POSIX Is Not a Shell

2026-06-28 | original

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Why This Matters

This article highlights the critical distinction between POSIX as a specification and the actual shells that implement it, emphasizing that scripts written for POSIX compliance may behave unpredictably across different shell environments. This matters to the tech industry and consumers because it underscores the importance of understanding shell dialects and standards to ensure portability and reliability in scripting and automation tasks.

Key Takeaways

POSIX is a standard, not a shell program.
Shell behaviors like 'echo' vary across implementations, affecting script portability.
Relying on POSIX compliance alone does not guarantee consistent script behavior across different shells.

POSIX Is Not A Shell

Alexandre Gomes Gaigalas – June 28, 2026

When someone says "write it in POSIX shell for portability," they mean well.

POSIX is a specification. Not a program. The thing that actually runs your script is bash, dash, ash, ksh, yash, or one of a dozen others. They each implement POSIX with their own gaps, extensions, and historical accidents.

Here is a small experiment. One line, no flags, no functions, nothing exotic:

echo "C:

ew"

On bash, ksh, and a handful of others, you get back exactly what you typed:

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