STUDY VAULT

Mohamed Attia
CS undergrad · visual study notes
Mohamedattiadev

Chapter 1 — Command Line Basics

7 min read

Time to study: ~45 min You will learn: echo, the difference between CLI and GUI, the terminal words everyone mixes up, how to make variables, the two kinds of quotes, and the Tab key that makes all of it faster.

← back to Terminal 101 | next: Chapter 2 →

1. the very basic command u can ever use

echo "Hello World"

this is basically the same as writing “Hello World” in a text editor, it prints Hello World to the terminal.

Hello World

2. why do i really need to learn the CLI

to get the idea of how to use the terminal we should first understand what is the CLI and what is the difference between the CLI and the GUI.

GUI (Graphical User Interface): we all already know what it is. when u open any application u have a window opened with a navbar, buttons and a scrolling bar, which is something u can navigate with ur mouse.

on the other hand,

CLI (Command Line Interface): is a way to interact with the computer, it is a text based interface, we can type commands in the terminal and execute them.

  • example: let’s say we have a python file we need to run. with the GUI we open vscode → we click the run button → the terminal opens and done.

if we want to do the same work with the terminal, we cd to the directory where the py file is located → then we type python filename.py → then Enter and we are done.

note: the cd command is used to change the directory, we will learn it in Chapter 3.

this is the main difference between the CLI and the GUI. the CLI is a text based interface where we type commands and execute them, the GUI is graphic based and can be navigated with the mouse.

3. some words that need to be differentiated

  • Terminal: the first time the word Terminal was used, it was for a physical device (a too old device). it was like a computer that had its own keyboard u can use to interact with it, and on the screen the very familiar black terminal screen with its white command line. but nowadays we use the word Terminal to refer to the terminal emulator programs.

  • Terminal Emulator: is basically a software program that emulates the behaviour of the old physical terminal device. the most known ones are Alacritty and ghostty. it can accept commands and render text on the screen.

  • Shell: is what happens behind the scenes. when we write a command in the terminal emulator, the shell interprets it and executes it then returns the result. it works with a very basic idea called REPL which is Read → Eval → Print → Loop, too ez. examples of shells: Bash, Zsh and Fish.

and this is the right moment to say it, because the next section is where it starts to matter: there is more than one shell, and they do not all speak the same language. this course teaches bash.

echo $SHELL
/usr/bin/fish

if that does not say bash, type bash and press Enter before u continue. u are now inside a bash session and everything in this course works as written. exit puts u back to ur normal shell.

  • i use fish myself, and in fish the very next section already fails. name="ati" gives u fish: Unsupported use of '='. so this is not something to skip.
  • the full explanation is in the course README, under “before u start”.

4. how to write variables in the terminal

in python we can use = to assign a value to a variable, the same in the terminal.

name="ati"
echo $name
ati

so we assign the value with = and we read it back by putting $ in front of the name.

  • note: no space before or after the =. name = "ati" is wrong, the right one is name="ati".

5. the two kinds of quotes

u just saw that $name gets replaced by its value. so now there is one thing u need to know before it bites u, because it will.

double quotes let the $ work. single quotes do not.

name="ati"
echo "hello $name"
hello ati

echo 'hello $name'
hello $name

look at the second one. with the single quotes nothing was replaced, u got $name back exactly as u typed it. with the double quotes u got the value.

  • so the rule is short: double quotes when u want the variable, single quotes when u want the text itself.

it looks like a small detail now. it is not, and u will meet it again in almost every chapter after this one.

and one more thing about quotes while we are here. if a file name has a space in it, u have to quote it, otherwise the terminal thinks u are talking about 2 different things:

touch my file.txt
ls
file.txt  my

that made 2 files, my and file.txt, which is not what we wanted. so:

touch "my file.txt"
ls
my file.txt
  • this is also the reason u will see developers avoid spaces in file names completely and write my-file.txt or my_file.txt instead. it is just less to think about.

NOTES

these are small commands and tricks u will use all the time, they are not a topic on their own but u should know them.

  1. the Tab key: this is the most useful key in the whole terminal and nobody tells u about it.

u start typing a file or a folder name, u press Tab, and the terminal finishes it for u.

cd Doc<Tab>
cd Documents/

u don’t type long paths. u type 3 letters and press Tab.

  • if more than one thing matches, nothing happens on the first press. press Tab again and it shows u the choices:
cd Do<Tab><Tab>
Documents/  Downloads/

then u type one more letter to make it clear which one, and press Tab again.

  • it works on commands too, not only files. type chm then Tab and u get chmod.
  • and it saves u from typos. if Tab does not complete the name, that usually means the file is not there or u are not in the folder u think u are in. it is a free spell check.
  1. ctrl + r: search backwards in ur history.

the arrow keys are fine for the last 3 commands. but when the command u want was 40 commands ago, u press ctrl + r and start typing any part of it:

(reverse-i-search)`grep': grep -rn "hello" ./notes

keep pressing ctrl + r to go further back through the matches, and press Enter to run the one u found, or the arrow keys to edit it first.

  1. history: shows u the last commands u used in ur terminal.
history
  1  echo "Hello World"
  2  name="ati"
  3  echo $name
  4  whoami
  5  history
  1. whoami: shows u the username of the current user.
whoami
ati

  1. the arrow keys: u can use the up and down arrows to go through the previous commands in ur history, so u don’t type the same long command again.

  2. clear: clears the screen of the terminal.

clear

Assignment

  1. Assign ur name, surname, age and ur job to variables.
  2. Print each variable with echo.
  3. Try to write this sentence in the terminal using ur variables: “Hello everyone! My name is Mohamed Ati. I am 23 years old, and I work as a programmer.”
  4. Use history to find the first command u typed in this chapter.
  5. Add more variables and print them together in one line.
  6. Print the sentence my variable is called $name on the screen, with the $name showing as text and not being replaced.
  7. Create a file with a space in its name, then delete it. Both times, quote it properly.
  8. Use Tab to move into ur Documents folder without typing the whole word.
  9. Use ctrl + r to find and re-run a command u typed earlier in this chapter.

stuck, or done and want to check? the solutions are here

← back to Terminal 101 | next: Chapter 2 →

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