Introduction to the Terminal

Introduction to the terminal/shell

Nearly all platforms you can get your hands on have a shell in one form or another, and many of them have several shells for you to choose from. While they may vary in the details, at their core they are all roughly the same: they allow you to run programs, give them input, and inspect their output in a semi-structured way.

-- Excerpt from the Missing Semester

For the most part, when people talk about the terminal, we normally talk about Bash, or POSIX-compliant shells. POSIX is just a fancy name for a set of rules that a shell should abide by, so that different shells can have similar behaviour. Other shells include:

• fish

• zsh

• nushell

• powershell

In modern operating systems, to open a shell prompt, you often need a terminal. Think of it as a nice GUI wrapping the textual interface (the shell). Your device should probably be shipped with one, or you should be able to install one easily.

The Shell Prompt

When you first launch a terminal, you will see a prompt, similar to or a slight variation of:

chun@legion:~$

This prompt tells you for example, that your username is chun on the machine named legion, and that you 'working directory' is ~ (short for home, we'll get to that). You should also see a blinking cursor, which you can type anything to, and when you hit Enter, it should execute the command you've typed out.

Here are some really simple commands you should be able to run (but some may not be installed depending on your machine):

• date - shows the date

Sun Sep  8 08:23:13 PM +08 2024`

• cal - a tiny calendar

   September 2024   
Su Mo Tu We Th Fr Sa
 1  2  3  4  5  6  7
 8  9 10 11 12 13 14
15 16 17 18 19 20 21
22 23 24 25 26 27 28
29 30               
              

• uptime - shows how long your computer has been powered on

 20:26:04  up   1:01,  2 users,  load average: 0.45, 0.62, 0.62

• echo - echoes what you typed

chun@legion:~$ echo hello
hello

Folders and directories

These commands are neat, but we can't really do much in the command line until we understand the concept of folders and directories.

All your files and directories on your system are stored in a structure known as a tree. This tree starts from a 'root' directory, this will be / on Linux and MacOS, and something like C:\ on Windows.

A path on the shell is just a list of directories, seperated by / on Linux and macOS and \ on Windows. For example:

C:\User\user\Documents - For Windows
~/Downloads - For Linux or MacOS

There are two types of paths:

• The absolute path is a path that starts from the root directory

• Relative paths are relative to your current working directory, or where you shell currently is

To see where you current directory is, use print working directory, or pwd for short in the terminal.

In a path, . refers to the current directory, and .. refers to the parent directory.

Common Commands

• to get the manual pages of a command

• to change directory

• to list files and directories

• to remove files and directories

• to copy file

• to move file

• to print working directory

Navigating the shell

bash has shortcuts that are based on emacs keybindings:

• Ctrl + a - beginning of line

• Ctrl + e - end of line

• Alt + b - move back one word

• Alt + f - move forward one word

• Ctrl + k - delete from cursor to end of line

• Ctrl + _ - undo

And some special keybindings:

• Ctrl + u - delete from cursor to the start of line

• Ctrl + w - delete from cursor to the start of word

• Ctrl + c - terminates the command

• Ctrl + z - suspends the command

• Ctrl + l - clears the screen

• Ctrl + s - stops the output to the screen

• Ctrl + q - allows output to the screen

You can find even more by doing man readline

readline(3) - Linux manual page

Editing in the terminal

There are a few ways you can make changes to your filesystem, be it editing files or directories/folders:

• mkdir to make a new empty directory

• touch <filename> to make a new empty file

• nano <filename> to open a editor to edit the file

  • Ctrl + o - to save

  • Ctrl + x - to exit

Finding commands

What if we want to find our previously used commands? If you haven't already realized, you should be able to use the up arrow to scroll through your previous commands, but it's not very efficient. This is where we can use a command called history.

chun@legion:~$ history

Composability

Remember what we said about the Unix Philosophy? A big part about it is the idea of programs working well together. The terminal allows this by allow the output of one program to be the input of another program. This is known as piping. To pipe we can do something like:

history | grep "echo"

• history prints out the entire command history as the output

• grep takes in an input and tries to filter for the keyword "echo"

Some other ways you could use pipes:

history | head ## Grabs the first 10 lines of the output
history | tail ## Grabs the last 10 lines of the output
## Opens output in a scrollable format, use you arrow keys to navigate
## and press q to quit
history | less 

There are more ways we can compose programs, which we'll go through in the scripting section!