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Writing portable Bash scripts across different UNIX flavors

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Writing Portable Bash Scripts Across Different UNIX Flavors

When it comes to scripting on UNIX-like systems, Bash (Bourne Again SHell) is one of the most popular shells due to its extensive availability and functional script processing capabilities. However, creating Bash scripts that run seamlessly across different UNIX flavors and Linux distributions can be challenging due to differences in installed packages, default shell environments, and specific system utilities. In this blog, we’ll explore effective ways to ensure that your Bash scripts are portable and provide guidance on managing package dependencies across various distributions using different package managers like apt, dnf, and zypper.

1. Understand the Environment

It’s important to recognize the commonalities and differences across UNIX-like systems. Linux distributions might use different init systems (e.g., Systemd, Upstart), file system layouts, and core utilities (GNU vs. BSD tools). Before writing your script, check the default shell, as not every system will link /bin/sh to /bin/bash. Use env to ensure your script uses bash when needed:

#!/usr/bin/env bash

2. Rely on POSIX Standards

To maximise compatibility, write your scripts following the POSIX (Portable Operating System Interface) standards. While Bash has many convenient extensions and features, they might not be supported on systems with a more minimal or strictly POSIX-compliant shell. For instance, instead of Bash arrays, consider using simple text processing tools like awk or sed that are generally available.

3. Manage External Dependencies Gracefully

Not all systems will have the packages your script needs, or they might be named differently across different package managers. Begin your script by checking for these dependencies or setting up the environment quietly. Here’s how you can ensure the essential packages are installed across different systems:

  • Debian/Ubuntu (apt):

    if ! command -v curl &> /dev/null; then
sudo apt update && sudo apt install curl -y
fi

  • Fedora/RHEL/CentOS (dnf):

    if ! command -v curl &> /dev/null; then
sudo dnf install curl -y
fi

  • openSUSE (zypper):

    if ! command -v curl &> /dev/null; then
sudo zypper install curl
fi

Use conditional blocks to differentiate between environments, or even better, use a case statement if multiple possibilities exist based on the $(. /etc/os-release && echo $ID) value.

4. Handle Variations in Utilities

Some utilities, such as sed, behave differently on GNU systems compared to their BSD counterparts (commonly found in systems like macOS). For example, the -i (in-place editing) option requires an explicit empty string argument sed -i '' on BSD-like systems, whereas on GNU systems the empty string is optional. To handle this, check the utility version in your script or manage divergent behavior through conditionals:

if sed --version 2>/dev/null | grep -q GNU; then
  sed -i "s/search/replace/g" file.txt
else
  sed -i '' "s/search/replace/g" file.txt
fi

5. Test Across Environments

Finally, the most crucial part of writing portable bash scripts is testing. Regularly test your scripts in as many different environments as possible. Tools like Docker or virtualization software can be immensely helpful to simulate different systems.

Conclusion

Writing portable Bash scripts requires careful planning and a bit of research into the environments where the scripts will run. It's crucial to adhere to common standards and practices, prepare for environment-specific differences, and include graceful degradation and dependency checks. Effectively managing these components can lead to successful script execution across various UNIX and Linux distributions, increasing the robustness and reliability of your scripts in diverse deployments.

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