Stop half-POSIXing your shell scripts

You cannot:

• use local to restrict a variable’s scope to the current function, which is bad news for functional programmers: Shell without local … is like Lisp without (let …).
  
• use mapfile or readarray to read contents into an array.
  
• use declare or typeset to set variable values and attributes (particularly relevant for array declaration and indirect references).
  
• use source or . to source files, which is as close as you can get to libraries around there.
  
• use caller to determine whether your script is being sourced, and from where — as used, e.g., at the very end of Ecos’ source.
  
• use compgen and complete, without which you can no longer configure and offer command line completion.
  
• use builtin printf’s additional flags — such as -v to store the result in a variable instead of printing.
  

That’s only a part of it. Much of the loss happens at the level of syntax.

If you haven’t yet read How to do things safely in Bash, make it a priority. Your Bash will improve.

When explaining why focus on Bash instead of other shells, it dismisses POSIX right off the bat:

This guide is here to show that bash can be used safely.
…
Clearly, bash is a bad choice, but other prevalent alternatives are not better:

• POSIX shell (a language subset that many shells support) lacks arrays. → Disqualified.
  
  • Hereunder: dash, busybox ash
    

…
Arrays is the feature that becomes absurdly impractical to program correctly without.
…
The recommendation of this guide must therefore be to not give POSIX compatibility a second thought. The POSIX shell standard is hereby declared unfit for our purposes. Likewise, sadly, for minimalistic POSIX compatible shells like Dash and Ash that don’t support arrays either. As for Zsh, it supports a superset of Bash’s array syntax, so it is good.

I can come up with counterexamples where a lack of arrays is not an issue, but this quickly gets harder in scripts beyond a certain (not very large) size or complexity.

So portability may come with a price whenever POSIX compatibility imposes this sort of trade-off. Your script may well reach more people — but less safely.

You’re shipping an installation script for a program that targets all sorts of architectures and operating systems. If you can write one good, safe, short, POSIX-compliant script that can find out about its environment to install the program, then you might not need to write a second one.

POSIX also seems perfectly adequate for tiny wrappers such as this one:

#!/bin/sh
export PATH=/usr/bin
exec gzip -9 "$@"

to be saved as $HOME/bin/gzip and chmod +x’d. When invoking gzip, this would be called instead of the binary /usr/bin/gzip, to which it then immediately surrenders control with the -9 flag passed. This assumes that $HOME/bin is prepended to your PATH.

This script is suggested by gzip itself as an example to replace the no longer recommended environmental variable GZIP — that is, instead of setting GZIP="-9" in, say, your .bash_aliases, you use that script.

Linting it as Bash, it outputs nothing: no error.

shellcheck -s bash -x mss

Linting it as POSIX sh, however...

shellcheck -s sh -x mss

In mss line 3:
maybe-she-said()
^-- SC3033 (warning): In POSIX sh, naming functions outside [a-zA-Z_][a-zA-Z0-9_]* is undefined.


In mss line 4:
{ local   foo bar zoo quux x face
  ^-----------------------------^ SC3043 (warning): In POSIX sh, 'local' is undefined.


In mss line 5:
  read -r foo bar zoo quux x face <<< "$@"
                                  ^-^ SC3011 (warning): In POSIX sh, here-strings are undefined.


In mss line 6:
  [[ -n "$face" && "${bar,,}" =~ ^[a-z]+$ ]] && ((x>foo)) &&
  ^-- SC3010 (warning): In POSIX sh, [[ ]] is undefined.
                    ^------^ SC3059 (warning): In POSIX sh, case modification is undefined.
                                                ^-------^ SC3006 (warning): In POSIX sh, standalone ((..)) is undefined.


In mss line 7:
      grep -q "${zoo:1}" < <(<<<"$quux" tr -d ,) && cat <<EOF
               ^------^ SC3057 (warning): In POSIX sh, string indexing is undefined.
                           ^-------------------^ SC3001 (warning): In POSIX sh, process substitution is undefined.
                             ^-^ SC3011 (warning): In POSIX sh, here-strings are undefined.


In mss line 8:
"I picked the number '$((RANDOM % 42))' randomly", she said,
                         ^----^ SC3028 (warning): In POSIX sh, RANDOM is undefined.


In mss line 9:
and returned to her ${BASH_REMATCH[0]}.
                    ^----------------^ SC3028 (warning): In POSIX sh, BASH_REMATCH is undefined.
                    ^----------------^ SC3054 (warning): In POSIX sh, array references are undefined.


In mss line 11:
  echo "🐱 = $_" ;}
            ^-- SC3028 (warning): In POSIX sh, _ is undefined.

This last warning implies: one cannot Ultimate Bash in POSIX.