Shell by Example: Variables

Shell by Example: Variables POSIX + Bash

Variables in shell scripts store values that can be referenced later. Unlike many programming languages, shell variables don’t require type declarations. To assign a variable, use `NAME=value` with no spaces around the `=` sign. This is important! To use a variable’s value, prefix it with `$`.	Edit `#!/bin/sh greeting="Hello" name="World" echo "$greeting, $name!"`
	Output: Hello, World!

You can also use curly braces for clarity, especially when the variable name could be ambiguous. Without braces, this would try to create a file with the contents “variable” and not “world”.	Edit `#!/bin/sh variable="world" echo "${variable}" >hello.txt cat hello.txt`
	Output: world

Variables can hold numbers too, though they are generally treated as strings.	Edit `#!/bin/sh count=42 echo "The count is $count"`
	Output: The count is 42

You can reassign variables at any time, even to different types. In this example, we assign a string to a variable, reassign it to a number, and then reassign it to a string again.	Edit `#!/bin/sh variable="world" echo "Hello, $variable!" variable=42 echo "Hello, $variable!" variable="world" echo "Hello, $variable!"`
	Output: Hello, world! Hello, 42! Hello, world!

Variable names can contain letters, numbers, and underscores. They cannot start with a number.	Edit `#!/bin/sh variable_1="hello" variable_2="world" # 3_variable="would cause an error" echo "variable_1: $variable_1" echo "variable_2: $variable_2"`
	Output: variable_1: hello variable_2: world

Unset variables expand to empty strings by default. You can use `unset` to remove an existing variable, returning it to the undefined state.	Edit `#!/bin/sh echo "Unset variable: '$undefined_var'" temp="temporary" echo "Before unset: $temp" unset temp echo "After unset: '$temp'"`
	Output: Unset variable: '' Before unset: temporary After unset: ''

Bash Bash provides powerful string manipulation within parameter expansion. These features are not available in POSIX sh. A few examples of string manipulation: String length with `${#var}`: Substring extraction with `${var:start:length}`: Remove prefix with `${var#pattern}` (shortest) or `${var##pattern}` (longest): Remove suffix with `${var%pattern}` (shortest) or `${var%%pattern}` (longest): Search and replace with `${var/find/replace}` (first) or `${var//find/replace}` (all):	Edit `#!/bin/bash message="Hello, World!" echo "Length: ${#message}" echo "${message:0:5}" echo "${message:7}" path="/home/user/docs/file.txt" echo "${path##/}" echo "${path%/}" filename="archive.tar.gz" echo "${filename%%.*}" text="hello hello hello" echo "${text/hello/hi}" echo "${text//hello/hi}"`
	Output: Length: 13 Hello World! file.txt /home/user/docs archive hi hello hello hi hi hi

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