Reading Environment Variables

In Go, environment variables can be accessed using the os package. This tutorial shows how to effectively read and use environment variables in your applications.

Basic Environment Variable Reading

You can use os.Getenv to retrieve the value of an environment variable. If the variable is not set, it returns an empty string.

package main

import (
	"fmt"
	"os"
)

func main() {
	path := os.Getenv("PATH")
	fmt.Println("Path:", path)

	home := os.Getenv("HOME")
	fmt.Println("Home:", home)

	// This will print an empty string if not set.
	unset := os.Getenv("NON_EXISTENT_VAR")
	fmt.Println("Unset:", unset)
}

Checking for Environment Variable Existence

To check whether an environment variable is set, compare the result from os.LookupEnv. It returns the value and a boolean indicating the presence of the variable.

package main

import (
	"fmt"
	"os"
)

func main() {
	if value, ok := os.LookupEnv("DATA_DIR"); ok {
		fmt.Println("DATA_DIR is set:", value)
	} else {
		fmt.Println("DATA_DIR is not set")
	}
}

Setting Environment Variables

Although not a common practice in production code, Go allows setting environment variables using os.Setenv.

package main

import (
	"fmt"
	"os"
)

func main() {
	err := os.Setenv("API_KEY", "73")
	if err != nil {
		fmt.Println("Error:", err)
	}

	value := os.Getenv("API_KEY")
	fmt.Println("API_KEY:", value)
}

Best Practices

• Use Default Values: Provide default values if environment variables are missing to ensure your application remains stable.
• Centralize Access: Implement a wrapper for accessing environment variables, which can simplify defaults and logging.
• Security Considerations: Be cautious with sensitive data stored in environment variables. Avoid logging these values.

Common Pitfalls

• Empty Strings: Getenv returns an empty string if the variable is unset, which might mislead if you're checking for empty values.
• Not Checking Presence: Failing to confirm if a variable is set can lead to unexpected behavior.
• Case Sensitivity: Environment variable names are case-sensitive on Unix but not on Windows, causing potential cross-platform issues.

Performance Tips

• Caching Values: If an environment variable is accessed multiple times, store it in a variable rather than repeatedly calling os.Getenv.
• Minimal Use: Limit the use of os.Setenv in performance-critical paths since it can affect program performance.
• Concurrent Access: When accessing environment variables concurrently, ensure thread safety if you choose to modify them during execution.