Levels 1–4: Beginner

Commands you will type every single day.

Command	What it does	Why it matters
`ls`	List directory contents	See what’s in a folder
`ls -l`	Long listing with permissions, owner, size	Inspect file details
`ls -a`	Show hidden files (dotfiles)	Config files are hidden by default
`pwd`	Print Working Directory	Know exactly where you are
`cd dir`	Change into a directory	Navigate the filesystem
`cd ~` or `cd`	Go to home directory	Quick return to your personal space
`mkdir name`	Create a directory	Build your workspace
`rmdir name`	Remove an empty directory	Clean up empty dirs

Key concept: The Linux filesystem is a tree rooted at /. Your home directory is /home/username (or /Users/username on macOS). ~ is a shell shortcut for it.

Level 2: File Operations

Create, read, copy, move, delete: and archive.

Command	What it does	Why it matters
`cat file`	Print file contents	Read small files instantly
`head -n N file`	Show first N lines	Preview large files
`tail -n N file`	Show last N lines	Check recent log entries
`touch file`	Create empty file / update timestamp	Create files without opening an editor
`cp src dst`	Copy a file	Backup before editing
`mv src dst`	Move or rename	Rename is just a move to the same directory
`rm file`	Delete a file (permanent, no undo)	Clean up: no recycle bin
`rm -r dir`	Delete directory and all contents	Remove non-empty directories
`tar -czf out.tar.gz dir/`	Create compressed archive	Bundle files for transfer or backup
`tar -tzf file.tar.gz`	List archive contents	Preview without extracting
`tar -xzf file.tar.gz`	Extract archive	Unpack in current directory
`tar -xzf file.tar.gz -C /target/`	Extract to specific directory	Control where files land

tar flags:

Flag	Meaning
`c`	Create archive
`x`	Extract archive
`t`	List contents
`z`	gzip compression
`j`	bzip2 compression
`J`	xz compression
`f`	Next argument is the filename

Level 3: Text & Search

Find what you need in files and directories.

Command	What it does	Why it matters
`grep pattern file`	Search for pattern in file	Filter log files, find config values
`grep -i pattern file`	Case-insensitive search	When you’re not sure of capitalisation
`grep -v pattern file`	Invert: show lines that do NOT match	Exclude noise from output
`grep -r pattern .`	Recursive search	Search all files in a directory tree
`find . -name '*.txt'`	Find files by name pattern	Locate files when you don’t know where they are
`find . -type d`	Find directories only	Browse just the directory structure
`wc -l file`	Count lines	How many records in a file?
`sort file`	Sort lines alphabetically	Order data for reading or for `uniq`
`uniq file`	Remove adjacent duplicates	Must sort first: uniq only removes consecutive duplicates
`cmd1 \| cmd2`	Pipe: send output of cmd1 to cmd2	Chain tools together: the Unix superpower

Pipe example:

grep ERROR app.log | sort | uniq -c | sort -rn

This finds ERROR lines, sorts them, counts each unique one, then sorts by frequency: showing the most common errors first.

Level 4: Permissions & Users

Every file has an owner. Every access is controlled.

Permission String

-rwxr-xr--
│├┤├─┤├─┤
│││ │  │
│││ │  └── Others: r-- = read only (4)
│││ └───── Group:  r-x = read + execute (5)
│└┤──────── Owner:  rwx = read + write + execute (7)
└────────── File type: - = file, d = directory, l = symlink

Command	What it does
`whoami`	Print current username
`id`	Print UID, GID, and all group memberships
`ls -l`	Show file permissions in long format
`chmod 755 file`	Set permissions numerically (rwxr-xr-x)
`chmod +x file`	Add execute permission for all
`chmod 600 file`	Owner read/write only (private keys)
`chown user:group file`	Change file owner and group

Octal permission values:

Value	Permission
`4`	Read (r)
`2`	Write (w)
`1`	Execute (x)
`7`	rwx (4+2+1)
`6`	rw- (4+2)
`5`	r-x (4+1)
`4`	r– (4)
`0`	— (none)