Shell by Example: Variables

Shell by Example: Variables POSIX

	`#!/bin/sh`
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!	`greeting="Hello" name="World"`
To use a variable’s value, prefix it with `$`.	`echo "$greeting, $name!"`
You can also use curly braces for clarity, especially when the variable name could be ambiguous.	`file="report" echo "${file}_2024.txt"`
Without braces, this would try to find a variable named `file_2024` which doesn’t exist.
Variables can hold numbers too (as strings).	`count=42 echo "The count is $count"`
You can reassign variables at any time.	`status="pending" echo "Status: $status" status="complete" echo "Status: $status"`
Variable names can contain letters, numbers, and underscores. They cannot start with a number.	`my_var="valid" myVar2="also valid" # 2invalid="would cause an error"`
Unset variables expand to empty strings by default.	`echo "Unset variable: '$undefined_var'"`
Use `unset` to remove a variable.	`temp="temporary" echo "Before unset: $temp" unset temp echo "After unset: $temp"`
Bash Bash provides powerful string manipulation within parameter expansion:
String length with ${#var}:	`message="Hello, World!" echo "Length: ${#message}" # 13`
Substring extraction with ${var:start:length}:	`echo "${message:0:5}" # Hello echo "${message:7}" # World!`
Remove prefix with ${var#pattern} (shortest) or ${var##pattern} (longest):	`path="/home/user/docs/file.txt" echo "${path##*/}" # file.txt (basename)`
Remove suffix with ${var%pattern} (shortest) or ${var%%pattern} (longest):	`echo "${path%/}" # /home/user/docs (dirname) filename="archive.tar.gz" echo "${filename%%.}" # archive`
Search and replace with ${var/find/replace} (first) or ${var//find/replace} (all):	`text="hello hello hello" echo "${text/hello/hi}" # hi hello hello echo "${text//hello/hi}" # hi hi hi`

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