Bash Shell

In this lesson, we explore various Linux shells and their unique features, with a focus on the widely used Bourne Again Shell (Bash). Linux supports several shells, including the traditional Bourne Shell (developed in the 1970s), C Shell, Korn Shell, Z Shell, and Bash. Although these shells vary in capabilities, they all enable users to communicate effectively with the operating system.

Identifying Your Current Shell

To check which shell you are currently using, execute the following command (note the uppercase letters in the variable name):

echo $SHELL
/bin/bash

Bob confirms his default shell is Bash. This shell is especially popular because it offers advanced features like Bash Completions and Brace Expansion that are not available in the original Bourne Shell.

Changing Your Default Shell

To change your default shell, use the chsh command. You will be prompted for your password and then to provide the new shell value. Remember, you must open a new terminal session for the changes to take effect.

[$] chsh
Password:
Changing the login shell for michael
Enter the new value, or press ENTER for the default
Login Shell [/bin/bash]: /bin/sh

Command Auto-Completion

Bash provides command auto-completion that completes commands, filenames, and directory names when you press the Tab key. This feature can significantly improve efficiency by saving time and reducing typing errors when navigating the filesystem.

For example:

[~]$ ls Documents
File1.txt file2.txt some_directory

Aliases and Command History

Bash allows you to create aliases to simplify frequent commands. For instance, you can create an alias named dt for the date command to quickly display the current date. Additionally, you can use the history command to view a list of previously executed commands, making it easier to reuse them.

To verify the shell in use, simply run:

echo $SHELL

Remember, the $SHELL variable stores the type of shell you are using. When printing an environment variable's value in Bash, the variable name should be prefixed with a dollar sign.

Environment Variables

Environment variables provide information about your login session to the shell. To view all active environment variables, use the env command:

bash
[~]$ echo $SHELL
/bin/bash

[~]$ env
LANG=en_CA.UTF-8
GDM_LANG=en_CA
DISPLAY=:0
GTK_OVERLAY_SCROLLING=1
COLORTERM=truecolor
XDG_VTNR=7
USER=bob
PWD=/home/bob
HOME=/home/bob
SSH_AGENT_PID=2023
QT_ACCESSIBILITY=1
XDG_SESSION_TYPE=x11
GJS_DEBUG_OUTPUT=stderr
GTK_MODULES=gail:atk-bridge
TERM=xterm-256color
SHELL=/bin/bash
VTE_VERSION=5202
XDG_SEAT_PATH=/org/freedesktop/DisplayManager/Seat0
LANGUAGE=en_CA:en
LOGNAME=bob
PATH=/home/bob/bin:/home/bob/.local/bin:/usr/local/sbin:/usr/local/bin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin

To create a new environment variable (for example, setting the value "Caleston" to office), use the export command:

export office=Caleston

This sets the variable for the current session and any child processes. Without export, the variable's scope remains limited to the current shell only. To ensure these variables persist after logging out or rebooting, add them to your .profile or .pam_environment file.

When executing an external command, Bash utilizes the PATH variable to locate the appropriate executable. To display the directories listed in your PATH, run:

bash
echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin

If a command is not in one of these directories, you may encounter an error. To check if a command resides within your PATH, use the which command. For instance, attempting to locate an application like OBS Studio that hasn’t been added to your PATH might result in a "command not found" error.

To add a new directory (e.g., the OBS Studio path /opt/OBS/bin) to your PATH, execute:

export PATH=$PATH:/opt/OBS/bin

After making changes, use the which command to verify that the new directory is recognized.

Customizing the Bash Prompt

The Bash prompt, typically seen as [~]$ when you log in, can be customized to display useful system and user information. This is particularly beneficial when managing multiple systems, as the prompt can include details like username, hostname, and current working directory.

For example, a customized prompt indicating the host might look like:

[michael@prod-server]$

The appearance of the Bash prompt is controlled by the PS1 environment variable. To view its current setting, run:

bash
[~]$ echo $PS1
[\W]$

Here, \W displays the current working directory. You can change the prompt by updating PS1. To set it to display "ubuntu-server:" followed by a colon, use:

bash
[~]$ PS1="ubuntu-server:"
ubuntu-server:

For a more informative prompt that includes the date, time, username, hostname, and working directory, set PS1 with special backslash-escaped characters:

bash
[~]$ PS1="[ \d \t \u@\h:\w ] $ "
[Thu Mar 12 22:12:54 bob@caleston:~ ] $

Refer to the Bash documentation for a complete list of special characters that can be used in your prompt.

This concludes chapter one of the Quick Start Guide. Bob encourages exploring Bash's features further through hands-on exercises. Enjoy diving deeper into the Bash shell!

Additional Resources

• Bash Reference Manual
• Linux Shell Scripting Tutorial
• Understanding Linux Paths