Linux cheat sheet

One page. Details live in README.md.

Who am I

Command	What it answers
`whoami`	Your login name
`id`	uid, gid, groups
`sudo -l`	Commands sudo will run for you
`uname -a`	Kernel and arch

Command	Notes
`ls -l`	type + `rwx` triad + owner + group
`chmod 600 file`	owner rw; group/other none
`chmod 755 dir`	owner rwx; others rx (enter the directory)
`chown user:group file`	needs root for another user
`mkdir -p path`	create parents
`df -h`	how full is each mount
`du -xh -d 1 DIR`	which child of DIR is large

Modes you will actually use: 644 files, 755 dirs and scripts, 600 secrets.

Command	Notes
`ps aux`	every process; `USER PID … COMMAND`
`ps -u "$USER"`	only yours
`kill PID`	SIGTERM (ask)
`kill -9 PID`	SIGKILL (last resort)
`systemctl status NAME`	running / failed / enabled
`systemctl start\|stop\|restart NAME`	now
`systemctl enable --now NAME`	now and on boot

Command	Notes
`ip addr`	interfaces and addresses
`ss -tuln`	listening TCP/UDP, numeric
`ss -tulpn`	same, with process (needs sudo for other users)
`journalctl -n 20 --no-pager`	last 20 journal lines
`journalctl -u NAME -e --no-pager`	one unit, jump to end
`ls /var/log`	classic log files

sudo apt update
sudo apt install -y PACKAGE
apt show PACKAGE

Fedora/RHEL: sudo dnf install -y PACKAGE.

sudo ufw allow 22/tcp
sudo ufw allow 80/tcp
sudo ufw status