How to check if a variable is set in Bash ?

To check if a variable is set in Bash, use parameter expansion or conditional expressions. Below are detailed methods depending on whether you want to check if a variable exists (declared) or exists and is non-empty.

1. Check if a Variable is Set (Declared)

Method 1: Use -v (Bash 4.2+)

Checks if the variable exists (even if empty).

if [[ -v variable_name ]]; then
  echo "Variable exists."
else
  echo "Variable is NOT set."
fi

Method 2: Use ${var+_} (Works in All Bash Versions)

Expands to a dummy value (_) if the variable is set.

if [ "${variable_name+_}" ]; then
  echo "Variable exists."
else
  echo "Variable is NOT set."
fi

**2. Check if a Variable is Set *and* Non-Empty**

Method 1: Use ${var:-}

Checks if the variable is non-empty.

if [ -n "${variable_name:-}" ]; then
  echo "Variable is set and non-empty."
else
  echo "Variable is unset or empty."
fi

Method 2: Use ${var:+_}

Expands to a dummy value (_) only if the variable is non-empty.

if [ "${variable_name:+_}" ]; then
  echo "Variable is set and non-empty."
else
  echo "Variable is unset or empty."
fi

3. Check if a Positional Parameter is Set (e.g., $1)

Use ${N:+_}, where N is the parameter position.

# Check if $1 (first argument) is set
if [ "${1+_}" ]; then
  echo "First argument exists."
else
  echo "First argument is missing."
fi

4. Edge Cases and Notes

• Empty vs. Unset:

  var=""    # Set but empty
  unset var # Unset

  [ -z "${var:-}" ] && echo "var is unset or empty"
  [ -z "${var+x}" ] && echo "var is unset (even if empty)"

• Arrays:

  arr=() # Declared but empty
  if [[ -v arr ]]; then
    echo "Array exists (even if empty)."
  fi

• Environment Variables:

  if [ -n "${PATH:-}" ]; then
    echo "PATH is set and non-empty."
  fi

Examples

Example 1: Check if a Variable is Set

name="John"
if [[ -v name ]]; then
  echo "name is set."
fi

Example 2: Check if a Variable is Unset

if [ -z "${age+x}" ]; then
  echo "age is unset."
fi

Example 3: Check for Non-Empty Variables in Scripts

# Check if $2 (second argument) is provided and non-empty
if [ -n "${2:-}" ]; then
  echo "Second argument: $2"
else
  echo "Error: Missing second argument."
  exit 1
fi

Common Pitfalls

• Quoting: Always quote variables ("${var}") to handle spaces/special characters.
• Legacy Syntax: Avoid [ -z "$var" ] to check emptiness—this fails to distinguish between unset and empty variables.
• Bash Version: [[ -v var ]] requires Bash 4.2+. Use ${var+_} for compatibility.

Summary

• [[ -v var ]]: Check if a variable is declared (Bash 4.2+).
• ${var+_}: Check if a variable is set (works in all versions).
• ${var:+_}: Check if a variable is set and non-empty.
• [ -n "${var:-}" ]: Safely check for non-empty variables.

Use these methods to handle script inputs, optional parameters, and environment variables robustly!