24.5 Shells, Scripting, Programming, and Compiling

24.5.1 Objective 1: Customize and Use the Shell Environment

• A shell presents an interactive Textual User Interface, an operating environment, a facility for launching programs, and a programming language.

• Shells can generally be divided into those derived from the Bourne shell, sh (including bash), and the C-shells, such as tcsh.

• Shells are distinct from the kernel and run as user programs.

• Shells can be customized by manipulating variables.

• Shells use configuration files at startup.

• Shells pass environment variables to child processes, including other shells.

24.5.1.1 bash

• bash is a descendant of sh.

• Shell variables are known only to the local shell and are not passed on to other processes.

• Environment variables are passed on to other processes.

• A shell variable is made an environment variable when it is exported.

• This sets a shell variable:

  # PI=3.14

• This turns it into an environment variable:

  # export PI

• This definition does both at the same time:

  # export PI=3.14

• Shell aliases conveniently create new commands or modify existing commands:

  # alias more='less'

• Functions are defined for and called in scripts. This line creates a function named lsps:

  # lsps (  ) { ls -l; ps; }

• bash configuration files control the shell's behavior. Table 24-3 contains a list of these files.

Table 24-3. Bash Configuration Files

File	Description
/etc/profile	The system-wide initialization file; executed when you log in.
/etc/bashrc	Another system-wide initialization file; may be executed by a user's .bashrc for each bash shell launched.
~/.bash_profile	If this file exists, it is executed automatically after /etc/profile when you log in.
~/.bash_login	If .bash_profile doesn't exist, this file is executed automatically when you log in.
~/.profile	If neither .bash_profile nor .bash_login exists, this file is executed automatically when you log in.
~/.bashrc	This file is executed automatically when bash starts.
~/.bash_logout	This file is executed automatically when you log out.
~/.inputrc	This file contains optional key bindings and variables that affect how bash responds to your keystrokes.

24.5.2 Objective 2: Customize or Write Simple Scripts

• Scripts are executable text files containing commands.

• Scripts must have appropriate execution bits set in the mode.

• Some scripts define the interpreter using the #!/bin/bash syntax on the first line.

24.5.2.1 Environment

• A script that starts using #!/bin/bash operates in a new invocation of the shell. This shell first executes standard system and user startup scripts. It also inherits exported variables from the parent shell.

• Like binary programs, scripts can offer a return value after execution.

• Scripts use file tests to examine and check for specific information on files.

• Scripts can use command substitution to utilize the result of an external command.

• Scripts often send email to notify administrators of errors or status.

• Refer to Chapter 17 for details on bash commands.