Making Unix shell scripts POSIX compliant

Question

I have been working on a shell script to automate some tasks. What is the best way to make sure the shell script would run without any issues in most of the platforms. For ex., I have been using echo -n command to print some messages to the screen without a trailing new line and the -n switch doesn't work in some ksh shells. I was told the script must be POSIX compliant. How do I make sure that the script is POSIX compliant. Is there a tool? Or is there a shell that supports only bare minimum POSIX requirements?

Answer 1

POSIX

One first step, which gives you indications of what works or not and why, is to set the shebang to /bin/sh and use shellcheck site to analyze your script.
For example, paste this script in the shellcheck editor window:

#!/bin/sh
read -r a b <<<"$1"
echo $((a+b))

to get an indication that: "In POSIX sh, here-strings are undefined".

As a second step, you can use a shell that is as compatible with POSIX as possible.
One shell that is compatible with most other simple shells, is dash, Debian default system shell, which is a derivative of the older BSD ash.

Another shell compatible with posix is posh.

However, dash and/or posh may not be available for some systems.

There is lksh (with a ksh flavor), with the goal to be compatible with legacy (old) shell scripts. From its manual:

lksh is a command interpreter intended exclusively for running legacy shell scripts.

But there is the need to use options when calling lksh, like -o posix and -o sh:

Note that it's strongly recommended to invoke lksh with at least the -o posix option, if not both that and -o sh, to fully enjoy better compatibility to the POSIX standard (which is probably why you use lksh over mksh in the first place) or legacy scripts, respectively.

You would call lksh -o posix -o sh instead of the simple lksh.

Using options is a way to make other shells become POSIX compatible. Like lksh, using the option -o posix, like bash -o posix.

In bash, it is even possible to turn the POSIX option inside an script, with:

shopt -o posix            # also with: set -o posix

It is also possible to make a local link to bash or zsh that makes both act like an old sh shell. Like this:

$ ln -s /bin/bash ./sh
$ ./sh

There are plenty of alternatives (dash, posh, lksh, bash, zsh, etc.) to get a shell that will work as a POSIX shell.

Portable

However, even so, all the above does not ensure "portability".

Unfortunately, making a shell script 'POSIX-compliant' is usually easier than making it run on any real-world shell.

The only real-world sensible recommendation is test your script in several shells.
Like the list above: dash, posh, lksh, and bash --posix.

Solaris is a world on its own, probably you will need to test against /bin/sh and xpg4/sh.

Followup:

How can I test for POSIX compliance for shell scripts?