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Linux is a widely adopted open-source operating system that has gained popularity among millions of users worldwide. Linux offers a wide range of shells for users to choose from, each with its own unique features. Linux includes a variety of shells. Users have the flexibility to switch to a different shell or install a new one, allowing for customization of their distribution.

Understanding the various types of available shells can give you a comprehensive understanding of the different options for changing your default shell.

Shells provide users with a way to interact with the operating system using text-based commands, similar to how a computer scientist would. Let’s dive into the fascinating world of Linux shells and discover the unique features of the 8 most common types.

What exactly is a shell and why is it necessary?

Whenever a user logs in to the system or opens a console window, a new shell instance is executed by the kernel. The kernel serves as the core component of every operating system.

It is crucial to manage and execute processes efficiently, while also ensuring optimal utilization of system resources.

A shell is a program that serves as a bridge between a user and the kernel. It enables a user to issue commands to the kernel and receive corresponding responses. By using a shell, we have the ability to run programs and utilities on the kernel. Therefore, at its essence, a shell is a program made use of to run other programs on our system.

Understanding how to interact with the kernel gives shells a significant advantage. Interacting with the kernel is crucial for users to access the utilities provided by their machine’s operating system.

Now, let’s delve into the primary shells that can be found in the Linux environment.

Understanding the significance of a Linux Shell

A shell is a program that interprets and executes commands on the operating system. This program represents the interactive interface of an operating system and the outermost layer of the kernel, also known as the shell. It enables users and programs to transmit signals and access the low-level utilities of an operating system.

Interacting with the system’s utilities is made possible through the terminal program (or terminal emulator). When running commands in the terminal, like ls or cat, the shell will analyze, evaluate, and search for the corresponding program before executing it, if it exists.

Exploring Different Linux Shells

Linux provides a variety of shell types that cater to different needs by offering distinct features. The shells that were developed alongside Unix frequently incorporated features from one another as development advanced.

Here is a brief overview of various shell types and the features they offer.

1. Bourne Shell (sh)

The Bourne shell, released in 1979, was the initial default shell on Unix systems. The shell program is commonly referred to as “sh” and is typically located in the “/bin/sh” directory. The prompt changes to $, while the root prompt becomes #.

Historically, the Bourne Shell was the default shell for Unix systems. Some older Unix-like systems may still use it as the default shell.

The Bourne shell gained popularity due to its compactness and speed. Unfortunately, it is missing some standard features, such as:

• Expanding on logical and arithmetic operations.
• Reviewing past commands.
• Additional advanced functionalities, like autocomplete, are available.

Modern Unix-like systems include the /bin/sh executable file. The program does not initiate the Bourne shell, but instead functions as an executable file that directs to the system’s default shell.

Typically, in most systems, the hard or symbolic link directs to bash. However, in the case of Ubuntu and Debian, the link is directed to dash. In both scenarios, the link closely resembles the Bourne shell.

2. C Shell (csh)

The C shell (csh) is a Linux shell that was developed in the late 1970s with a focus on enhancing interactive use and emulating the C language. Given that the Linux kernel is primarily coded in C, the shell strives to maintain a uniform style throughout the system.

The C Shell is the default shell in some BSD systems, such as FreeBSD.

The location of the C shell executable is /bin/csh. The prompt makes use of the % symbol for regular users and the # symbol for the root user.

Introducing exciting new interactive features:

• Retrieve the previous command’s history.
• Custom aliases for programs.
• Home directory, denoted by “~”.
• Expression grammar is included as a built-in feature.

There are several significant disadvantages associated with the C shell:

• Inconsistencies in syntax.
• Lacks support for standard input/output (stdio) file handles or functions.
• Not being fully recursive can limit the ability to handle complex commands.

The C shell enhanced both readability and performance when compared to the Bourne shell. The interactive features and innovations in csh had a significant impact on all subsequent Unix shells.

3. The TENEX C Shell (tcsh)

The tcsh, an extension of the csh, was merged in the early 1980s. The shell maintains compatibility with csh while incorporating extra features and concepts from the TENEX OS.

The tcsh is an enhanced version of the C Shell and is the default shell on some Unix systems like FreeBSD and its derivatives.

The executable path for the TENEX C shell is located in /bin/tcsh. The user prompt is displayed as hostname:directory, while the root prompt is shown as hostname:directory#. Early versions of Mac OS and the default root shell of FreeBSD employ tcsh.

Other features of the shell include:

• Enhanced command history.
• Efficient auto-completion functionality.
• Matching using wildcards.
• Managing job control.
• Command for built-in “where” function.

Although tcsh is an extension of the C shell, there are still several drawbacks that persist in the extended version.

4. KornShell (ksh)

The KornShell (ksh) is a Unix shell and language that was developed in the early 1980s, based on the Bourne shell (sh). The location can be found in /bin/ksh or /bin/ksh93, and the prompt is identical to that of the Bourne shell ($ for a user and # for root).

The KornShell is the default shell on some commercial Unix systems, such as AIX (IBM) and Solaris (Oracle).

The shell incorporates functionalities from the C shell and Bourne shell, with a focus on interactive commands and programming capabilities. The KornShell introduces additional functionalities, including:

• Includes a range of mathematical functions and supports floating-point arithmetic.
• Object-oriented programming is a fundamental concept in software development.
• Increased flexibility of pre-installed commands.
• Works seamlessly with the Bourne shell.

The shell is more efficient than both the C shell and the Bourne shell.

5. The Debian Almquist Shell (dash)

The Debian Almquist Shell (dash) is a Unix shell that was developed in the late 1990s as a derivative of the Almquist shell (ash). It was later ported to Debian and given a new name.

Dash is well-known as the default shell for Ubuntu and Debian. The shell is designed to be minimal and POSIX compliant, making it highly suitable for OS startup scripts. 

The executable path is /bin/dash, and /bin/sh points to /bin/dash on Ubuntu and Debian. The default and root user prompt remains consistent with the Bourne shell.

Here are some of the features of Dash:

• Experience lightning-fast execution speeds, surpassing those of bash and other shells by up to 4 times.
• Makes use of significantly less disk space, CPU, and RAM in comparison to other options.

One downside is that the dash is not compatible with bash. Features that are not included in dash are referred to as “bashisms.” 

Thus, bash scripts need some modifications to eliminate bash-specific features in order to run smoothly.

6. The Bourne Again Shell (bash)

Created in 1989, the Bourne Again Shell is a Unix shell and command language that builds upon the functionality of the Bourne shell (sh). The shell program serves as the default login shell for numerous Linux distributions and older iterations of macOS.

Bash is the default shell on most Linux distributions, including but not limited to, Debian, Ubuntu, Fedora, CentOS, and openSUSE. It is also the default shell in macOS.

The shell name is shortened to bash, and its location is /bin/bash. Similar to the Bourne shell, the bash prompt uses $ for a regular user and # for root.

Bash introduces additional features that are not present in the Bourne shell, including:

• Expanding braces.
• Auto-suggestion.
• Debugging and signal handling are fundamental aspects of computer systems analysis.
• Reviewing past commands.
• Conditional commands, such as the if and case statements in bash.
• Support for heredoc syntax.

Keep in mind that Bash incorporates certain features that have been borrowed from other shells.

Given that bash is a superset of the Bourne shell, most sh scripts can be executed in bash without requiring any additional modifications.

7. Z Shell (zsh)

The Z shell (zsh) was developed in the early 1990s as an extension for the Bourne shell, a Unix shell. The shell is designed with a focus on functionality, drawing inspiration from ksh and tcsh to provide a robust and user-friendly alternative.

The zsh has become the default shell in macOS starting from macOS Catalina (10.15).

The location of the executable is /bin/zsh. The prompt format for regular users is user@hostname location %, while for the root user it is hostname#. The Z shell is the default shell for Kali Linux and Mac OS.

There are several new features that have been added to the zsh.

• Сommon history for all shell sessions that are currently active.
• Improved array and variable management.
• Improving spelling accuracy and automatically suggesting command names.
• Multiple compatibility modes available.
• Increased functionality with the use of plugins.

The shell can be easily customized and configured thanks to the extensive support provided by the Oh My Zsh framework, which is driven by a dedicated community.

8. User-friendly Interactive Shell (fish)

The Friendly Interactive Shell (fish) is a Unix shell that was released in the mid-2000s, prioritizing usability as its main focus. The shell comes fully equipped with all the necessary features, so there’s no need for any extra setup. This makes it incredibly easy to use right from the beginning.

The fish is not typically the default shell on any major distribution, but it is available in the repositories of most Linux distributions and can be set as the default shell by users.

The executable path is set to /usr/bin/fish by default. The user prompt is displayed as user@hostname location, while the root prompt is shown as root@hostname location#.

The shell has several features:

• Enhanced recommendations and auto-fill options derived from the historical record of the current directory.
• Providing syntax highlighting and clear error messages to assist users.
• Configuring settings through a web interface.
• Accessing and managing command history with advanced search options.

One major issue with fish is its lack of POSIX compliance. Nevertheless, the developers are determined to enhance the imperfect designs from POSIX.

In summary

The diverse landscape of Linux shells offers a toolkit tailored for various user needs, from system scripting to interactive use. Whether optimizing for performance with `dash`, enhancing script capabilities with `bash`, or exploring advanced user features in `zsh` and `fish`, each shell brings unique strengths to the table. This versatility not only underscores the adaptability of Linux environments but also empowers users to fine-tune their command line interface for an efficient, customized experience.

Having familiarized yourself with various shell types in this article, you now have a comprehensive understanding of the Linux shells that are at your disposal. There are various shells available for specific purposes, each designed to tackle different challenges.