Checking if a variable is set

To determine whether a variable is set or not in Bash, you can use various methods and conditional checks.

Using test command or [ ] brackets

The test command, or its equivalent [ ] brackets, can be used to check if a variable is set.

# Declare the variable
variable="Hello"

# Check if the variable is set
if test -n "$variable"; then
  echo "Variable is set."
fi

or

# Declare the variable
variable="Hello"

# Check if the variable is set
if [ -n "$variable" ]; then
  echo "Variable is set."
fi

In both cases, the -n flag checks if the length of the variable is greater than zero, which indicates that the variable is set.

Using [[ ]] double brackets (preferred method)

The double brackets [[ ]] provide extended test capabilities and are generally preferred in Bash scripts.

# Declare the variable
variable="Hello"

# Check if the variable is set
if [[ -n "$variable" ]]; then
  echo "Variable is set."
fi

The [[ ]] approach works similarly to the test command but offers better readability and additional features.

Using isset function (custom function)

You can create a custom function to check if a variable is set.

# Define the isset function
isset() {
  [[ ${!1+x} ]]
}

# Declare the variable
variable="Hello"

# Check if the variable is set
if isset variable; then
  echo "Variable is set."
fi

The isset function uses indirect expansion (${!1+x}) to check if the variable exists.

Using :- operator for default value

You can use the :- operator to assign a default value if the variable is not set.

# Unset variable
# variable=""

# Check if the variable is set, otherwise assign a default value
echo "Value: ${variable:-default_value}"

If the variable is not set, it will print default_value; otherwise, it will print the value of the variable.

Using declare command

The declare command can be used to check if a variable is set.

# Declare the variable
variable="Hello"

# Check if the variable is set
if declare -p variable &> /dev/null; then
  echo "Variable is set."
fi

The declare -p command prints the attributes and values of variables. By redirecting the output to /dev/null, we suppress the output and only use the return status to check if the variable is set.