⚖️ POSIX vs. Non-POSIX

This is the most important distinction in shell scripting.

📜 What is POSIX?

Portable Operating System Interface. A set of standards that defines how a UNIX-like OS should behave. - Goal: "Write once, run anywhere." - Benefit: Your script runs on Linux, BSD, Solaris, macOS without changes. - Cost: No fancy features (no arrays, no [[ ]], no {1..10}).

🎪 The "Bashisms" Trap

Bashisms are features specific to Bash that break in sh/dash.

Feature	❌ POSIX (sh)	✅ Bash (bash)	Why it matters
Conditionals	if [ "$a" = "$b" ]; then	if [[ $a == $b ]]; then	[[ ]] prevents word splitting bugs.
Arrays	❌ Impossible	arr=(a b c)	Critical for data structures.
Brace Expansion	❌ Impossible	echo {1..10}	Great for loops/filenames.
Process Substitution	❌ Impossible	diff <(cmd1) <(cmd2)	Powerful pipe alternative.
Functions	func() { }	function func { }	Syntax difference.

🎯 The Golden Rule

If you use #!/bin/sh, you are in a straightjacket. If you use #!/bin/bash, you are safe on 99% of Linux servers.

Shebang	When to use?
#!/bin/sh	Init scripts, Docker Alpine, embedded systems.
#!/bin/bash	General automation, CI/CD, personal scripts.
#!/usr/bin/env bash	Preferred. Finds bash wherever it is installed.