Shell FundamentalsLesson 1.2

Bash variables and quoting rules explained

variable assignment, dollar-sign expansion, single vs double quotes, unset variables, readonly, local scope, word splitting

Variables in Bash

Bash variables have no types — everything is a string. Assignment uses = with no spaces around it (spaces break parsing).

name="Alice"
count=42
path_to_logs=/var/log/app

echo $name        # Alice
echo "Hello $name" # Hello Alice
echo 'Hello $name' # Hello $name  ← literal, no expansion

Quoting Rules — Get These Wrong, Break Everything

Double quotes "...": allow variable and command expansion. Use for almost everything.
Single quotes '...': treat everything literally. No expansion at all.
No quotes: enables word splitting and glob expansion — dangerous with filenames containing spaces.

file="my report.txt"
rm $file      # WRONG: tries to rm 'my' and 'report.txt' separately
rm "$file"    # CORRECT: treats as one argument

Special Variables

$0   # Script name
$1, $2 ... $9  # Positional arguments
$#   # Number of arguments
$@   # All arguments (as separate words)
$?   # Exit code of last command
$$   # PID of current shell

Always quote "$@" when passing arguments forward — it preserves argument boundaries even if arguments contain spaces. This single habit prevents an entire class of script bugs.