Understanding Shell Scripts: The Difference Between `sh` and `bash`

Introduction

In the realm of Unix-like operating systems, shell scripting is a powerful tool for automating tasks. Two primary shells often discussed are sh (Bourne shell) and bash (Bourne Again SHell). Understanding the differences between these two can help you choose the right one for your scripting needs.

What is sh?

The Bourne Shell, commonly referred to as sh, is a command language interpreter described by the POSIX standard. This means it adheres to specifications that ensure compatibility across different Unix-like systems. There are several implementations of sh, such as ksh88 and dash. On many systems, /bin/sh acts as a symbolic link pointing to one of these implementations.

Key Characteristics of sh:

• Standardized: As part of the POSIX specification, it ensures scripts run consistently across compliant systems.
• Simplicity: It offers basic scripting capabilities without the extensive features found in more modern shells like bash.
• Portability: Being a standard, it is widely available and supported on Unix-like systems.

What is bash?

Bash started as an implementation of sh, but over time it has evolved to include numerous extensions and additional functionalities. These enhancements make Bash more versatile for scripting, offering features like arrays, associative arrays, and scoped variables that are not present in the original sh.

Key Characteristics of bash:

• Feature-rich: It includes many advanced features that simplify complex scripting tasks.
• POSIX Compliance Mode: While it is not fully POSIX-compliant by default, Bash can operate in a more compliant mode using the --posix switch.
• Ubiquity: On most Linux systems, /bin/sh points to bash, making it a familiar choice for many users.

Differences Between sh and bash

While both shells are used for scripting, there are notable differences:

1. Standardization vs. Extensions:

   • sh is strictly POSIX-compliant, ensuring scripts behave the same across all compliant systems.
   • bash includes extensions that may not be portable but provide more powerful scripting capabilities.

2. Complexity and Learning Curve:

   • sh is simpler and easier to learn due to its minimalistic design.
   • bash offers a richer set of features, which can make it more complex but also more capable for advanced scripts.

3. Portability:

   • Scripts written in sh are guaranteed to run on any POSIX-compliant system.
   • bash-specific features may not work on systems where Bash is not installed or if /bin/sh does not point to Bash.

Choosing Between sh and bash

When deciding which shell to use for scripting, consider the following:

• Portability: If your script needs to run across various Unix-like systems without modification, using sh is a safer choice.
• Feature Requirements: For scripts that benefit from advanced features like arrays or improved string manipulation, bash may be more suitable.
• Environment Compatibility: Check which shell /bin/sh points to on the target system. If it’s Bash, using #!/bin/bash as your shebang line is generally safe.

The Shebang Line

The shebang line at the top of a script specifies the interpreter that should execute the script. For example:

• #!/bin/sh ensures the script uses whatever shell /bin/sh points to.
• #!/bin/bash explicitly uses Bash, if available.

Checking Shell Implementations

To determine which shell is being used by /bin/sh, you can use:

file -h /bin/sh  # For symbolic links
find -L /bin -samefile /bin/sh  # For hard links or to find all linked files

Conclusion

Understanding the differences between sh and bash is crucial for writing effective shell scripts. While sh offers simplicity and portability, bash provides a rich set of features that can make scripting more efficient. Choose the shell that best fits your needs based on the script’s requirements and the target environment.