🐧 Linux Command Cheat Sheet

Everything from today's workshop in one place. Keep it open during the recap mission. The mission instructions are at the bottom.

Paths in 10 seconds

• / is the top of the filesystem. ~ is your home folder.
• . is the current folder. .. is the folder above it.
• An absolute path starts with / (for example, /home/alice/notes.txt) and works from anywhere.
• A relative path starts from where you are now (for example, notes.txt or ../notes.txt).

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1. Navigating Filesystems

Command	What it does
pwd	Print the full path of the folder you're in
ls	List files and folders
ls -l	Long listing: permissions, owner, size, date
ls -a	Show all files, including hidden ones (names starting with .)
ls -la	Both at once. Probably the listing you'll use most
ls -lh	Sizes in human units (K, M, G)
ls <folder>	List another folder without going into it
cd <folder>	Go into a folder
cd ..	Go up one level
cd ~ or cd	Go to your home folder
cd -	Go back to the folder you were just in
cd /	Go to the top of the filesystem
tree	Show folders and files as a tree (if it's installed)

Reading ls -l: -rwxr-xr-x means: the first character is - for a file or d for a directory. After that come read/write/execute permissions for the owner, the group, and everyone else. An x means the file can be run.

2. Text Display

Command	What it does
cat <file>	Print the whole file
cat -n <file>	Print it with line numbers
head <file>	First 10 lines
head -n 5 <file>	First 5 lines
tail <file>	Last 10 lines
tail -n 5 <file>	Last 5 lines
tail -f <file>	Keep printing new lines as they're added, e.g. a live log. Stop with Ctrl+C
less <file>	Scroll through a big file. Use the arrow keys or Space, /word to search, and q to quit
echo "text"	Print text
echo $USER / echo $HOME	Print a variable
clear	Clear the screen (or Ctrl+L)

💡 Don't cat a file with thousands of lines. Use head, tail, less or grep instead.

3. File Management

Command	What it does
touch <file>	Create an empty file, or update the timestamp of an existing one
mkdir <folder>	Create a folder
mkdir -p a/b/c	Create nested folders in one go
cp <src> <dest>	Copy a file (the original stays)
cp -r <src> <dest>	Copy a folder and everything in it
mv <src> <dest>	Move a file. The original is gone afterwards
mv old.txt new.txt	Rename: moving within the same folder
mv dir/old.txt other/new.txt	Move and rename in one command
rm <file>	Delete a file. ⚠️ There is no recycle bin
rm -r <folder>	Delete a folder and everything in it. ⚠️ Double-check first
rmdir <folder>	Delete an empty folder
echo "text" > file	Write text into a file. ⚠️ Overwrites what was there
echo "text" >> file	Append text to the end of a file
command > file	Save any command's output to a file, e.g. ls -la > listing.txt

4. Searching

Command	What it does
grep word <file>	Show lines containing word. Case-sensitive: Error ≠ error
grep -i word <file>	Ignore upper/lower case
grep -n word <file>	Also show line numbers
grep -c word <file>	Count the matching lines
grep -v word <file>	Show lines that don't match
grep -r word <folder>	Search every file in a folder
wc <file>	Count lines, words and characters
wc -l <file>	Count lines only
wc -w <file>	Count words only
find . -name "*.txt"	Find files by name under the current folder

Pipes |: chaining commands

A pipe sends one command's output into the next command:

ls | wc -l                    # how many files are here?
cat notes.txt | grep todo     # lines in notes.txt that contain "todo"
grep -i warn app.log | head -n 3   # the first 3 warnings

5. Running Scripts

Command	What it does
python3 script.py	Run a Python script
bash script.sh	Run a shell script
./script	Run an executable script in the current folder. The ./ tells the shell where to look
./script arg1 arg2	Give the script values (arguments)
./script 'a {tricky} value'	Quote arguments that contain spaces or symbols
chmod +x script	Make a script executable, if you get "Permission denied"

Handy extras

Command / key	What it does
Tab	Auto-complete file and folder names. Use it constantly!
↑ / ↓	Scroll through the commands you've typed before
Ctrl+C	Stop the running command
history	List your previous commands
whoami	Print your username
man <command>	The manual for a command (press q to quit)
<command> --help	A quick summary of a command's options

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🕵️ The Recap Mission

Your mission is to find 4 secret fragments hidden in a set of folders and files, then combine them into a flag. You are being timed. The clock starts when you run the start script and stops when you submit the correct flag.

Start:

cd ~/linux-workshop-mission      # the folder you cloned, where start_mission.py is
python3 start_mission.py

Only if your instructor gives you a server address: before python3 start_mission.py, run export MISSION_SERVER=http:// followed by that address, e.g. export MISSION_SERVER=http://10.0.0.5:8000. This creates a different folder, ~/linux_mission (with an underscore). That's where the mission happens: go in there and read the README.

Submit (from ~/linux_mission, keeping the single quotes):

./submit.py 'FLAG{fragment1-fragment2-fragment3-fragment4}'

Stuck?

• Read every file the mission gives you. All the instructions are in there.
• Every script prints a hint when something's wrong. Read what it says!
• Broke something? Run python3 start_mission.py again (from ~/linux-workshop-mission). It rebuilds your folder from the start, so you'll need to open the vault again. Your timer doesn't reset, and your flag stays the same.
• Don't delete ~/.linux_mission_token. It proves to the server that you are you.
• Still stuck? Put your hand up.

Good luck, agent. 🐧