Linux files and directories cheat sheet

The chapter on one page: what to remember, the commands and patterns to have at your fingertips, and flashcards for quick revision.

The chapter in eight sentences

Linux keeps every file in one tree under /: configuration in /etc, users' files in /home, software in /usr, variable data in /var, runtime data in /run, and scratch space in /tmp.
An absolute path starts at /; a relative path starts from the current directory, and ., .. and ~ stand for this directory, its parent and your home.
pwd shows where you are, cd moves you (cd alone goes home, cd - goes back), and ls -la lists everything, including hidden dot files.
mkdir -p creates whole paths; cp (with -r for directories) copies, mv moves and renames, and both put files inside a destination that is an existing directory.
rm (-r for directories, -i to confirm) removes for good: there is no wastebasket. rmdir removes only empty directories.
A hard link is a second name for the same inode; a symbolic link is a small file that points to a name, can reach directories and other file systems, and dangles if its target goes.
Bash expands patterns (*, ?, […]), braces ({a,b}, {1..9}), ~, $VARIABLES and $(commands) before a command runs; echo shows the result.
Double quotes keep spaces and patterns but still expand $; single quotes and backslashes stop expansion altogether.

Command	Does
`pwd`	Print the current directory
`cd DIR`, `cd`, `cd -`, `cd ..`	Go to DIR / home / back / up one level
`ls -l`, `-a`, `-h`, `-R`, `-d`, `-t`, `-i`	Long, all, human sizes, recursive, the directory itself, newest first, inode numbers
`touch FILE`	Create an empty file, or update its time stamp

Command	Does
`mkdir DIR…`, `mkdir -p A/B/C`	Create directories; with all missing parents
`cp SRC DST`, `cp -r DIR DST`	Copy files; directories need `-r`
`cp -p`, `-i`, `-v`	Keep times and permissions; ask before overwriting; report
`mv SRC DST`	Move, or rename
`rm FILE`, `rm -r DIR`, `rm -i`	Remove files; directories; ask first
`rmdir DIR`	Remove an empty directory
`ln FILE NAME` / `ln -s TARGET NAME`	Hard link / symbolic link
`ln -sfn TARGET NAME`	Repoint an existing symbolic link
`readlink -f NAME`	Where a link finally leads

Write	Expands to
`*` `?` `[abc]` `[!abc]` `[[:digit:]]`	Matching file names (a pattern that matches nothing stays as written)
`{a,b,c}` `{1..10}` `{01..10}` `{a..e}`	Generated words, files or not
`~`, `~user`	Home directories
`$VAR`, `${VAR}`	A variable's value
`$(command)`	A command's output
`"…"` / `'…'` / `\x`	Only `$` expands / nothing expands / take the next character literally

Where does system-wide configuration live?: `/etc`, almost all as text files.
Which directory is emptied at every boot?: `/run`, which holds runtime data for running programs.
What makes a path absolute?: It starts with `/`. Anything else is relative to the current directory.
What do `.`, `..` and `~` mean in a path?: This directory, its parent, and your home directory.
Go back to the previous directory?: `cd -`
Create a/b/c when none of them exist?: `mkdir -p a/b/c`
Copy a directory and everything in it?: `cp -r SRC DST`
Rename report.txt to final.txt?: `mv report.txt final.txt`
What does a link count of 2 mean?: The file has **two names** (hard links) for the same inode.
Two things a symbolic link can do that a hard link cannot?: Point to a **directory**, and point **across file systems**.
What happens to a symbolic link when its target is deleted?: It **dangles**: it still exists, but opening it fails.
Why doesn't `*` match `.bashrc`?: Patterns don't match a leading dot unless they start with one, such as `.*`.
What does Bash do with a pattern that matches nothing?: Passes it on **unchanged** to the command.
Double quotes or single quotes to keep `$HOME` literal?: **Single** quotes. Double quotes still expand ` Linux files and directories Cheat Sheet (RHCSA) .