What is a Shell?

Shell: Takes input from the user in the form of commands, processes it, and then gives an output.

• Interface through which a user can work on the programs, commands, and scripts.
• Accessed by a terminal which runs it.
• While a kernel is the innermost part of an operating system, a shell is the outermost part.
  • Like a wrapper for the UNIX kernel, protecting it (hence, shell)
• Functionality:
  • Interactive use
  • Customization of your UNIX session
  • Programming
• Automatically executed when you log on and provides a CLI to the user.
• A program like any other.

Basic Form of Shell:

while (read command) {
  parse command
  execute command
}

Shell Families

Two main shell families in UNIX:

1. The Bourne Shell: Prompt is $.
   • POSIX Bourne Shell: sh
     • /bin/sh
   • Korn Shell: ksh
     • /bin/ksh
   • Bourne Again Shell: bash
     • /bin/bash
2. C Shell: Prompt is %.
   • C Shell: csh
     • /bin/csh
   • Trusted C Shell: tcsh
     • /bin/tcsh

Note: Z Shell (zsh) is inspired by both Korn shell and Trusted C Shell, and thus doesn’t fit within a single shell family.

History

The original Bourne shell distributed with V7 Unix in 1979.

• Became the standard shell for writing shell scripts.
• Written by Steve Bourne
• Still to be found in /bin/sh on many commercial Unix systems.

Berkeley C shell (csh) offered features that were more pleasant for interactive use.

• Additional features:
  • Command history
  • Job control.
• For a long time the standard practice was to use the Bourne shell for programming and the C shell for daily use.

David Korn at Bell Labs enhanced the Bourne shell by adding csh-like features to it.

• Additional features:
  • History
  • Job control
  • Additional programmability.
• Eventually, the Korn shell’s feature set surpassed both the Bourne shell and the C shell, while remaining compatible with the Bourne shell for shell programming.

Today, the POSIX standard defines the “standard shell” language and behavior based on the System V Bourne shell, with a selected subset of features from the Korn shell.

The Free Software Foundation, in keeping with its goal to produce a complete Unix work-alike system, developed a clone of the Bourne shell written from scratch, the Bourne-Again Shell.

• Over time, Bash has become a POSIX-compliant version of the shell, with many additional features.
• A large part of these additional features overlap the features of the Korn shell, but Bash is not an exact clone of the Korn shell.

Shell Source Code

Common Features in Bash, Korn, and C Shells

Different Features in Bash, Korn, and C Shells

Learning a Shell

• Invoking the shell
• Syntax
• Functions
• Variables
• Arithmetic expressions
• Command history
• Job control
• Command execution
• Restricted shells
• Built-in commands

Shell Script

Shell Script: A regular text file that contains shell or UNIX commands.

• Useful for automating repetitive tasks and administrative tools and for storing commands for later execution.
• Needs execution permissions.
• Spawns a child process irrespective of the default shell.
  • The child process is a non-interactive shell that runs the commands in the shell script.

Shebang (#!)

Shebang: Specifies shell to execute a program.

• Begins with #!.
• As the absolute path of shells can differ from system-to-system, it can be beneficial to use the env utility to search the $PATH environment variable for the location of a shell.
  • A bit overkill for Bash, but can be useful for other binaries like Python, which can be located in a wide variety of places.

Examples

• #!/bin/sh
• #!/bin/bash
• #!/bin/csh
• #!/usr/bin/tcsh
• #!/usr/bin/env bash

Recommended Extensions

• .sh: Bourne shell or Bourne Again Shell script
• .csh: C shell script
• ksh: Korn shell script

Remember: File extensions don’t matter in UNIX, but can be helpful for organization.

Comments

The hash symbol (#) can be used to create comments.

Example:

#!/usr/bin/env bash
echo "Hello, world!" # This is a comment