Add GitHub Actions with ShellCheck #951

Fix SC2197: fgrep is non-standard and deprecated. Use grep -F instead. https://www.shellcheck.net/wiki/SC2197

Fix SC2164: Use 'cd ... || exit' or 'cd ... || return' in case cd fails. https://www.shellcheck.net/wiki/SC2164

Fix SC2128: Expanding an array without an index only gives the first element. https://www.shellcheck.net/wiki/SC2128

Ignore SC1090: ShellCheck can't follow non-constant source. Use a directive to specify location. https://www.shellcheck.net/wiki/SC1090 See the following discussion for why SC1090 is disabled and not fixed: koalaman/shellcheck#507

Fix SC2166: Prefer [ p ] && [ q ] as [ p -a q ] is not well defined. https://www.shellcheck.net/wiki/SC2166

Fix SC2162: read without -r will mangle backslashes. https://www.shellcheck.net/wiki/SC2162

Fix SC2091: Remove surrounding $() to avoid executing output (or use eval if intentional). https://www.shellcheck.net/wiki/SC2091

Ignore SC2034: foo appears unused. Verify use (or export if used externally). https://www.shellcheck.net/wiki/SC2034

Ignore SC2154: yellow is referenced but not assigned. https://www.shellcheck.net/wiki/SC2154

Fix SC2155: Declare and assign separately to avoid masking return values. https://www.shellcheck.net/wiki/SC2155

Fix SC1018: This is a unicode non-breaking space. Delete and retype it. https://www.shellcheck.net/wiki/SC1018

Fix SC2124: Assigning an array to a string! Assign as array, or use * instead of @ to concatenate. https://www.shellcheck.net/wiki/SC2124

Fix SC2199: Arrays implicitly concatenate in [[ ]]. Use a loop (or explicit * instead of @). https://www.shellcheck.net/wiki/SC2199

Fix SC2181: Check exit code directly with e.g. 'if mycmd;', not indirectly with $?. https://www.shellcheck.net/wiki/SC2181

Fix SC2046: Quote this to prevent word splitting. https://www.shellcheck.net/wiki/SC2046

Ignore SC2139: This expands when defined, not when used. Consider escaping. https://www.shellcheck.net/wiki/SC2139

Fix SC2140: Word is of the form "A"B"C" (B indicated). Did you mean "ABC" or "A\"B\"C"? https://www.shellcheck.net/wiki/SC2140

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