Setting Environment Variables from a File of Key/Value Pairs in Bash

Introduction

In many scripting and development scenarios, managing environment variables effectively is crucial. This becomes especially relevant when dealing with configuration files that define key-value pairs for environment variables. In this tutorial, we’ll explore how to set environment variables from a file containing such key/value pairs using Bash.

Understanding Environment Variables in Shell Scripts

Environment variables are dynamic-named values used by processes and programs on your computer. They can influence the behavior of running processes on a shell or command-line interface. For instance, they might determine the directories where system looks for executable files (PATH) or set application-specific settings.

In Bash scripting, you often need to export these variables so that child processes can access them. This is typically done with the export command. However, when dealing with configuration files, a more efficient approach is required to avoid manual exports and ensure portability across different environments and platforms.

Approach 1: Using source or . (dot)

The simplest way to set environment variables from a file in Bash is by using the source command (or its shorthand .). This executes the script in the current shell context, meaning any exported variables become available immediately:

# Example configuration file: config.env
MINIENTREGA_FECHALIMITE="2011-03-31"
MINIENTREGA_FICHEROS="informe.txt programa.c"
MINIENTREGA_DESTINO="./destino/entrega-prac1"

# Source the file in your script or terminal session
source config.env

# Verify that variables are set
echo $MINIENTREGA_FECHALIMITE

Note: Ensure all variable assignments in the configuration file use export if they need to be available to child processes.

Approach 2: Using xargs with Export

If you prefer not to modify your configuration files, another method is to leverage xargs. This can parse the key=value pairs and export them directly:

# Assuming config.env contains simple key-value assignments without spaces in values:
export $(cat config.env | xargs)

# For handling spaces within variable values (GNU systems):
export $(grep -v '^#' config.env | xargs -d '\n')

# On BSD systems, use:
export $(grep -v '^#' config.env | xargs -0)

Approach 3: Using set with -a Option

The set -a option in Bash automatically exports variables as they are set. This is a powerful feature for configuration files:

# Set the options to automatically export all variables defined after this point.
set -a

# Source your file
. config.env

# Reset the behavior by turning off automatic export
set +a

Considerations and Best Practices

• Sensitive Information: Be cautious when dealing with sensitive information in configuration files, such as API keys or passwords. Avoid version controlling such files.
• Portability: When writing scripts intended for multiple environments (Linux vs BSD), consider using platform detection to handle nuances like xargs behavior:

#!/bin/sh

unamestr=$(uname)
if [ "$unamestr" = 'Linux' ]; then
  export $(grep -v '^#' config.env | xargs -d '\n')
elif [ "$unamestr" = 'FreeBSD' ] || [ "$unamestr" = 'Darwin' ]; then
  export $(grep -v '^#' config.env | xargs -0)
fi

• Debugging: While sourcing files directly is convenient, it can obscure the actual environment setup if there’s an issue. Consider logging or echoing variables post-import for verification.

Conclusion

By understanding these methods, you can efficiently manage environment variables in your Bash scripts, making them more robust and portable across different systems. Each method has its trade-offs, so choose based on your specific needs—be it simplicity with source, automation using xargs or set -a, or platform compatibility.