Shell shell-basics

Variables store values. Assign with VAR=value (no spaces around =). Access with $VAR or ${VAR}. Variables are case-sensitive and untyped—everything is a string unless used in arithmetic.

$ NAME="Alice"
$ echo $NAME                     # prints: Alice
$ echo ${NAME}_suffix           # prints: Alice_suffix
$ X=5; Y=$((X + 3)); echo $Y   # arithmetic: prints 8

Unset variables are empty strings. Quote variable references to preserve spaces: "$VAR" vs $VAR.

Quoting controls how the shell interprets special characters. Double quotes "..." preserve most meaning but allow expansion of $VAR and backticks. Single quotes '...' prevent all expansion. Backslash \ escapes a single character.

$ echo "Hello $USER"            # expands $USER
$ echo 'Hello $USER'            # literal: Hello $USER
$ echo "Cost: \$5"              # literal: Cost: $5
$ echo $'Hello\nWorld'          # ANSI-C quoting: interprets \n

Use single quotes for literals, double quotes when you need expansion.

Expansion is how the shell transforms text before running commands. Types include:

• Parameter expansion: $VAR, ${VAR}, ${VAR:-default}
• Arithmetic expansion: $((2 + 2))
• Command substitution: $(command) or `command`
• Brace expansion: {a,b,c} expands to a b c
• Tilde expansion: ~ becomes $HOME
• Pathname expansion (globbing): *.txt matches files ending in .txt

$ echo {1,2,3}                  # expands to: 1 2 3
$ echo /home/*/Desktop          # expands to matching directories
$ echo "Today is $(date +%Y)"   # command substitution

Expansions happen after quoting, so "$VAR" expands but '$VAR' does not.