create a Makefile that replaces strings inside a .sh file from an env file

To create a Makefile that replaces strings inside a .sh file using values from an .env file, we will leverage shell utilities like sed for substitution, while Makefile rules will help manage this process efficiently. Here’s a step-by-step guide to achieving this.

Understanding the Components

.env File

Typically, an .env file is formatted with key-value pairs that represent environment variables. For instance:

# .env file
DATABASE_URL=mysql://user:password@localhost/dbname
API_KEY=secret_api_key

Target .sh Script

Let’s assume you have a shell script file named script.sh that contains placeholder values for these variables:

#!/bin/bash

echo "Connecting to database at ${DATABASE_URL}"
echo "Using API Key: ${API_KEY}"

Makefile Setup

The Makefile will read the .env file, export those variables, and use them to replace placeholders in script.sh.

Sample Makefile

Below is an example of how to set up your Makefile to replace strings in the .sh file based on the values from the .env file:

# Makefile

# Define the target shell script and the env file
SCRIPT = script.sh
ENV_FILE = .env

# Default target
all: replace

# Target to replace strings in the shell script
replace: $(ENV_FILE)
	@echo "Replacing placeholders in $(SCRIPT) using $(ENV_FILE)..."
	@set -a; . $(ENV_FILE); set +a; \
    sed -e "s|\${DATABASE_URL}|$$DATABASE_URL|g" \
        -e "s|\${API_KEY}|$$API_KEY|g" \
        $(SCRIPT) > tmp_script.sh && \
    mv tmp_script.sh $(SCRIPT) && \
    echo "Replacements done."

# Clean target
clean:
	rm -f $(SCRIPT)

.PHONY: all replace clean

Explanation of the Makefile

• Variables: SCRIPT and ENV_FILE are defined as the names of your shell script and environment file, respectively.

• Default Target: The all target, which is invoked when you run make, depends on the replace target.

• Replace Target: This contains the commands that will:

  • Load environment variables from .env using the set -a and set +a commands, enabling the export of variables for use within the same shell.
  • Use sed with the -e flag to perform regex-based replacements in script.sh. The g at the end of each sed command option indicates a global replacement of the placeholders.
  • Redirect the output to a temporary file (tmp_script.sh), which is then moved back to the original script.sh.

• Clear Target: This optional clean target can be used to remove the original .sh file, in case you need to reset your script before running replacements again.

Running the Makefile

To execute the replacement process:

1. Save the .env file and script.sh in the same directory as your Makefile.
2. Open your terminal, navigate to the directory containing these files, and run:

make

After running this command, your script.sh should now have the actual values from the .env file embedded in it.

Conclusion

This approach allows you to dynamically replace placeholders in your shell scripts with actual values defined in an environmental file. By utilizing Makefiles, you can streamline your build and configuration processes effectively, making your deployments more consistent and manageable.

Feel free to modify the variables and commands in the Makefile according to your specific requirements to enhance its functionality, such as handling additional environment variables or scripts.