Linux Command Line and Shell Scripting Bible, 3rd Edition

Understanding the Shell

IN THIS CHAPTER

Investigating Shell Types

Understanding the Parent/Child Shell Relationship

Using Subshells Creatively

Investigating Built-in Shell Commands

Now that you know a few shell basics, such as reaching the shell and rudimentary shell commands, it is time to explore the actual shell process. To understand the shell, you need to understand a few CLI basics.

A shell is not just a CLI. It is a complicated interactive running program. Entering commands and using the shell to run scripts can raise some interesting and confusing issues. Understanding the shell process and its relationships helps you resolve these issues or avoid them altogether.

This chapter takes you through learning about the shell process. You see how subshells are created and their relationship to the parent shell. The varied commands that create child processes are explored as well as built-in commands. You even read about some shell tips and tricks to try.

Exploring Shell Types

The shell program that the system starts depends on your user ID configuration. In the /etc/passwd file, the user ID has its default shell program listed in field #7 of its record. The default shell program is started whenever the user logs into a virtual console terminal or starts a terminal emulator in the GUI.

In the following example, user christine has the GNU bash shell as her default shell program: ...