Variables and Quoting

Scope: assignment, expansion, and the quoting rules that prevent most Bash bugs. All examples were run with Bash 5.2.

Assignment: no spaces

name="Ada"
age=36

No spaces around =. name = "Ada" tries to run a command called name — the single most common beginner error, and the error message won't hint at the cause.

Expansion: $name

echo "Hello, $name"
echo "Next year: $((age + 1))"

• $name / ${name} — the value. Braces disambiguate: "${name}s" vs "$names" (different variables!).
• $((...)) — arithmetic.
• $(...) — command substitution (output of a command).

The quoting rules (memorize these)

Quote	Expands $var?	Expands $(cmd)?	Use for
"double"	Yes	Yes	Almost everything
'single'	No	No	Literal text
none	Yes, then word-split	Yes	Almost never

file="my notes.txt"
cat "$file"    # one argument: my notes.txt  ✓
cat $file      # TWO arguments: my, notes.txt  ✗

Unquoted variables get word-split on spaces — the classic source of "works until a filename has a space." Rule: quote every expansion, every time, unless you specifically want splitting or globbing.

Single quotes are for literals: 'Cost: $5' prints Cost: $5 with no expansion.

Special variables

Variable	Meaning
$0	Script name
$1, $2, ...	Positional arguments
$#	Number of arguments
$@	All arguments (quoted: "$@")
$?	Exit code of the last command
$$	This script's process ID

#!/usr/bin/env bash
echo "Script: $0"
echo "Args: $#"
echo "First: $1"

Run ./args.sh one two → Script: ./args.sh, Args: 2, First: one.

Always "$@", never $@ or $*, when passing arguments along — only "$@" preserves arguments containing spaces.

Exports and environment

export EDITOR="nano"   # visible to child processes
EDITOR="nano"          # visible only in this shell

export marks a variable for child processes. Scripts usually don't need it unless calling programs that read the variable.

readonly and defaults

readonly VERSION="1.0"              # cannot be reassigned
echo "${MISSING:-default}"          # "default" if unset
echo "${MISSING:?must be set}"      # error out if unset

${var:-default} is handy for optional settings; ${var:?message} fails fast on required ones.

Check

• [ ] You write name="x" with no spaces and can say why
• [ ] You quote every "$variable" by reflex
• [ ] You can explain "$@" vs $* in one sentence

Next: Control Flow.