Every program, once it is executed, exists in the environment of the operating system. The program receives input and provides output to this environment. The operating system also needs to communicate with the program to let it know what's happening outside. And finally, the program needs to respond with appropriate actions.

This chapter will walk you through the basics of the discovery of the system environment, the program parameterization via program arguments, and the concept of the operating system signals. You will also learn how to execute and communicate with the child process.

While building a program, it is a good practice to log the environment settings, build version, and runtime version, especially if your application is more complex. This helps you to analyze the problem, in case something goes wrong.

Besides the build version and, for example, the environmental variables, the Go version by which the binary was compiled could be included in the log. The following recipe will show you how to include the Go runtime version into such program information.

Install and verify the Go installation. The following steps could help:

1. Download and install Go on your machine.
2. Verify that your GOPATH and GOROOT environmental variables are set properly.
3. Open your Terminal and execute go version. If you get output with a version name, then Go is installed properly.
4. Create a repository in the GOPATH/src folder.

The following steps cover the solution:

1. Open the console and create the folder chapter01/recipe01.
2. Navigate to the directory.
3. Create the main.go file with the following content:

        package main
        import (
          "log"
          "runtime"
        )
        const info = `
          Application %s starting.
          The binary was build by GO: %s`

        func main() {
          log.Printf(info, "Example", runtime.Version())
        }

4. Run the code by executing the go run main.go.
5. See the output in the Terminal:

The runtime package contains a lot of useful functions. To find out the Go runtime version, the Version function could be used. The documentation states that the function returns the hash of the commit, and the date or tag at the time of the binary build.

The Version function, in fact, returns the runtime/internal/sys .The Version constant. The constant itself is located in the $GOROOT/src/runtime/internal/sys/zversion.go file.

This .go file is generated by the go dist tool and the version is resolved by the findgoversion function in the go/src/cmd/dist/build.go file, as explained next.

The $GOROOT/VERSION takes priority. If the file is empty or does not exist, the $GOROOT/VERSION.cache file is used. If the $GOROOT/VERSION.cache is also not found, the tool tries to resolve the version by using the Git information, but in this case, you need to initialize the Git repository for the Go source.

The most simple way to parameterize the program run is to use the command-line arguments as program parameters.

Simply, the parameterized program call could look like this: ./parsecsv user.csv role.csv. In this case, parsecsv is the name of the executed binary and user.csv and role.csv are the arguments, that modify the program call (in this case it refers to files to be parsed).

1. Open the console and create the folder chapter01/recipe02.
2. Navigate to the directory.
3. Create the main.go file with the following content:

        package main
        import (
          "fmt"
          "os"
        )

        func main() {

          args := os.Args

          // This call will print
          // all command line arguments.
          fmt.Println(args)

          // The first argument, zero item from slice,
          // is the name of the called binary.
          programName := args[0]
          fmt.Printf("The binary name is: %s \n", programName)

          // The rest of the arguments could be obtained
          // by omitting the first argument.
          otherArgs := args[1:]
          fmt.Println(otherArgs)

          for idx, arg := range otherArgs {
            fmt.Printf("Arg %d = %s \n", idx, arg)
          }
        }

4. Build the binary by executing go build -o test.
5. Execute the command ./test arg1 arg2. (Windows users can run test.exe arg1 arg2).
6. See the output in the Terminal:

The Go standard library offers a few ways to access the arguments of the program call. The most generic way is to access the arguments by the Args variable from the OS package.

This way you can get all the arguments from the command line in a string slice. The advantage of this approach is that the number of arguments is dynamic and this way you can, for example, pass the names of the files to be processed by the program.

The preceding example just echoes all the arguments that are passed to the program. Finally, let's say the binary is called test and the program run is executed by the Terminal command ./test arg1 arg2.

In detail, the os.Args[0] will return ./test. The os.Args[1:] returns the rest of the arguments without the binary name. In the real world, it is better to not rely on the number of arguments passed to the program, but always check the length of the argument array. Otherwise, naturally, if the argument on a given index is not within the range, the program panics.

If the arguments are defined as flags, -flag value, additional logic is needed to assign the value to the flag. In this case, there is a better way to parse these by using the flag package. This approach is part of the next recipe.

The previous recipe describes how to access the program arguments by a very generic approach.

This recipe will provide a way of defining an interface via the program flags. This approach dominates systems based on GNU/Linux, BSD, and macOS. The example of the program call could be ls -l which will, on *NIX systems, list the files in a current directory.

The Go package for flag handling does not support flag combining like ls -ll, where there are multiple flags after a single dash. Each flag must be separate. The Go flag package also does not differentiate between long options and short ones. Finally, -flag and --flag are equivalent.

1. Open the console and create the folder chapter01/recipe03.
2. Navigate to the directory.
3. Create the main.go file with the following content:

        package main
        import (
          "flag"
          "fmt"
          "log"
          "os"
          "strings"
        )

        // Custom type need to implement
        // flag.Value interface to be able to
        // use it in flag.Var function.
        type ArrayValue []string

        func (s *ArrayValue) String() string {
          return fmt.Sprintf("%v", *s)
        }

        func (a *ArrayValue) Set(s string) error {
          *a = strings.Split(s, ",")
          return nil
        }

        func main() {
 
          // Extracting flag values with methods returning pointers
          retry := flag.Int("retry", -1, "Defines max retry count")

          // Read the flag using the XXXVar function.
          // In this case the variable must be defined
          // prior to the flag.
          var logPrefix string
          flag.StringVar(&logPrefix, "prefix", "", "Logger prefix")

          var arr ArrayValue
          flag.Var(&arr, "array", "Input array to iterate through.")

          // Execute the flag.Parse function, to
          // read the flags to defined variables.
          // Without this call the flag
          // variables remain empty.
          flag.Parse()

          // Sample logic not related to flags
          logger := log.New(os.Stdout, logPrefix, log.Ldate)

          retryCount := 0
          for retryCount < *retry {
            logger.Println("Retrying connection")
            logger.Printf("Sending array %v\n", arr)
            retryCount++
          }
        }

4. Build the binary by executing the go build -o util.
5. From the console, execute ./util -retry 2 -prefix=example -array=1,2.

6. See the output in the Terminal:

For the flag definition in code, the flag package defines two types of functions.

The first type is the simple name of the flag type such as Int. This function will return the pointer to the integer variable where the value of the parsed flag is.

The XXXVar functions are the second type. These provide the same functionality, but you need to provide the pointer to the variable. The parsed flag value will be stored in the given variable.

The Go library also supports a custom flag type. The custom type must implement the Value interface from the flag package.

As an example, let's say the flag retry defines the retry limit for reconnecting to the endpoint, the prefix flag defines the prefix of each row in a log, and the array is the array flag that will be send as an payload to server. The program call from the Terminal will look like ./util -retry 2 -prefix=example array=1,2.

The important part of the preceding code is the Parse() function which parses the defined flags from Args[1:]. The function must be called after all flags are defined and before the values are accessed.

The preceding code shows how to parse some data types from the command-line flags. Analogously, the other built-in types are parsed.

The last flag, array, demonstrates the definition of the custom type flag. Note that the ArrayType implements the Value interface from the flag package.

The flag package contains more functions to design the interface with flags. It is worth reading the documentation for FlagSet.

By defining the new FlagSet, the arguments could be parsed by calling the myFlagset.Parse(os.Args[2:]). This way you can have flag subsets based on, for example, the first flag.

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:as@example.com/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:as@127.0.0.1/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.