Linux Command Line and Shell Scripting Bible. Christine Bresnahan
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a terminal for silence.
Finally, we covered the xterm terminal emulator package. xterm was the first terminal emulator available for Linux. It can emulate older terminal hardware such as the VT and Tektronix terminals.
In the next chapter, you start looking at the Linux command line commands. It walks you through the commands necessary to navigate around the Linux filesystem, and to create, delete, and manipulate files.
Chapter 3
Basic bash Shell Commands
Interacting with the shell
Using the bash manual
Traversing the filesystem
Listing files and directories
Managing files and directories
Viewing file contents
The default shell used in many Linux distributions is the GNU bash shell. This chapter describes the basic features available in the bash shell, such as the bash manual, tab auto-completion and how to display a file's contents. You will walk through how to work with Linux files and directories using the basic commands provided by the bash shell. If you're already comfortable with the basics in the Linux environment, feel free to skip this chapter and continue with Chapter 4 to see more advanced commands.
Starting the Shell
The GNU bash shell is a program that provides interactive access to the Linux system. It runs as a regular program and is normally started whenever a user logs in to a terminal. The shell that the system starts depends on your user ID configuration.
The /etc/passwd file contains a list of all the system user accounts, along with some basic configuration information about each user. Here's a sample entry from a /etc/passwd file:
Each entry has seven data fields, with fields separated by colons. The system uses the data in these fields to assign specific features for the user. Most of these entries are discussed in more detail in Chapter 7. For now, just pay attention to the last field, which specifies the user's shell program.
Note
Though the focus is on the GNU bash shell, additional shells are reviewed in this book. Chapter 23 covers working with alternative shells, such as dash and tcsh.
In the earlier /etc/passwd sample entry, the user christine has /bin/bash set as her default shell program. This means when christine logs into the Linux system, the bash shell program is automatically started.
Although the bash shell program is automatically started at login, whether a shell command line interface (CLI) is presented depends on which login method is used. If a virtual console terminal is used to log in, the CLI prompt is automatically presented, and you can begin to type shell commands. However, if you log into the Linux system via a graphical desktop environment, you need to start a graphical terminal emulator to access the shell CLI prompt.
Using the Shell Prompt
After you start a terminal emulation package or log in to a Linux virtual console, you get access to the shell CLI prompt. The prompt is your gateway to the shell. This is the place where you enter shell commands.
The default prompt symbol for the bash shell is the dollar sign ($). This symbol indicates that the shell is waiting for you to enter text. Different Linux distributions use different formats for the prompt. On this Ubuntu Linux system, the shell prompt looks like this:
On the CentOS Linux system, it looks like this:
Besides acting as your access point to the shell, the prompt can provide additional helpful information. In the two preceding examples, the current user ID name, christine, is shown in the prompt. Also, the name of the system is shown, server01. You learn later in this chapter about additional items shown in the prompt.
Tip
If you are new to the CLI, keep in mind that, after you type in a shell command at the prompt, you need to press the Enter key for the shell to act upon your command.
The shell prompt is not static. It can be changed to suit your needs. Chapter 6, “Using Linux Environment Variables,” covers modifying your shell CLI prompt configuration.
Think of the shell CLI prompt as a helpmate, assisting you with your Linux system, giving you helpful insights, and letting you know when the shell is ready for new commands. Another helpful item in the shell is the bash Manual.
Interacting with the bash Manual
Most Linux distributions include an online manual for looking up information on shell commands, as well as lots of other GNU utilities included in the distribution. You should become familiar with the manual, because it's invaluable for working with commands, especially when you're trying to figure out various command line parameters.
The man command provides access to the manual pages stored on the Linux system. Entering the man command followed by a specific command name provides that utility's manual entry. Figure 3.1 shows an example of looking up the xterm command's manual pages. This page was reached by typing the command man xterm.
Figure 3.1 Manual pages for the xterm command
Notice the xterm command DESCRIPTION paragraphs in Figure 3.1. They are rather sparse and full of technical jargon. The bash manual is not a step-by-step guide, but instead a quick reference.
Tip
If you are new to the bash shell, you may find that the man pages are not very helpful at first. However, get into the habit of using them, especially to read the first paragraph or two of a command's DESCRIPTION section. Eventually, you will learn the technical lingo, and the man pages will become more helpful to you.
When you use the man command to view a command's manual pages, they are displayed with something called a pager. A pager is a utility that allows you to page through displayed text. Thus, you can page through the man pages by pressing the spacebar, or you can go line by line using the Enter key. In addition, you can use the arrow keys to scroll forward and backward through the man page text (assuming that your terminal emulation package supports the arrow key functions).
When you are finished with the man pages, press the q key to quit. When you quit the man pages, you receive a shell CLI prompt, indicating the shell is waiting for your next command.
Tip
The bash manual even has reference information on itself. Type man man to see manual pages concerning the man pages
The manual page divides information about a command into separate sections. Each section has a conventional naming standard as shown in Table 3.1.
Table 3.1 The Linux man Page Conventional Section Names
Not every command's man page has all the section names described in Table 3.1. Also, some commands have section names that are not listed in the conventional standard.
Tip
What if you can't remember the command name? You can search the man pages using keywords. The syntax is man – k keyword. For example, to find commands dealing with the terminals, you type man – k terminal.
In addition to the conventionally named sections for a man page, there are man page section areas. Each section area has an assigned number, starting at 1 and