger -t TAG "msg" → /var/log/messages	logger -t TAG "msg" → /var/log/syslog
Log a whole script	exec >> /path/script.log 2>&1 (near the top) same on both

Substitution & subshells

Task	Rocky / RHEL	Ubuntu / Debian
Insert a command’s output	echo "Today: $(date +%A)" old style: `date +%A` same on both
Save output in a variable	n=$(ls | wc -l) x=$(cmd 2>&1) (include errors) same on both
Keep lines & spaces	"$(cmd)" (quote it!) same on both
Math	$(( 6 * 7 )) $(( n + 1 )) $(( 7 % 2 )) same on both
Variable tricks	${f##*/} ${f%/*} ${f%.txt} ${#f} ${X:-default} same on both
Do it “over there”	(cd /var/log && ls) (subshell: you don’t move) same on both
Group outputs	{ date; uptime; } > report.txt same on both
Output as a file	diff <(ls dir1) <(ls dir2) same on both
/bin/sh is…	bash (everything works)	dash (no <( ), [[ ]], ${x//a/b})
Force bash	#!/bin/bash bash -c '…' SHELL=/bin/bash in crontab same on both

Shell scripting

Task	Rocky / RHEL	Ubuntu / Debian
First line	#!/bin/bash (then chmod +x script.sh) same on both
Arguments	$1 $2 … $# "$@" shift ${1:-default} same on both
Tests	[ -f f ] [ -d d ] [ -z "$s" ] [ "$a" = "$b" ] [ "$n" -gt 5 ] same on both
Decide	if …; then …; elif …; else …; fi cmd && ok || fail same on both
Choose	case "$x" in a|b) … ;; *) … ;; esac same on both
Loops	for x in a b c; do …; done while [ … ]; do …; done same on both
File line by line	while IFS=: read -r a b rest; do …; done < file same on both
Functions	name() { local x="$1"; …; return 0; } same on both
Debug / strict	bash -x script.sh set -euo pipefail same on both
Both families in one script	. /etc/os-release; case "$ID" in rocky|rhel) dnf … ;; ubuntu|debian) apt … ;; esac same on both

POSIX & portability

Task	Rocky / RHEL	Ubuntu / Debian
/bin/sh is…	bash (lenient)	dash (strict)
Instead of [[ ]] / == / source	[ ] · = · . same on both
Instead of echo -e / -n	printf 'text\n' same on both
Instead of {1..5}	for i in 1 2 3 4 5 $(seq 5) same on both
Instead of &>	> file 2>&1 same on both
Check a script	sudo dnf install ShellCheck (EPEL) → shellcheck s.sh	sudo apt install shellcheck → shellcheck s.sh checkbashisms s.sh
POSIX version / limits	getconf _POSIX_VERSION getconf ARG_MAX same on both

Distro cousins

Task	Rocky / RHEL	Ubuntu / Debian
Rocky’s twin	AlmaLinux: same commands, ID="almalinux" same on both
Amazon Linux 2023	Red Hat family (from Fedora): dnf, ec2-user, no EPEL, no firewalld, no cron by default same on both
Which family?	grep -E '^(ID|ID_LIKE)=' /etc/os-release (check ID_LIKE) same on both
Container images	rockylinux/rockylinux:9 · almalinux:9 · amazonlinux:2023 · ubuntu:24.04 · debian:13 · alpine same on both
Alpine	apk add PKG no bash (BusyBox sh) same on both

Alpine & containers

Task	Rocky / RHEL	Ubuntu / Debian
Get podman	sudo dnf install podman	sudo apt install podman (or docker.io, with sudo)
Run Alpine	podman run -it docker.io/library/alpine:3.22 (exit to leave) same on both
One command, then clean up	podman run --rm IMAGE COMMAND same on both
Containers / images	podman ps -a podman images podman rm NAME same on both
Alpine packages	apk update apk add PKG apk del PKG apk search WORD apk add --no-cache PKG same on both
Alpine basics	shell = ash (apk add bash) tools = BusyBox libc = musl services = OpenRC same on both
OpenRC (real Alpine)	rc-service NAME start rc-update add NAME default rc-status same on both

Build & ship images (DevOps)

Task	Rocky / RHEL	Ubuntu / Debian
Build	podman build -t NAME:TAG . (the dot = build context) sudo docker build -t NAME . same on both
Containerfile	FROM image:tag RUN cmd COPY src dest WORKDIR dir ENV K=V EXPOSE port CMD ["prog", "arg"] same on both
Run with a port	podman run -d --name web -p HOST:CONTAINER IMAGE (rootless: HOST ≥ 1024) same on both
Look after it	podman ps -a podman logs NAME podman exec -it NAME sh podman port NAME same on both
Replace it	podman stop NAME podman rm NAME run the new tag podman rmi IMAGE same on both
Open the port	sudo firewall-cmd --add-port=8080/tcp --permanent ; sudo firewall-cmd --reload	sudo ufw allow 8080/tcp
Share it	podman login REGISTRY podman tag IMG REGISTRY/you/IMG:TAG podman push REGISTRY/you/IMG:TAG same on both

Reliability numbers (SRE)

Task	Rocky / RHEL	Ubuntu / Debian
The words	SLI = what you measure SLO = your target SLA = a promise with a penalty KPI = a business number same on both
Nines per 30 days	99% = 7h12m 99.5% = 3h36m 99.9% = 43m 99.99% = 4m19s 99.999% = 26s same on both
Availability from a log	awk '$9 < 500 {g++} END {printf "%.2f%%\n", 100*g/NR}' access.log same on both
Percentiles	awk '{print $NF}' access.log | sort -n | awk '{t[NR]=$1} END {print t[int(NR*0.95)]}' same on both
Error budget	budget = 1 − SLO burn rate 1 = spends it in exactly 30 days same on both

Monitoring (Prometheus)

Task	Rocky / RHEL	Ubuntu / Debian
Ports	node_exporter :9100/metrics Prometheus :9090 Alertmanager :9093 same on both
Check before (re)loading	promtool check config /etc/prometheus/prometheus.yml promtool check rules FILE sudo systemctl reload prometheus same on both
Query	curl -s 'localhost:9090/api/v1/query?query=up' | jq promtool query instant http://localhost:9090 'EXPR' same on both
PromQL	up rate(node_cpu_seconds_total{mode="idle"}[5m]) avg by (instance) (…) predict_linear(x[6h], 86400) same on both
Alerts	curl -s localhost:9090/api/v1/alerts | jq pending → firing after for: same on both
See the UI safely	ssh -L 9090:localhost:9090 you@server → http://localhost:9090 same on both

Incidents

Task	Rocky / RHEL	Ubuntu / Debian
Order	acknowledge → confirm impact → mitigate (roll back!) → communicate → resolve → postmortem same on both
What changed?	sudo journalctl -t sudo --since "1 hour ago" ls -lt /etc/httpd/conf.d	sudo journalctl -t sudo --since "1 hour ago" ls -lt /etc/apache2/sites-available
Config test	sudo apachectl configtest	sudo apache2ctl configtest
Update format	time (UTC) · what users see · what we know · what we’re doing · next update at same on both

Limits & OOM

Task	Rocky / RHEL	Ubuntu / Debian
OOM evidence	dmesg | grep -i -E "oom|killed process"	sudo dmesg | grep -i -E "oom|killed process"
Per-service usage	systemd-cgtop -m systemctl status NAME (Memory:) systemctl show NAME -p MemoryCurrent same on both
Set limits now + saved	sudo systemctl set-property NAME MemoryMax=300M CPUQuota=50% same on both
Other settings	sudo systemctl edit NAME → [Service] OOMScoreAdjust=-500 / LimitNOFILE=65536 same on both
cgroup files	cat /sys/fs/cgroup/system.slice/NAME.service/memory.max same on both

Capacity & load testing

Task	Rocky / RHEL	Ubuntu / Debian
Get ab	sudo dnf install httpd-tools	sudo apt install apache2-utils
Load test (your own server only!)	ab -n 2000 -c 50 http://localhost/ (note the trailing /) | grep -E 'Requests per second|Failed| 95%' same on both
Little’s Law	in flight = throughput × latency same on both
Growth	awk 'BEGIN { print log(target/now) / log(1 + rate) }' → months same on both

Performance triage (SRE)

Task	Rocky / RHEL	Ubuntu / Debian
Get the tools	sudo dnf install sysstat	sudo apt install sysstat
The checklist	uptime dmesg | tail vmstat 1 5 mpstat -P ALL 1 3 pidstat 1 3 iostat -xz 1 3 free -m sar -n DEV 1 3 sar -n TCP,ETCP 1 3 top same on both
Kernel messages	dmesg | tail	sudo dmesg | tail
Disk hogs	pidstat -d 1 3 iostat -xz 1 3 (w_await, aqu-sz, %util) same on both
Signs of trouble	load > CPUs vmstat b > 0 or wa high si/so ≠ 0 %util ≈ 100 state D in top same on both
Which service owns a PID?	systemctl status PID ps -o pid,ppid,stat,cmd -p PID same on both
Timers	systemctl list-timers systemctl cat NAME.timer OnCalendar=*-*-* 02:00:00 sudo systemctl daemon-reload same on both

Compose

Task	Rocky / RHEL	Ubuntu / Debian
Get it	sudo dnf install epel-release ; sudo dnf install podman-compose	sudo apt install podman-compose (or docker-compose-v2 → docker compose)
Run	podman-compose up -d ps logs [SERVICE] exec SERVICE sh restart SERVICE same on both
Change	up -d (recreates what changed) up -d --build (new code) same on both
Stop	down (keeps volumes) down -v (deletes volumes = data!) same on both
Networking	containers reach each other by SERVICE NAME localhost = the container itself same on both

CI (GitHub Actions)

Task	Rocky / RHEL	Ubuntu / Debian
Where	.github/workflows/NAME.yml → on: [push] jobs: runs-on: steps: uses / run same on both
Pass or fail	exit code 0 = pass anything else = fail same on both
From the terminal	gh run list gh run view gh run view ID --log-failed same on both
Lint scripts	sudo dnf install ShellCheck (EPEL) ; shellcheck *.sh	sudo apt install shellcheck ; shellcheck *.sh

Ansible

Task	Rocky / RHEL	Ubuntu / Debian
Install	sudo dnf install epel-release ; sudo dnf install ansible	sudo apt install ansible
Inventory	[web] web1 ansible_host=192.168.1.61 ansible-inventory --graph same on both
Ad-hoc	ansible all -m ping ansible web -a 'uptime' ansible web -b -m package -a 'name=tree' same on both
Playbooks	ansible-playbook site.yml --check --diff ansible-playbook site.yml -l web1 same on both
Both families	when: ansible_facts['os_family'] == "RedHat" / "Debian" or [rocky]/[ubuntu] groups + group_vars same on both
Variables	group_vars/GROUP.yml host_vars/HOST.yml ansible-inventory --host HOST -e var=value (wins) same on both
Loops & handlers	loop: "{{ list }}" + {{ item }} notify: Name → handlers: - name: Name same on both
Roles	ansible-galaxy role init --init-path roles NAME roles: [NAME] defaults/ (low) vs vars/ (high) same on both
Vault	ansible-vault encrypt|view|edit|decrypt FILE --ask-vault-pass vault_password_file = ~/.vault_pass same on both
Rolling updates	serial: 1 max_fail_percentage: 0 uri + assert health check -l web1 -i inventories/production.ini same on both
Checks	ansible-playbook X --syntax-check ansible-lint run twice → changed=0 --list-tasks --tags TAG same on both

Secrets & config

Task	Rocky / RHEL	Ubuntu / Debian
.env	echo "DB_PASSWORD=$(openssl rand -hex 16)" > .env ; chmod 600 .env ; echo .env >> .gitignore same on both
Compose	REDIS_PASSWORD: ${DB_PASSWORD:?set it in .env} same on both
Services	/etc/sysconfig/NAME (600) + EnvironmentFile=/etc/sysconfig/NAME	/etc/default/NAME (600) + EnvironmentFile=/etc/default/NAME
Find leaks	git log -p | grep -i pass gitleaks trufflehog same on both
After a leak	ROTATE: new secret everywhere, old one must stop working same on both

Deploys

Task	Rocky / RHEL	Ubuntu / Debian
Proxy switch	edit ProxyPass port ; sudo apachectl configtest ; sudo systemctl reload httpd	edit ProxyPass port ; sudo apache2ctl configtest ; sudo systemctl reload apache2
Smoke test green first	curl localhost:8082/health curl localhost:8082/api/… same on both
Strategies	recreate · rolling · blue-green (instant rollback) · canary (small slice first) same on both
Rocky + proxy	sudo setsebool -P httpd_can_network_connect 1 same on both

AWS: basics

Task	Rocky / RHEL	Ubuntu / Debian
Install CLI v2	sudo dnf install unzip ; curl "https://awscli.amazonaws.com/awscli-exe-linux-x86_64.zip" -o awscliv2.zip ; unzip awscliv2.zip ; sudo ./aws/install	sudo apt install unzip ; curl "https://awscli.amazonaws.com/awscli-exe-linux-x86_64.zip" -o awscliv2.zip ; unzip awscliv2.zip ; sudo ./aws/install
Sign in / who am I	aws configure aws configure --profile NAME aws sts get-caller-identity same on both
Pick fields	--query 'Items[].Field' --output text|table|json X=$(aws … --query Id --output text) same on both
Region	--region us-east-1 export AWS_REGION=… aws configure get region same on both
IAM	aws iam create-user / create-group / add-user-to-group / attach-group-policy --policy-arn … create-access-key same on both
Test a policy	aws iam simulate-principal-policy --policy-source-arn USER-ARN --action-names s3:PutObject same on both

AWS: network & servers

Task	Rocky / RHEL	Ubuntu / Debian
VPC	aws ec2 create-vpc --cidr-block 10.0.0.0/16 create-subnet --vpc-id … --cidr-block 10.0.1.0/24 --availability-zone us-east-1a same on both
Public subnet	create-internet-gateway + attach-internet-gateway create-route-table + create-route 0.0.0.0/0 → igw + associate-route-table same on both
Security group	create-security-group authorize-security-group-ingress --protocol tcp --port 22 --cidr MY-IP/32 --source-group SG same on both
Latest AMI	aws ssm get-parameter --name /aws/service/ami-amazon-linux-latest/al2023-ami-kernel-default-x86_64	aws ssm get-parameter --name /aws/service/canonical/ubuntu/server/24.04/stable/current/amd64/hvm/ebs-gp3/ami-id
Launch	aws ec2 run-instances --image-id $AMI --instance-type t3.micro --key-name K --subnet-id … --security-group-ids … --user-data file://web.sh same on both
Wait & find	aws ec2 wait instance-running --instance-ids $ID describe-instances --query 'Reservations[].Instances[].PublicIpAddress' same on both
Log in	ssh -i key.pem ec2-user@IP (Amazon Linux)	ssh -i key.pem ubuntu@IP
First-boot log	tail /var/log/cloud-init-output.log same on both
Lifecycle	stop-instances (new IP on start) · start-instances · terminate-instances (gone) same on both

AWS: S3

Task	Rocky / RHEL	Ubuntu / Debian
Buckets & files	aws s3 mb s3://UNIQUE-NAME aws s3 cp FILE s3://B/ aws s3 ls s3://B --recursive aws s3 sync DIR s3://B same on both
Delete	aws s3 rm s3://B/KEY aws s3 rb s3://B --force same on both
Website	aws s3 website s3://B --index-document index.html --error-document error.html → http://B.s3-website-REGION.amazonaws.com same on both
Go public (careful!)	aws s3api put-public-access-block … BlockPublicPolicy=false,RestrictPublicBuckets=false put-bucket-policy --policy file://p.json same on both
Share one file	aws s3 presign s3://B/KEY --expires-in 300 same on both
Versions	aws s3api put-bucket-versioning --bucket B --versioning-configuration Status=Enabled list-object-versions same on both

AWS: scale, DNS, alarms

Task	Rocky / RHEL	Ubuntu / Debian
Load balancer	elbv2 create-target-group · register-targets · create-load-balancer --subnets A B · create-listener --default-actions Type=forward,TargetGroupArn=… same on both
Why unhealthy?	aws elbv2 describe-target-health --target-group-arn $TG (Timeout = security group) same on both
Auto Scaling	create-launch-template (UserData = base64 -w0) · autoscaling create-auto-scaling-group --min-size --max-size --desired-capacity same on both
Scale on CPU	put-scaling-policy --policy-type TargetTrackingScaling … 'TargetValue':50 describe-scaling-activities same on both
DNS records	aws route53 change-resource-record-sets --hosted-zone-id Z --change-batch file://c.json (CREATE · UPSERT · DELETE) same on both
Check DNS	dig +short NAME dig NS DOMAIN dig @ns-X.awsdns-Y.com NAME same on both
Alarm → email	sns create-topic · sns subscribe --protocol email · cloudwatch put-metric-alarm … --alarm-actions TOPIC-ARN same on both
Guard the bill	billing alarm in us-east-1 (AWS/Billing EstimatedCharges) aws budgets create-budget same on both

AWS: costs & clean-up

Task	Rocky / RHEL	Ubuntu / Debian
This month	aws ce get-cost-and-usage --time-period Start=…,End=… --granularity MONTHLY --metrics UnblendedCost --group-by Type=DIMENSION,Key=SERVICE same on both
Find by tag	aws resourcegroupstaggingapi get-resources --tag-filters Key=Project,Values=NAME same on both
Every region	for r in $(aws ec2 describe-regions --query 'Regions[].RegionName' --output text); do aws ec2 describe-instances --region $r …; done same on both
Usual leftovers	describe-addresses 'Addresses[?AssociationId==null]' · describe-volumes status=available · load balancers · hosted zones · buckets same on both
Delete order	ASGs → instances → load balancers → target groups → IPs/volumes → buckets → DNS → network same on both

Git

Task	Rocky / RHEL	Ubuntu / Debian
Install	sudo dnf install git	(preinstalled) sudo apt install git
Who am I	git config --global user.name "Name" user.email you@example.com same on both
Start / copy a repo	git init git clone URL same on both
Save a snapshot	git status git add FILE git commit -m "message" git commit -am "…" same on both
Look back	git log --oneline git diff git diff --staged git show HASH same on both
Undo	git restore FILE git restore --staged FILE git restore --source=HASH FILE same on both
Branches	git branch git switch -c NAME git switch main git merge NAME same on both
Conflict	edit the file (remove <<< === >>>) → git add FILE → git commit same on both
Share	git remote add origin URL git push -u origin main git pull same on both
Never commit	passwords & keys → list them in .gitignore same on both

Python & AI

Task	Rocky / RHEL	Ubuntu / Debian
Python tools	sudo dnf install python3-pip	sudo apt install python3-venv python3-pip
Make & use a venv	python3 -m venv ~/ai-env source ~/ai-env/bin/activate deactivate same on both
Install Ollama	curl -fsSL https://ollama.com/install.sh | sh same on both
Chat with a model	ollama run llama3.2 (/bye to quit) same on both
Models: list / remove	ollama list ollama rm NAME same on both

Help!

Task	Rocky / RHEL	Ubuntu / Debian
Manual page	man COMMAND (q to quit) same on both
Quick help	COMMAND --help same on both
Which command does…?	man -k WORD apropos WORD same on both
Cancel / clear screen	Ctrl+C clear (or Ctrl+L) same on both
Previous commands	↑ history same on both
Auto-complete	Tab same on both

Attack surface

Task	Rocky / RHEL	Ubuntu / Debian
What's listening (0.0.0.0 = everyone)	sudo ss -tlnp same on both
Which services run	systemctl list-units --type=service --state=running same on both
Accounts that can log in	getent passwd | awk -F: '$7 !~ /(nologin|false)$/ {print $1}' same on both
Who is an admin	getent group wheel	getent group sudo
Turn a service off for good	sudo systemctl disable --now NAME same on both
Lock an account (keep it for now)	sudo usermod -L NAME sudo usermod -s /sbin/nologin NAME sudo gpasswd -d NAME wheel	sudo usermod -L NAME sudo usermod -s /sbin/nologin NAME sudo gpasswd -d NAME sudo

SSH hardening

Task	Rocky / RHEL	Ubuntu / Debian
Where the SSH server settings live	/etc/ssh/sshd_config /etc/ssh/sshd_config.d/*.conf (read first; first value wins) same on both
Give a key a passphrase (new or existing)	ssh-keygen -t ed25519 ssh-keygen -p -f ~/.ssh/id_ed25519 same on both
Unlock the key once per session	ssh-add ssh-add -l same on both
Keys only, no root login	printf 'PasswordAuthentication no\nPermitRootLogin no\n' | sudo tee /etc/ssh/sshd_config.d/00-hardening.conf same on both
Test the config (silence = OK)	sudo sshd -t same on both
What sshd really uses	sudo sshd -T | grep -iE 'passwordauth|permitroot' same on both
Apply it	sudo systemctl reload sshd	sudo systemctl reload ssh
Prove passwords are off (from the laptop)	ssh -o PubkeyAuthentication=no user@server (must be refused) same on both
Only these users may log in	AllowUsers student maria (in the drop-in) same on both
The drop-in that overrides you	01-permitrootlogin.conf (PermitRootLogin yes)	50-cloud-init.conf (PasswordAuthentication yes)

Firewalls in depth

Task	Rocky / RHEL	Ubuntu / Debian
See the rules	sudo firewall-cmd --get-active-zones sudo firewall-cmd --list-all	sudo ufw status verbose sudo ufw status numbered
Refuse everything by default	(the public zone already does)	sudo ufw default deny incoming
SSH only from your network	sudo firewall-cmd --permanent --zone=internal --add-source=192.168.1.0/24 sudo firewall-cmd --permanent --zone=public --remove-service=ssh sudo firewall-cmd --reload	sudo ufw allow from 192.168.1.0/24 to any port 22
Block one address	sudo firewall-cmd --permanent --add-rich-rule='rule family="ipv4" source address="203.0.113.45" drop'	sudo ufw insert 1 deny from 203.0.113.45
Remove a rule	--remove-service=NAME --remove-rich-rule='…'	sudo ufw delete NUMBER
Save what's running now	sudo firewall-cmd --runtime-to-permanent	(ufw saves every change)
Rate-limit SSH	rich rule: rule service name="ssh" limit value="3/m" accept	sudo ufw limit OpenSSH
Log what's blocked	sudo firewall-cmd --set-log-denied=all sudo journalctl -k | grep REJECT	sudo ufw logging on sudo journalctl -k | grep 'UFW BLOCK'