In Bash, you can assign default values to variables in a single command using parameter expansion syntax. This allows you to set a fallback value if the variable is unset or null. Below are the key methods with examples:
1. Assign Default if Variable is Unset or Null
Use the :- operator to set a default value temporarily (only for the current expansion).
To permanently assign the default to the variable, combine it with an assignment:
# Syntax: Assign default if variable is unset or empty
variable="${variable:-default_value}"
# Example:
name="${name:-John}" # If $name is unset/null, set it to "John"2. Assign Default if Variable is Unset (Ignore Null Values)
Use the - operator to set a default only if the variable is unset (ignores null/empty values):
# Syntax: Assign default only if variable is unset (not for empty values)
variable="${variable-default_value}"
# Example:
count="${count-0}" # Sets $count to 0 only if it was never defined3. Assign and Export in One Line
Use := to set a default and update the variable permanently in a single step.
This works even for variables that are null (empty):
# Syntax: Permanently assign default if variable is unset or null
: "${variable:=default_value}" # Colon (:) is a no-op command that evaluates the assignment
# Example:
: "${PATH:=/usr/local/bin}" # Set $PATH to /usr/local/bin if it's unset/null4. Assign Defaults for Positional Parameters
Set defaults for script arguments ($1, $2, etc.):
# Syntax: Set default for the first argument
param1="${1:-default}"
# Example (script.sh):
#!/bin/bash
name="${1:-Guest}" # Use "Guest" if no argument is provided
echo "Hello, $name!"Run it:
./script.sh # Output: Hello, Guest!
./script.sh Alice # Output: Hello, Alice!5. Nested Defaults
Chain defaults for multiple fallback values:
# Use the first non-empty value in a chain
value="${VAR1:-${VAR2:-fallback}}"
# Example:
color="${USER_COLOR:-${DEFAULT_COLOR:-blue}}"Key Operators
| Operator | Description |
|---|---|
${var:-x} | Use x if var is unset or null (does not modify var). |
${var-x} | Use x only if var is unset (ignores null values). |
${var:=x} | Set var to x if unset or null (permanently updates var). |
${var=x} | Set var to x only if unset (ignores null values). |
Examples
Set a Default Port
: "${PORT:=8080}" # Set $PORT to 8080 if unset/null
echo "Using port: $PORT"Handle Optional Configurations
# Use $USER_CONFIG if set, otherwise "config/default.yml"
config_file="${USER_CONFIG:-config/default.yml}"Fallback for Empty Strings
# If $message is empty/null, use "No message provided"
echo "${message:-No message provided}"Why This Matters
- Avoid Errors: Prevent “unbound variable” errors in scripts.
- Simplify Code: Replace verbose
ifchecks with one-liners. - Flexibility: Set defaults dynamically based on conditions.
By mastering these techniques, you can write cleaner, more robust Bash scripts!