Shell Scripting - Intro

You, Sat Jan 08 2022 • shell scripting

What is a shell script?

A shell script is a text file that contains a series of commands. This means that any work you can do on the command line can be automated by a shell script.

Every shell script file has the extension *.sh, and you can create it using an editor text - nano or vim, for example (or if you want to keep it simple at first, simply use VS Code or other application).

First steps

Imagine that you want to create the first shell scripting file. Making sure you have nano installed, let's run the following command to start editing the file -> nano day1.sh:

#!/bin/bash
echo "The first steps into shell scripting"

Note: you will want to start each shell script with a shebang (#!) followed up by the path to an interpreter (/bin/bash), that is what determines what program executes the commands listed in the file. Otherwise, the script will be eexecutedusing the current shell.

After saving it, run chmod +x day1.sh to make sure you have permission to execute the program.

Then, simply run ./day1.sh and you'll get in the console the phrase above! Right, you just executed a script :)

Let's dive into more details, starting with simple commands.

Simple commands

Type	Description	Notes
`cat <file>`	Short for concatenate	By default it displays the contents of files to the terminal
`chmod <options> <file>`	Short for change mode	You can change the file permissions using this command
`cd <path>`	Short for change directory	Used to navigate throughout your directory
`pwd`	Short for present working directory	Get the present working directory's path
`type <command>`	Get shell built-in command	Indicates if the given command is a shell built-in or not
`echo <string>`	Print in the console what you wrote	Can be seen as the `console.log('')` of shell
`su`	Short for superuser	Changes the current account to the system root account
`sudo`	Short for superuser do	Executes an action as the system root account

Using variables

Let's now start creating a second file to start mmanipulatingwith variables, running this command -> nano day2.sh :

#!/bin/bash
SKILL="shell scripting"
echo "I'm getting better at ${SKILL}! Yeahhh!"

After running it, you'll check that the variable has been replaced by the text, getting the following message:

"I'm getting better at shell scripting! Yeahhh!"

Very simple, right? Note that you don't need to specify its type, you simply need to assign the value to the variable. It's very important to know that there are no spaces around the equal sign, otherwise, you'll get an error.

Variable considerations

Variable names can contain letters, digits and underscores. However, variables can only start with letters or underscores.
- Example of valid variables:
  - SKILL
  - SKILL5
  - _SKILL
- Example of invalid variables:
  - 1SKILL
  - A-SKILL
  - E@VAR
By convention, and since they are case sensitive, variable names are in UPPERCASE. However, they can be also written in lowercase, camelCase, PascalCase or snake_case, but not in kebab-case.

To reassign the value of a variable, just write it again with the desired value:

#!/bin/bash
SKILL="shell scripting"
echo "I'm getting better at ${SKILL}! Yeahhh!"
SKILL="variable reassignment"
echo "I'm getting better at ${SKILL}! Yeahhh!"

Adding comments

To write a comment in a shell scripting file, just write a # with the desired comment. When executed, the script will ignore this line of code -the shebang is the only exception.

#!/bin/bash

# The following line will print a string
echo "Just print this line!"

Don't forget to use blank lines to improve code readability.

Writing simple scripts

Imagine you want a script that prints some information about the system. That's what we'll do in the following script. Also, you'll learn some more commands.

#!/bin/bash

# This script displays information about the system on which it is executed.

# Tell the user the script is starting.
echo "Starting the script file."

# Display the hostname of the system.
hostname

# Display the current date and time when this information was collected.
date

# Display the kernel release followed by the architecture
uname -r
uname -m

# Display the disk usage in a human-readable format.
df -h

# End the script by letting the user know that it's done.
echo "End of the script file."

`IF` statement

Let's learn how to execute an if statement in a script file. This one will output if you are the root user or not. Let's call it is-root-user.sh.

#!/bin/bash

# Determine if the user executing this script is the root user or not.

# Display the UID
echo "Your UID is ${UID}."

# Display if the user is the root user or not.
# Zero indicates that the user is rooted in UNIX systems.
# Spaces in if statements are important!
if [[ "${UID}" -eq 0 ]]
then
  echo "You are root."
else
  echo "You are not root."
fi

And that's it! Hope you enjoy it!

This article was written based on this crash course by Jason Cannon.