Shell Variables

You can set variables using the = operator (no spaces around =):

name=value
ROOM_NUMBER=237
name=Flavio
nickname="Flavio"

Print a variable by prepending $:

echo $ROOM_NUMBER
echo $name

Strings

Always use quotes around strings when working with them:

dogname="Roger"

Get string length with ${#variable}:

echo ${#dogname}  # outputs: 5

Compare strings:

"$dogname" == "$anotherdogname"
"$dogname" != "$anotherdogname"

Environment Variables

Environment variables are values you can set outside of programs to alter their execution. An API key or configuration value, for example.

Set an environment variable:

export VARIABLE=something

With spaces, use quotes:

export VARIABLE="variable value"

This works the same in Bash and Zsh. To persist them:

• Bash: add to .bashrc or .bash_profile
• Zsh: add to .zshrc

After editing a dot file, apply changes with:

source ~/.bashrc

System Environment Variables

The system sets up some environment variables for you:

• $HOME - your home folder
• $LOGNAME - your user name
• $SHELL - the path to your default shell
• $PATH - where the shell looks for commands
• $PWD - current working directory

Inspect their values:

echo $HOME     # /Users/flavio
echo $LOGNAME  # flavio
echo $SHELL    # /bin/bash

The $PATH Variable

$PATH is a list of folders where the shell looks for commands, separated by colons:

echo $PATH
# /usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin

Add a folder to your path in .bashrc or .zshrc:

export PATH="$PATH:/Users/flavio/bin"

Operators

Bash implements arithmetic operators:

• + add
• - subtract
• * multiply
• / divide
• % modulo
• ** exponentiation

Comparison operators for numbers:

• -lt lower than
• -gt greater than
• -le lower or equal
• -ge greater or equal
• -eq equal to
• -ne not equal to

Logical operators:

• && logical AND
• || logical OR

Lessons in this unit:

0:	Introduction
1:	Introduction to Shells
2:	Bash Basics
3:	Writing Shell Scripts
4:	▶︎ Variables and Environment Variables
5:	Loops and Arrays
6:	Shell Script Functions
7:	Creating Aliases
8:	Tips and Tricks
9:	The Fish Shell
10:	Persist aliases and other configuration in Fish Shell
11:	How to add a path to Fish Shell
12:	Fish Shell, how to avoid recording commands to history
13:	Fish Shell, how to remove the welcome message
14:	How to replace all filenames with space with underscore using a shell script
15:	How to update all npm packages in multiple projects that sit in subfolders