What is the Difference Between Bash and Sh on Linux or UNIX Systems?

When working with Linux and UNIX systems, it's important to understand the differences between the Bourne Shell and the Bash Shell. This article will explore the differences, compatibility, and common usage of these shells in the context of shell scripts.

Overview of Bourne Shell /bin/sh

The Bourne Shell, represented by /bin/sh, is the traditional Unix system command processor and login shell. It is a lightweight and simple shell designed for efficient command-line use. Despite its simplicity, it has served as the foundation for many other shells, including Bash.

Introduction to Bash Shell /bin/bash

The Bourne-Again Shell, commonly referred to as /bin/bash, is the GNU/Linux system command processor and login shell. It was developed as a rewrite of the original Unix shell and is highly compatible with the Bourne Shell while introducing many new features. Bash is widely used in both Linux and UNIX systems due to its robustness and extensive functionality.

Key Differences Between Bash and Bourne Shell

The primary differences between Bash and the Bourne Shell lie in their features and compatibility:

1. Functionality and Features

Bash has many more built-in operators and commands compared to the Bourne Shell. This allows for more efficient and versatile script writing. Additionally, Bash offers several enhancements that make it more user-friendly and powerful, such as extended globbing and advanced string manipulation functions. These features are not available in the Bourne Shell.

2. POSIX Compliance

Bash is not fully compliant with the POSIX standard due to its additional features. However, it can be made to conform to POSIX behavior by setting the POSIXLY_CORRECT environment variable. Debian-based systems often use /bin/sh as a symlink to /bin/dash, which is more compliant with POSIX, while Red Hat-based systems typically link /bin/sh to /bin/bash.

3. Performance

Bash is generally faster than the Bourne Shell when running scripts due to its more optimized internal mechanisms and built-in commands. This performance difference is often negligible for most casual users but significant for more intensive scripting tasks.

Shell Selection and Portability

In Unix systems, the shell is user-selectable, which means multiple shells can coexist on the same system. Red Hat-like Linux distributions typically use /bin/sh as a symlink to /bin/bash, ensuring a seamless experience for most users. However, it's a best practice when writing scripts to use #! /bin/sh as the shebang line. This guarantees that the script will work correctly regardless of the underlying shell being used on different systems.

Other Shell Choices

While Bash is the default shell for many Linux distributions, other shells like zsh and dash are also popular. For instance, MacOSX uses zsh and Debian-based systems may use /bin/sh as a symlink to /bin/dash. These shells often have distinctive features and configurations that suit different user preferences:

zsh

As an alternative to Bash, zsh offers some syntactic differences and a broader range of features, such as enhanced configurability, faster filename completion, and more flexible aliases. It is also more POSIX-compliant than Bash. Zsh replaces Bash in some systems due to its advanced capabilities and faster performance in certain scenarios.

dash

/bin/dash is a BASH-like shell that is compatible with the Bourne Shell and is more POSIX-compliant than Bash. Debian-based systems often use it as a fallback shell for /bin/sh to ensure a higher degree of portability across different systems.

Conclusion

While both the Bourne Shell and Bash are powerful command processors, Bash offers many additional features and is more widely used in modern Linux distributions. However, using /bin/sh with #! /bin/sh in scripts ensures better portability and compatibility across different systems. The choice of shell often depends on the specific requirements and user preferences.