A shell is a special user program that provides an interface for the user to use operating system services. Shell accepts human-readable commands from users and converts them into something which the kernel can understand. It is a command language interpreter that executes commands read from input devices such as keyboards or from files. The shell gets started when the user logs in or starts the terminal.

1. Bash (Bourne Again Shell)

• Description: Bash is the default shell on most Linux distributions and a powerful scripting language for command-line operations and automation.
• Features:
  • Widely supported across Unix-like systems, making scripts highly portable.
  • Built-in commands for control flow (if, for, while, case) and file handling.
  • Supports arrays, functions, and arithmetic operations.
• Common Use Cases: System automation, task scheduling, data processing, and general scripting.

2. Zsh (Z Shell)

• Description: Zsh is similar to Bash but offers additional features and improved usability. Many users prefer it for interactive use due to its customization options.
• Features:
  • Extended globbing (pattern matching) for advanced file matching.
  • Better auto-completion and auto-correction.
  • Customizable prompt and themes with frameworks like Oh My Zsh.
  • Built-in support for plugins to extend functionality.
• Common Use Cases: Interactive shell, customization for power users, scripting with enhanced syntax.

3. Ksh (KornShell)

• Description: Ksh was developed as an enhanced version of the original Bourne shell (sh) and combines elements from both the Bourne and C shells.
• Features:
  • Powerful scripting capabilities similar to Bash, with some unique syntax.
  • Support for associative arrays (hash tables).
  • Built-in floating-point arithmetic support.
• Common Use Cases: Advanced scripting tasks in enterprise environments, especially where performance is critical.

4. Tcsh (TENEX C Shell)

• Description: Tcsh is an enhanced version of the C Shell (csh) with additional features for interactivity.
• Features:
  • Syntax based on C language, making it easier for C programmers to pick up.
  • Command-line editing and history, which were not part of the original csh.
  • Auto-completion and spelling correction.
• Common Use Cases: Interactive shell sessions, though less common for scripting due to limited portability and features compared to Bash and Zsh.

5. Dash (Debian Almquist Shell)

• Description: Dash is a minimal POSIX-compliant shell used mainly for system scripts in Debian-based distributions.
• Features:
  • Fast and lightweight, with low memory usage.
  • Designed strictly for POSIX compliance, making it more portable than Bash.
  • Commonly used as the default /bin/sh shell on Debian-based systems.
• Common Use Cases: System initialization scripts, where performance is crucial, and strict POSIX compliance is required.

6. Fish (Friendly Interactive Shell)

• Description: Fish focuses on user-friendliness and simplicity, providing a modern alternative to traditional shells.
• Features:
  • Built-in syntax highlighting and autosuggestions.
  • No need for configuration files (like .bashrc or .zshrc) since it has sensible defaults.
  • Web-based configuration for customization.
• Common Use Cases: Interactive shell sessions, especially for users who want a shell that “just works” without complex configuration.

7. ASH (Almquist Shell)

• Description: ASH is a lightweight and POSIX-compliant shell, originally developed for embedded systems.
• Features:
  • Minimalist design for low-resource environments.
  • Fast execution, making it suitable for small, embedded devices.
• Common Use Cases: Embedded Linux systems, such as those found in routers and other networking equipment.

Each of these shells has its niche, balancing between ease of use, portability, and scripting capabilities. For scripting, Bash remains the most commonly used, but Zsh and Fish have gained popularity among users who spend a lot of time in the shell due to their interactive features and customizability.