Configuring shell behavior

Your shell is the most important Linux program you use because it provides access to your other programs and files. You want your interactions with bash to be comfortable and productive.

Bash can be configured in many ways to provide the Linux experience you want and to let you work quickly and easily. Regular Linux users make customizing bash a priority task when logging into a new computer (often by copying their preferred configuration from another machine).

The .bashrc file

You begin a shell session each time you log into a computer or open a new terminal window. Shell sessions are independent of one another. The work you did yesterday (or a few minutes ago) doesn't affect your new session.

You may want every shell session to start with certain actions - tweaking a setting or running a start-up command. You can tell bash to take these actions at the start of every session (without retyping them) by editing a special configuration file named .bashrc. You find this file in your home directory (many Linux systems create it for you when your account is set up). You can check for it using ls -a (you need -a to see it becaues it's a dotfile), or open it in a text editor.

The text of .bashrc is a shell script, a program written with shell commands. Bash runs the script (and performs each command) at the start of every shell session. You can add commands (or edit existing ones) to customize your shell sessoin.

Changes to your .bashrc (notably, to environment variables like your PATH) don't automatically apply to the current session. The source command is used to re-run the file in the current shell.

The newest version of .bashrc is automatically used when you start a new session - no source required.

Customizing your shell

The customizations in this section can be typed directly into your shell prompt to customize the current session. They are also frequently added to the .bashrc file so they take effect automatically in every session.

Command aliases

As you spend more time working with Linux, you may find yourself typing the same command - maybe with specific flags or arguments - again and again (and again...). An alias is a user-defined "command" that stands for another (usually longer) command, like a desktop shortcut to your favorite program.

The alias command is used to define a new alias, or to print the current definition of an existing alias.

The value is a command (including any command line options or arguments) enclosed in quotation marks. Whitespace matters to bash, so make sure there are no spaces between the name and equals sign or between the equals sign and value.

If you type the name of an existing alias without a matching value, bash will print the current definition of that alias. If you type alias with no other arguments, bash will print all currently-defined aliases and their definition.

Configuring your PATH

Your PATH environment variable tells bash where to look for commands you run. Your PATH is initially set by a system-wide script to include the location of common commands (such as cp).

You may want your PATH to include other directories containing programs you've installed (or written yourself). You can modify your PATH with a line in your .bashrc file. The syntax looks like string concatenation and variable assignment in a programming langauge (because that's exactly what it is).

Breaking down this line piece-by-piece:

• The PATH (on the left side of the assignment) tells bash to assign a value to PATH
• The = assignment operator must not have spaces on either side
• The $PATH (on the right side of the assignment) tells bash to use the current value of PATH (note the dollar sign here!)
• The colon (right after $PATH separates entries in the path
• The remainder of the line is the directory you're adding to the path
• The export at the beginning ensures the new value of PATH is available to other programs started by bash.

This adds a new directory to the back of the PATH so bash will look there after searching the other directories in the path. The order of the concatenation matters - if you reverse them, bash would look in the new directory first because it's at the front of the path.

Customizing your prompt

Your shell prompt is where you type every command you give to bash. It can also give you information about the current state of your shell or computer. Typical shell prompts include your username, the machine hostname, and the current working directory.

Your prompt's appearance is described by an environment variable named PS1, whose contents you can view using echo.

The value of PS1 is mostly literal characters, with special escaped characters representing special information.

• \u is your username
• \h is the first part of the hostname ("cs-class-1")
• \W is the name of the current working directory
• \$ is "$" if you're not root, and "#" if you are

You can assign a new value to PS1 that includes the current date or time; displays information about the current directory (such as the status of a git repository); and uses colors for different parts of the prompt. The Web is full of examples of such prompts to suit your individual tastes.

Changing your default shell

Shells are regular programs and not part of Linux. This means you can swap one shell for another to change how you interact with Linux (or even write your own shell program!).

Bash is arguably the most popular shell and is the default on many Linux computers. Some other widely-available shells include:

• tcsh (a successor to csh), also called the "C shell". Written by Bill Joy in 1978, this shell uses a syntax closer to the C programming language when writing shell scripts (programs consisting of shell commands).
• ksh, the "Korn shell" was written by David Korn in 1983. This shell was proprietary ATT software for many years, limiting its use outside certain Unix distributions.
• zsh is a "newer" shell (written in 1990) which aimed to combine the features of tcsh and ksh. It was adopted as the default shell in macOS Catalina (10.15) in 2019 (replacing bash)
• fish calls itself a "friendly interactive" shell and was first released in 2005. Fish is an exotic shell - it does not adhere as closely to the POSIX standard that other shells follow.

You can use the chsh command to select which shell you want to use (or reset to the system default).

You must provide the path to a valid login shell for the chsh command to succeed. A list of available (valid) login shells is kept in the /etc/shells file - you can cat this file to see the list.