The previous recipe, Creating a program interface with the flag package, describes how to use flags as program parameters.

The other typical way of parameterization, especially for larger applications, is the configuration with the use of environment variables. Environment variables as a configuration option significantly simplify the deployment of the applications. These are also very common in cloud infrastructure.

Usually, the configuration of a database connection for a local and for an automated build environment is different.

If the configuration is defined by the environment variables, it is not necessary to change the application config files or even the application code. The exported environment variables (for example, DBSTRING) are all we need. It is also very practical to default the configuration if the environmental variable is not in place. This way, the life of the application developers is much easier.

This recipe will demonstrate how to read, set and unset the environment variable. It will also show you how to implement the default option if the variable is not set.

1. Open the console and create the folder chapter01/recipe04.
2. Navigate to the directory.

3. Create the get.go file with the following content:

        package main

        import (
          "log"
          "os"
        )

        func main() {
          connStr := os.Getenv("DB_CONN")
          log.Printf("Connection string: %s\n", connStr)
        }

4. Execute the code by calling DB_CONN=db:/user@example && go run get.go in the Terminal.
5. See the output in the Terminal:

6. Create the lookup.go file with the following content:

        package main

        import (
          "log"
          "os"
        )

        func main() {

          key := "DB_CONN"
    
          connStr, ex := os.LookupEnv(key)
          if !ex {
            log.Printf("The env variable %s is not set.\n", key)
          }
          fmt.Println(connStr)
        }

7. Execute the code by calling unset DB_CONN && go run lookup.go in the Terminal.
8. See the output in the Terminal:

9. Create the main.go file with the following content:

        package main
        import (
          "log"
          "os"
        )

        func main() {

          key := "DB_CONN"
          // Set the environmental variable.
          os.Setenv(key, "postgres://as:[email protected]/pg?
                          sslmode=verify-full")
          val := GetEnvDefault(key, "postgres://as:as@localhost/pg?
                                     sslmode=verify-full")
          log.Println("The value is :" + val)

          os.Unsetenv(key)
          val = GetEnvDefault(key, "postgres://as:[email protected]/pg?
                                    sslmode=verify-full")
          log.Println("The default value is :" + val)

        }
 
        func GetEnvDefault(key, defVal string) string {
          val, ex := os.LookupEnv(key)
          if !ex {
            return defVal
          }
          return val
        }

10. Run the code by executing go run main.go.
11. See the output in the Terminal:

The environment variables are accessed by the Getenv and Setenv functions in the os package. The names of the functions are self-explanatory and do not need any further description.

There is one more useful function in the os package. The LookupEnv function provides two values as a result; the value of the variable, and the boolean value which defines if the variable was set or not in the environment.

The disadvantage of the os.Getenv function is that it returns an empty string, even in cases where the environment variable is not set.

This handicap could be overcome by the os.LookupEnv function, which returns the string as a value of the environment variable and the boolean value that indicates whether the variable was set or not.

To implement the retrieval of the environment variable or the default one, use the os.LookupEnv function. Simply, if the variable is not set, which means that the second returned value is false, then the default value is returned. The use of the function is part of step 9.