Practical Go

CHAPTER 2Advanced Command-Line Applications

In this chapter, you will learn how to use the flag package to implement command-line applications with sub-commands. Then, you will see how you can enforce predictable behavior in your command-line applications using contexts. Finally, you will learn how to combine contexts and handling operating system signals in your application. Let's jump in.

Implementing Sub-commands

Sub-commands are a way to split the functionality of your command-line application into logically independent commands having their own options and arguments. You have a top-level command—your application—and then you have a set of sub-commands, each having its own options and arguments. For example, the Go toolchain is distributed as a single application, go, which is the top-level command. As a Go developer, you will interact with its various functionalities via dedicated sub-commands such as build, fmt, and test .

You will recall from Chapter 1 that to create a command-line application, you first created a FlagSet object. For creating an application with sub-commands, you will create one FlagSet object per sub-command. Then, depending on which sub-command is specified, the corresponding FlagSet object is used to parse the remaining command-line arguments (see Figure 2.1).

Figure 2.1: The main application looks at the command-line arguments and invokes the ...