Linux for Developers

One tree, no drive letters

Everything hangs under a single root /. Paths use forward slashes and are case-sensitive: App.log and app.log are different files.

Path	Contents
/home/<user>	Home directories (~ is a shortcut for yours)
/etc	System-wide configuration
/var/log	Log files
/usr/bin, /bin	Programs
/tmp	Temporary files, may be deleted at any time
/opt	Optionally installed software
/proc, /dev	Virtual files for processes and devices

Users and permissions

Every file has an owner, a group and permission bits for owner / group / others: read (r, 4), write (w, 2) and execute (x, 1).

ls -l deploy.sh          # -rwxr-xr-- 1 anna dev 220 ... deploy.sh
chmod +x deploy.sh       # make it executable
chmod 640 secrets.conf   # owner rw, group r, others nothing
chown anna:dev file      # change owner and group (needs privileges)
sudo systemctl restart nginx   # run one command as root

Avoid running everything as root, and avoid chmod 777, which gives everyone full access.

Processes and services

ps aux | grep myapp      # find a process
top                      # live view of CPU and memory (htop if installed)
kill <pid>               # ask a process to stop (kill -9 forces it)
systemctl status myapp   # state of a service
journalctl -u myapp -f   # follow a service's logs

Most distributions use systemd to start and supervise services. Logs are in /var/log or in the journal.

Packages and getting help

Install software with the distribution’s package manager (apt on Debian/Ubuntu, dnf on Fedora/RHEL). Use man <command> or <command> --help for documentation.

Try it

Open a Linux shell (a WSL distribution works). Create a script, run it, then fix the “Permission denied” error with chmod. Find your five busiest processes with top and read the last 20 lines of a file in /var/log with tail -n 20.