Chapter Objective

Log in to a Red Hat Enterprise Linux system's graphical and command-line environments, and run simple commands using the bash shell correctly.

What Is the Shell?

The shell is a command interpreter that reads what you type, runs the corresponding program, and displays the results. On Red Hat Enterprise Linux, the default shell is bash (the Bourne Again SHell).

Every time you open a terminal, you're running an instance of the shell. It reads commands one line at a time, executes them, and prints a new prompt when it's ready for the next command.

The Shell Prompt

The default prompt shows your username, hostname, and current directory, ending in $ for a regular user or # for the root user.

# Regular user prompt
[student@server1 ~]$

# Root user prompt (note the #)
[root@server1 ~]#

Accessing a Terminal

RHEL provides several ways to reach a shell prompt, depending on whether the system has a graphical desktop installed.

Switching Virtual Consoles

RHEL provides multiple virtual consoles. From the graphical desktop, switch to a text console with Ctrl+Alt+F3 (through F6), and return to the graphical session with Ctrl+Alt+F1 or F2.

Command Syntax

Most commands follow the same general pattern:

command [options] [arguments]

• command — the program to run
• options — modify how the command behaves, usually prefixed with - or --
• arguments — the files, directories, or values the command acts on

# A command with no options or arguments
pwd

# A short option
ls -l

# A long option
ls --all

# Short options can often be combined
ls -la

# A command with an option and an argument
ls -l /etc

Quoting and Special Characters

The shell treats spaces, quotes, and characters like *, $, and > specially. Quote arguments that contain spaces so the shell treats them as a single value.

# Without quotes, the shell sees two arguments
echo Hello World

# With quotes, the shell sees one argument
echo "Hello World"

# Single quotes prevent the shell from expanding variables
echo '$HOME stays literal here'

# Double quotes still allow variable expansion
echo "$HOME expands here"

Command-Line Editing and History

Bash keeps a history of commands you've run and provides keyboard shortcuts, inherited from the Emacs editing model, for editing the current line without retyping it.

Working with History

# Show your recent command history
history

# Re-run command number 482 from history
!482

# Re-run the most recent command
!!

# Search history interactively (press Ctrl+R, then type)
(reverse-i-search)`ssh': ssh student@server1

Files, Paths, and Tab Completion

Every command that expects a file or directory understands both absolute paths (starting from /) and relative paths (starting from your current directory).

# Show your current working directory
pwd

# Absolute path — always starts from root
cd /etc/ssh

# Relative path — starts from wherever you currently are
cd ../ssh

# Shortcuts
cd ~        # your home directory
cd -        # the previous directory
cd ..       # the parent directory

Tab Completion

Press Tab while typing a command, file name, or path and bash will complete it automatically. Press Tab twice to list all possible matches when more than one exists.

# Type this, then press Tab
cd /etc/ss[Tab]
# bash completes to:
cd /etc/ssh/

Getting Help at the Command Line

RHEL ships extensive built-in documentation. You rarely need to leave the terminal to learn how a command works.

# Read the full manual page for a command
man ssh

# Quick reference of a command's options
ssh --help

# Search man page descriptions for a keyword
apropos partition

# Find the exact program a command runs
which ssh

Navigating a man Page

Key Terms for Chapter 2

shell

A command interpreter that reads, executes, and returns output for commands

bash

The Bourne Again SHell; the default shell on RHEL

prompt

The text shown when the shell is ready for input; ends in $ (user) or # (root)

virtual console

A separate text-mode login session, reached with Ctrl+Alt+F3–F6

option

A flag that modifies a command's behavior, usually prefixed with - or --

argument

The file, directory, or value a command acts on

absolute path

A file path that starts from the root directory /

relative path

A file path interpreted from the current working directory

tab completion

Bash feature that auto-completes commands, files, and paths when Tab is pressed

man page

The built-in manual page documenting a command, file format, or system call

apropos

Searches man page descriptions for a keyword

history

The list of previously run commands in the current shell session

Review Questions

1. What is the difference between a $ prompt and a # prompt?
2. Name two ways to reach a shell prompt on a RHEL system that has no graphical desktop installed.
3. What is the difference between how single quotes and double quotes handle a variable like $HOME?
4. You want to re-run the command you ran three commands ago without retyping it. Name two different ways to do this.
5. What does pressing Tab twice do when there is more than one possible completion?
6. What is the difference between an absolute path and a relative path? Give an example of each.
7. You run man passwd but get the wrong page for what you need. What command would get you the file-format version instead?