How do you write a portable shell script? The programs invoked by your shell script need to behave the same everywhere. Even fundamental things like cp, rm, etc. don’t universally behave the same across the various Unix and Unix-like systems.
The joke is that your shell is actually more portable than your shell script :)
Most of the shell utilities described by posix have standard flags, and then gnu/bsd extra flags.
If you use the standard ones (and use the "posix mode" flags when available) you're mostly ok.
Also, a shell script can have logic to handle different tools available (either different flavours of the same tool or even different tools that do similar things).
If the basic syntax it uses (or the shebang) are bash specific then you need bash to run it.
The joke is that your shell is actually more portable than your shell script :)
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