Bash Parameter Expansion With Default Value

Bash parameter expansion with default value

Bash parameter expansion with default value is a useful feature for handling variables that may or may not be set. The syntax is:

${variable-default}

This expands to the value of variable if it's set and not null. If variable is unset or null, it expands to default. It's commonly used to provide fallback values for environment variables or function parameters, improving script robustness and readability.

A slight variation is ${variable:-default}, which uses the default even if variable is set but empty.

Examples

1. Basic usage:

echo ${USER-anonymous}

This prints the value of USER if set, otherwise "anonymous".

1. In a script:

#!/bin/bash
NAME=${1-"World"}
echo "Hello, $NAME!"

This script greets the first argument, defaulting to "World" if no argument is provided.

1. With environment variables:

BACKUP_DIR=${CUSTOM_BACKUP_DIR-"/var/backups"}
cp important_file.txt $BACKUP_DIR

This uses a custom backup directory if set, otherwise defaults to "/var/backups".

1. Function parameters:

function greet() {
    local greeting=${1-"Hello"}
    local name=${2-"Guest"}
    echo "$greeting, $name!"
}

greet                 # Output: Hello, Guest!
greet "Hi"            # Output: Hi, Guest!
greet "Hey" "Alice"   # Output: Hey, Alice!

1. Nested default values:

echo ${FOO-${BAR-"default"}}

This uses FOO if set, otherwise BAR if set, otherwise "default".

Use cases

Here's a list of situations where bash parameter expansion with default values can be particularly useful:

1. Script arguments: Provide default values for optional command-line arguments.

2. Configuration management: Set default configuration values that can be overridden by environment variables.

3. Function parameters: Define default values for optional function parameters.

4. Fallback paths: Specify default file or directory paths when custom locations aren't provided.

5. User customization: Allow users to customize behavior through environment variables without modifying scripts.

6. Default usernames: Provide a default username when one isn't specified.

7. API tokens: Use a default or test API token when a custom one isn't provided.

8. Temporary directories: Set a default temp directory location that can be overridden.

9. Logging levels: Specify a default logging level that can be changed via an environment variable.

10. Database connections: Provide default database credentials that can be overridden for different environments.

11. Default timeouts: Set default timeout values for operations that can be customized.

12. Fallback commands: Use alternative commands if preferred ones aren't available on the system.

13. Default language or locale settings: Specify default language settings that can be overridden.

14. Build environments: Set default build flags or environments that can be customized.

15. Default resource allocations: Specify default CPU or memory allocations that can be adjusted.