Linux commands

How to read a command line#

Every call has three parts: the program name, options and arguments. Options come short (-l) and long (--long); short ones can be combined, so ls -l -a and ls -la are the same thing. Arguments are what you work on: paths, process names, addresses.

Two commands help when you forget an option:

ls --help          # a short summary right in the terminal
man ls             # the full manual, quit with q

One more habit saves hours: Tab completes file and command names, the up arrow brings back the previous command, Ctrl+C stops a stuck program and Ctrl+D ends input.

You do not always need superuser rights. The rule is simple: your own home directory — no sudo; system directories and services — with sudo. If a command from this cheat sheet answers Permission denied, that is your hint.

Files and directories#

Command	What it does	Example
pwd	prints the current directory	pwd
ls	lists a directory's contents	ls -lh /var/log
cd	changes directory	cd ~/projects
mkdir	creates a directory	mkdir -p src/api/v1
cp	copies a file or a tree	cp -r src backup
mv	moves and renames	mv notes.txt archive/
rm	deletes	rm -r build
touch	creates an empty file	touch README.md
cat	prints a whole file	cat /etc/hostname
less	pages through a file	less /var/log/syslog
head / tail	first and last lines	tail -n 50 app.log
tail -f	follows lines as they are appended	tail -f app.log

-p on mkdir creates the whole chain of directories at once and stays quiet if the directory already exists. -h on ls prints sizes in kilobytes and megabytes instead of raw bytes. rm has no undo: there is no trash can in the terminal, so before rm -r it pays to run ls on the same path.

Three path shortcuts save typing: ~ is your home directory, . the current one, .. the parent. cd - takes you back to where you were before the last change.

Permissions#

Every file has three sets of permissions — for the owner, the group and everyone else — and each set has three bits: read (4), write (2), execute (1). That is where the familiar numbers come from: 7 = 4+2+1 (everything), 6 = 4+2 (read and write), 4 — read only.

Command	What it does	Example
chmod	changes permissions	chmod 644 notes.txt
chmod +x	makes a file executable	chmod +x deploy.sh
chown	changes owner and group	sudo chown www-data:www-data site
ls -l	shows permissions as a string	ls -l notes.txt
id	prints your uid, gid and groups	id
sudo	runs a command as the superuser	sudo systemctl restart nginx

chmod 644 is rw-r--r--: the owner reads and writes, everyone else only reads. For a directory, the execute bit means the right to enter it, which is why directories usually get 755, not 644. You almost never need 777: if it "fixes" a problem, the real cause is the wrong owner.

Processes#

Command	What it does	Example
ps aux	a snapshot of every process	ps aux | grep nginx
top	a live table of load	top
kill	sends a process a termination signal	kill 4821
kill -9	kills a process unconditionally	kill -9 4821
pkill	kills by name	pkill -f "python app.py"
jobs / fg / bg	jobs in the current shell	bg %1
systemctl status	the state of a service	systemctl status nginx
journalctl	service logs	journalctl -u nginx -n 100

A plain kill asks a process to close cleanly — finish writing a file, release a port. kill -9 gives it no such chance, so it is the last resort, not the first. To run a long program in the background, put & at the end of the line; to bring it back to the foreground, use fg.

Networking#

Command	What it does	Example
ip a	addresses of network interfaces	ip a
ping	checks whether a host is reachable	ping -c 4 example.com
curl	makes an HTTP request from the terminal	curl -i https://example.com
wget	downloads a file	wget https://example.com/file.zip
ss -tulpn	open ports and who holds them	sudo ss -tulpn
dig	asks DNS about a name	dig example.com
ssh	logs in to a remote machine	ssh user@10.0.0.5
scp	copies a file over ssh	scp dump.sql user@10.0.0.5:/tmp/

-c 4 on ping stops after four packets — otherwise it runs forever. ss -tulpn with sudo shows the name of the process that took a port: it is the first command to run when a service says "address already in use". What curl returns and how to read status codes is covered in what is an API.

Packages and disk space#

Command	Distributions	Example
apt	Debian, Ubuntu	sudo apt update && sudo apt install git
apt remove	Debian, Ubuntu	sudo apt remove git
dnf	Fedora, RHEL	sudo dnf install git
pacman	Arch	sudo pacman -S git
apt list --installed	Debian, Ubuntu	apt list --installed | grep python
df / du	everywhere	df -h and du -sh ~/Downloads

apt update refreshes the list of available packages, apt upgrade upgrades the packages themselves; mixing them up is a typical first-week mistake. df -h shows free space per partition, du -sh folder shows how much one folder takes. When the disk is full, this pair finds the culprit in two calls.

Finding files and text#

Command	What it does	Example
find	finds files by name, size, date	find . -name "*.log" -mtime -1
grep	finds a string inside files	grep -rn "TODO" src/
grep -i	case-insensitive search	grep -i error app.log
which	shows the path to a program	which python3
wc -l	counts lines	wc -l access.log
sort / uniq	sorting and collapsing duplicates	sort ips.txt | uniq -c

For grep, -r means "go into every subdirectory" and -n means "show line numbers". The search pattern is a regular expression, and knowing the syntax turns grep into a tool of its own: classes, quantifiers and anchors are explained in the article on regular expressions.

A special strength of the terminal is the pipe |: the output of one command becomes the input of the next. A classic chain for analysing a log looks like this:

grep "500" access.log | awk '{print $1}' | sort | uniq -c | sort -rn | head

Step by step: pick the lines with status 500, take the first field (the address) from each, group identical ones, count them, sort in descending order and show the top of the list.

Common mistakes#

A space in a path without quotes. cd My files tries to go to a directory called "My". The right way is cd "My files".

rm -rf with a variable. If the variable turns out to be empty, the path $DIR/ becomes just /. Check the value with echo before deleting.

Copying a directory without -r. cp src dst on a directory answers omitting directory; a tree needs cp -r.

Editing a system file without a backup. Before sudo nano /etc/…, run sudo cp /etc/file /etc/file.bak.

It is safer to practise commands in a throwaway container than on your working system: any mistake there is undone by a restart — see what is Docker. For a full study plan, see learn Linux from scratch.

Step-by-step plan

1. Learn to move aroundWalk the directory tree with pwd, ls -la and cd without using a file manager.
2. Understand permissionsCreate a file, look at ls -l, change the permissions to 600 and 755 and explain every letter in the output.
3. Find a process and a portStart a long-running program, find it with ps aux, stop it with kill, check the port with ss -tulpn.
4. Build a pipelinePick lines from a log with grep, count them with wc -l and group them with sort | uniq -c.
5. Write your own cheat sheetList the twenty commands you actually used this week, with an example for each.

Check yourself

Sources

• GNU Coreutils manualThe primary source for ls, cp, sort and the other basic commands
  free
• Ubuntu: The Linux command line for beginnersA short official tutorial with exercises
  free
• The Linux Command Line (William Shotts)A free book that goes from the first command to shell scripts
  free

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