shells and scripts part 1

date published: 2023-08-28

Hey folks!

This post contains my notes from the third chapter of Learning Modern Linux book written by Michael Hausenblas. Let’s just jump into it!

Ways of working with a CLI

There are two ways in which a user can work with Linux via a CLI:

Manual via a terminal or a pseudo-terminal such as tty, where a human inputs commands and checks their output
Automatic by utilising shell scripts, which come in handy for repetitive tasks

Terminal and Terminal Emulator

Terminal is an electronic device which generally consists of a keyboard and a monitor for inputting commands and data into a computer and inspecting the output of entered commands respectively.

Terminal Emulator is an application that mimics the behaviour of a physical terminal usually via a graphical user interface or a full-screen console.

Shell

Shell is a program that interprets and executes incoming commands, handles input/output data and enables the user to enter these commands both manually and automatically via scripts.

Streams

Definition

Streams serve as a means of transporting input, output, and error data. In Linux and other Unix-like systems, every process has access to three file descriptors:

0 for stdin
1 for stdout
2 for stderr

Redirecting to other destinations

By default, stdin is connected to the keyboard and the other two are connected to the screen. It’s possible to redirect a stream elsewhere by adding $FD>, where $FD is an appropriate file descriptor (see: list above).

If you omit $FD, stdout will get redirected to the specified destination. If you want to redirect both stdout and stderr to the same destination, use &>. And if you want to ignore a certain stream, redirect it to /dev/null.

Special characters

& (ampersand) will run a command in the background
\ (backslash) allows you to continue writing a command in a new line
| (pipe) allows you to pipe one command’s stdout into the following command’s stdin

Variables

Shell variables, just like in pretty much any other programming language, are used to store a piece of data under a symbolic name.

We can specify two types of variables:

Environment variables that are used to configure a value in some script from outside said code
Shell variables that are limited to the execution context of a given script

Here’s how to create a shell variable:

set NICE_NUMBER=69

And here’s how to create an environment variable:

export ANOTHER_NICE_NUMBER=420

Here’s how to print a variable’s value:

echo $VARIABLE_NAME_HERE

Select environment and shell variables:

PATH - list of paths to search for executables to run
HOSTNAME - name of the device
PWD - absolute path to the current working directory
USER - current user’s username
$ - current PID
? - last task’s exit code

Speaking of exit codes…

Exit Codes

Every process is expected to return a status code after exiting. Code 0 means a successful execution, whereas every code greater than or equal 1 means some sort of a failure.

Job Control

By default, every command you execute runs in the foreground, meaning it takes control over the keyboard and the screen. If you want to start a task in the background, you have to append a & to the command, or press Ctrl+Z while the task is running in the foreground.

Modern command replacements and other utils

exa as a replacement for ls -l
bat as a replacement for cat
ripgrep (rg) as a replacement for… well, grep
gtop as a performance monitor in your CLI
curlie as a modern enhancement of curl

Feel free to check out the modern-unix repo for more examples.

Here’s a list of some key combinations to help you navigate around your CLI:

Ctrl + A moves the cursor to the start of the line
Ctrl + E moves the cursor to the end of the line
Left Alt + F moves the cursor forward by a word
Left Alt + B moves the cursor backward by a word
Ctrl + U removes all the characters on the left-hand side of a cursor
Ctrl + K removes all the characters on the right-hand side of a cursor

File content CRUD operations

Create a file

touch cool-file.txt

Read a file

cat cool-file.txt

Overwrite a file’s content

echo 'New file content' > cool-file.txt

Append content to a file

echo 'Even more new content' >> cool-file.txt

(There’s no need to add a newline character manually)

Delete a file

rm cool-file.txt