Chapter 3: Using the Shell

This chapter introduces the Linux shell known as the Bash shell, short for Bourne Again shell. There are other shells, such as the C shell (csh) popular among BSD UNIX users, the Korn shell (ksh) popular in UNIX System V, Dash (Ubuntu’s default shell, faster than Bash), Tcsh (an improved C shell), and Ash (very similar to the Bourne shell).

3.1 Shell and Terminal Windows

1. Using Terminal Windows

Through the GUI, you can open a terminal emulator (sometimes called a Terminal window) to start a shell.

Most systems open it via the Ctrl+Shift+T shortcut. In Fedora, look for it in the applications or press Alt+F2 and type gnome-terminal.

2. Using Virtual Consoles

Most Linux systems with a GUI run multiple virtual consoles at startup. Besides the one running the GUI, you can open multiple shell sessions.

Switch virtual consoles using Ctrl+Shift+F1~F6. In Fedora, tty1 is gdm (login screen), tty2 is the first desktop, tty3 is the second (plain text), and so on.

• Command Prompt

For regular users, the default prompt is a simple dollar sign:

$

For the root user, the default prompt is a pound sign (also known as a number sign or hash tag):

#

A vivid illustration of the US root being in the UK.

3.2 Choosing a Shell

Use the who command to display the current logged-in username, the virtual console, and the login time.

Use grep username /etc/passwd. The end of the output shows the default shell being used.

You can switch shells by typing the command for it, such as ksh, tcsh, csh, sh, dash, or others (assuming they are installed).

Learning Bash is important not just because it’s the default in most installs, but because it’s the standard for most Linux certification exams.

3.3 Running Commands

While many commands run just by typing their name, it’s common to follow them with options and arguments to change their behavior.

3.3.1 Understanding Command Syntax

• Most commands have one or more options to modify behavior.

Options are usually a single letter preceded by a hyphen -. You can combine multiple single-letter options or use a hyphen for each.

Some options are full words, usually preceded by a double hyphen --. For example, to use the help option, you type --help. Typing -help would be interpreted as four options: -h, -e, -l, and -p. While some commands don’t follow the double-hyphen rule, most do for word-based options.

• Most commands also accept arguments after options or at the end of the line.

An argument is a block of extra info, like a filename, directory, username, or device. You can usually use any number of arguments as long as you don’t exceed the total character limit of the command line.

Sometimes an argument is associated with an option and must follow it immediately. For single-letter options, the argument is usually after a space. For full-word options, the argument follows an equals sign =.

Example:

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tar -cvf backup.tar /home/yexca

The options mean create (c) a file (f) named backup.tar containing all files in /home/yexca, and show verbose details (v) once finished. Since backup.tar is an argument for the f option, it must follow it.

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ls --hide=Desktop

The --hide option tells ls not to show the file or directory named Desktop. Note there is no space between the option and the argument.

• Other commands to try:

The uname command shows the system type. Add the -a option to see the hostname and kernel version.

When logged in, Linux assigns you an identity including a username, group name, user ID (UID), and group ID (GID). It also tracks the session time and location. Use the id command to see identity info.

Distros with SELinux (Security Enhanced Linux) enabled show extra info at the end of id output, like:

context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023

SELinux provides a way to strictly lock down Linux system security.

Use who -uH to add idle time and process ID info (u) and print headers (H).

IDLE shows how long the shell has been open without input. PID is the Process ID of the login shell. COMMENT shows the remote computer name (if logging in over a network) or the local X Display name (if using a Terminal window), such as :0.0.

3.3.2 Finding Commands

To find the command you typed, the shell searches through a “path”. For commands not in the path, you must provide the full path to the executable.

Use echo $PATH to see the shell’s PATH environment variable.

Reference: Linux bin Directory - yexca’Blog

Unlike some other OSs, Linux does not check the current directory for executables by default before searching the path. It only runs an executable in the current directory if it’s in the PATH or if you provide an absolute or relative path.

Not all commands are in the PATH directories. Some are built into the shell. You can also override commands using aliases. Here is the order the shell uses to check input:

1. Aliases: Names set by the alias command. Type alias to see current ones.

2. Shell Reserved Words: Words reserved by the shell for special purposes.

3. Functions: Groups of commands that run together in the current shell.

4. Built-in Commands: Commands built into the shell with no external file, like cd, echo, exit, pwd, history, fg (bring background command to foreground), set (set shell options), and type (show command location).

5. File System Commands: Commands stored in the computer’s file system (represented by the PATH variable).

To find where a command comes from, use type or which. Use type -a to show all known locations of a command.

If a command isn’t in the PATH, use locate to find it. locate can access any reachable part of the system.

locate searches the entire file system, not just command directories. If it can’t find a recently added file, run updatedb as root to update the database.

3.4 Using Command History

Repeating long, complex, or error-prone commands saves a lot of trouble.

3.4.1 Command Line Editing

By default, Bash uses Emacs-based command line editing. If you prefer vi, add set -o vi to your .bashrc file.

• Navigation Keys:

• Editing Keys:

• Cut and Paste Keys:

3.4.2 Command Line Completion

To reduce typing, Bash provides tab completion. Type a few characters and hit Tab. Bash can complete:

• Commands, aliases, or functions.
• Variables: If the text starts with $.
• Usernames: If the text starts with ~. ~username represents that user’s home directory.
• Hostnames: If the text starts with @, using entries from /etc/hosts.

To add hostnames from other files, set the HOSTFILE variable to that filename (using the same format as /etc/hosts).

If there are multiple matches, double-tap Tab to list them all.

3.4.3 Repeating History Commands

Every line you type is saved to the shell’s history list.

Use history to see the list, or add a number to see the most recent entries.

Use the exclamation mark ! to rerun commands. These execute immediately without confirmation.

• !n — Run command by number. e.g., !255.
• !! — Run the previous command.
• !?string? — Run the command containing the string.

Besides running history directly, you can find and edit specific commands:

You can also use the fc command. Typing fc followed by a number or range (e.g., fc 233 255) opens those commands in a text editor (default is vi). When you close the editor, the commands run sequentially.

When the shell closes, history is saved to .bash_history in your home directory (default limit is 1000 lines).

To exit without saving history, use kill -9 PID on the shell process.

Setting HISTFILE to /dev/null or clearing HISTSIZE won’t stop the save if the shell exits normally.

3.5 Connecting and Expanding Commands

The real power of the shell lies in redirecting input/output between commands or files.

Shells use metacharacters to link or expand requests. These include |, &, ;, ), (, <, and >.

3.5.1 Pipes Between Commands

The pipe | connects the output of one command to the input of another. Example:

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cat /etc/passwd | sort | less

This lists the contents of /etc/passwd, sorts them, and sends the result to less for viewing.

3.5.2 Sequential Commands

Use a semicolon ; to separate multiple commands on one line. Example:

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date ; troff -me VertLargeDocument | lpr ; date

This formats a large document and shows the time before and after.

3.5.3 Background Commands

For long-running tasks, use the ampersand & to run them in the background. Example:

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troff -me VertLargeDocument | lpr &

Don’t close the shell or kill the process before it finishes, or the task will terminate.

3.5.4 Command Expansion

Command substitution lets you use the output of one command as an argument for another.

The two forms are $(command) and command (backticks).

Example:

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vi $(find /home | grep xyzzy)

The shell runs the substitution first: find looks in /home, grep filters for “xyzzy”, and vi opens the resulting files one by one.

Note: Don’t run this from the root directory or you might try to edit thousands of files.

3.5.5 Arithmetic Expansion

To pass arithmetic results to a command, use $[expression] or $((expression)). Example:

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echo "I am $[2022-1957] years old"

Output: I am 65 years old.

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echo "There are $((ls | wc -w)) files in this directory"

This counts the files (ls | wc -w) and outputs the number.

3.5.6 Variable Expansion

Use the dollar sign $ to expand variables. It prints the value of the variable, not the name. Example:

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ls -l $BASH

Prints the long listing for the bash command: -rwxr-xr-x. 1 root root 1390064 Jan 20 2022 /usr/bin/bash.

3.6 Using Shell Variables

The shell uses variables to store session info. Use set to see all variables.

A subset of these are environment variables, which are passed to new shells opened from the current one. Use env to see these.

System files also set variables for mail, paths, and config locations. Prefix the name with $ to reference the value.

Many variables are preset at startup. Here are some common ones:

3.6.1 Creating and Using Aliases

Use alias to list or create aliases.

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alias p='pwd ; ls -CF'

Running p now runs pwd then ls -CF. Use unalias to remove it.

3.6.2 Exiting the Shell

Type exit or press Ctrl+D.

3.7 Customizing Your Shell Environment

3.7.1 Shell Configuration

Multiple config files determine shell behavior:

To change /etc/profile or /etc/bashrc, you need root access. It’s better to create /etc/profile.d/custom.sh for system-wide changes instead of editing the main files.

• nano Editor

A simple text editor. Ctrl+O to save, Ctrl+X to exit.

3.7.2 Setting the Prompt

The prompt is set by the PS1 variable. Special characters include:

Temporary change: export PS1="[\t\w]\$"

Permanent change: Add the PS1 value to ~/.bashrc.

Read more: Bash Prompt HOWTO

3.7.3 Adding Environment Variables

You might want to add variables to .bashrc:

• TMOUT — Seconds before auto-logout.
• PATH — Search directories. To add /home/yexca/bin: PATH=$PATH:/home/yexca/bin ; export PATH This reads the current PATH, appends the new one, and exports it.

Never add . (current directory) to your PATH for security reasons.

• Custom Variables: Create shortcuts for your workflow. MYWORKDIR=/home/yexca/work ; export MYWORKDIR Then just use cd $MYWORKDIR.

3.8 Getting Info on Commands

Some commands are built into the shell. Use help | less or help command for info.

Most commands support the --help or -h option.

Use info and man command for in-depth documentation.

Manual (man) pages are the standard for Linux documentation. They are divided into 8 sections:

Use man -k to search the database. If there are no results, run mandb as root to initialize it.

Use man 5 passwd to view section 5 specifically.

Navigate using Page Up/Down or Enter/Arrow keys. Search using / followed by the string. n for next match, N for previous. Press q to exit.