Envsubstr

Understanding the envsubst Command and Unset Variables
The envsubst command is a powerful tool for substituting environment variables within text. However, when a variable is not set, it can lead to unexpected behavior or errors.
The Problem:
When you use envsubst with an unset variable, it typically replaces the variable reference with an empty string. This might not be the desired behavior, especially if you expect an error or a default value.
Solutions:
Here are a few approaches to handle unset variables gracefully:
1. Using Bash's Parameter Expansion:
 * Default Value:
   unset MY_VAR
echo "${MY_VAR:-default_value}"  # Output: default_value

 * Error on Unset:
   unset MY_VAR
echo "${MY_VAR:?Variable MY_VAR is not set}"  # Output: Variable MY_VAR is not set

2. Combining envsubst with Bash's Parameter Expansion:
 * Default Value:
   unset MY_VAR
MY_VAR="${MY_VAR:-default_value}"
echo "$MY_VAR" | envsubst

 * Error on Unset:
   unset MY_VAR
MY_VAR="${MY_VAR:?Variable MY_VAR is not set}"
echo "$MY_VAR" | envsubst

3. Using a More Robust Approach with envsubst and a Temporary File:
# Create a temporary file with the template
cat > template.txt <<EOF
This is a template with a variable: ${MY_VAR}
EOF

# Set a default value or error on unset
MY_VAR="${MY_VAR:-default_value}"  # Or use "${MY_VAR:?Variable MY_VAR is not set}"

# Use envsubst to substitute the variable
envsubst < template.txt > output.txt

# Clean up the temporary file
rm template.txt

Choosing the Right Approach:
The best approach depends on your specific use case and desired behavior:
 * If you want a default value when the variable is unset, use parameter expansion with a default value.
 * If you want to error out when the variable is unset, use parameter expansion with the error option.
 * For more complex scenarios, consider using a temporary file and envsubst to ensure proper handling of unset variables.
By understanding these techniques, you can effectively handle unset variables in envsubst and avoid unexpected behavior.
Do you have a specific use case or scenario you'd like to discuss? I can provide more tailored advice based on your needs.