Reference · Command Anatomy & Navigation

The shape of every command

lsCommand — what to run -lOption — how to run it logsArgument — what to run it on

The shell splits the line on spaces. First word = command. Everything after = handed to it.

Short option: one dash, one letter — -l. Bundles: -la is -l -a.

Long option: two dashes, a word — --color. GNU/Linux convention; often absent on macOS.

Options usually precede arguments, though most commands tolerate either order.

Looking things up

Move	When to use it
man cmd	The offline manual. `Space` pages down, `/word` searches, `q` quits.
cmd --help	Quick summary. Works on Linux/GNU tools; often fails on macOS.
which cmd	Shows which file actually runs when you type that name.
type cmd	Like `which`, but also reveals shell builtins and aliases.
explainshell.com	Paste a whole command; it labels every part. Best tool for decoding someone else's script.

Move

When to use it

man cmd

The offline manual. Space pages down, /word searches, q quits.

cmd --help

Quick summary. Works on Linux/GNU tools; often fails on macOS.

which cmd

Shows which file actually runs when you type that name.

type cmd

Like which, but also reveals shell builtins and aliases.

explainshell.com

Paste a whole command; it labels every part. Best tool for decoding someone else's script.

Paths

Form	Means
logs/app.log	Relative — starts from your current directory.
/var/www/html	Absolute — starts at the top of the disk. Same everywhere. What scripts use.
~	Your home folder. `~/Desktop` works from anywhere.
.	The current directory.
..	The parent directory. `cd ..` goes up one level.
-	With `cd` only: the directory you were in before. A fast toggle.

Form

Means

logs/app.log

Relative — starts from your current directory.

/var/www/html

Absolute — starts at the top of the disk. Same everywhere. What scripts use.

Your home folder. ~/Desktop works from anywhere.

The current directory.

The parent directory. cd .. goes up one level.

With cd only: the directory you were in before. A fast toggle.

Getting around

Command	Does
pwd	Print working directory — where am I?
ls	List contents. `-l` long form, `-a` include hidden, `-h` human-readable sizes.
cd dir	Change directory. Bare `cd` returns home.
cat file	Dump a whole file to the screen. Fine for short files.
less file	Page through a long file. Same keys as `man`; `q` quits.
head / tail file	First / last 10 lines. `tail -f` follows a log live.
grep pat file	Print lines matching a pattern. `-c` counts, `-i` ignores case, `-n` shows line numbers.

Command

Does

pwd

Print working directory — where am I?

List contents. -l long form, -a include hidden, -h human-readable sizes.

cd dir

Change directory. Bare cd returns home.

cat file

Dump a whole file to the screen. Fine for short files.

less file

Page through a long file. Same keys as man; q quits.

head / tail file

First / last 10 lines. tail -f follows a log live.

grep pat file

Print lines matching a pattern. -c counts, -i ignores case, -n shows line numbers.

Terminal survival keys

Key	Does
Ctrl-C	Cancel the running command. The universal escape hatch.
Ctrl-D	End of input / exit the shell.
Ctrl-L	Clear the screen.
Ctrl-A / Ctrl-E	Jump to start / end of the line.
Tab	Autocomplete a name. Press twice to list options. Use constantly — it prevents typos.
Up arrow	Previous command. `history` lists them all.
q	Quit `man` and `less`. Worth burning into memory.

Key

Does

Ctrl-C

Cancel the running command. The universal escape hatch.

Ctrl-D

End of input / exit the shell.

Ctrl-L

Clear the screen.

Ctrl-A / Ctrl-E

Jump to start / end of the line.

Tab

Autocomplete a name. Press twice to list options. Use constantly — it prevents typos.

Up arrow

Previous command. history lists them all.

Quit man and less. Worth burning into memory.

macOS vs Linux — where copied commands break

Verified on this machine

Your Mac's tools come from BSD; most servers use GNU. Same names, different flags.

Thing	Your Mac	Linux server
default shell	zsh	usually bash
bash version	3.2.57 (2007)	5.x
ls --help	fails	works
sed -i	needs an argument: `sed -i ''`	`sed -i` alone
declare -A, mapfile, **	unavailable (bash 3.2)	available

Thing

Your Mac

Linux server

default shell

zsh

usually bash

bash version

3.2.57 (2007)

5.x

ls --help

fails

works

sed -i

needs an argument: sed -i ''

sed -i alone

declare -A, mapfile, **

unavailable (bash 3.2)

available

To get a modern bash locally: brew install bash. Not needed yet.

Before you press Enter

Destructive — no undo, no trash

rm deletes permanently. There is no recycle bin. rm -rf dir removes a folder and everything inside without asking.

Habits that save you: run ls on a path before you rm it; use rm -i to be prompted; never paste an rm, sudo, >, or curl … | bash line you can't read aloud. > silently overwrites a file — it is quietly destructive too.