Re: Which shell and how to get started handling arguments

 Christian Weisgerber <naddy@mips.inka.de>:

On 2024-04-15, James Harris <james.harris.1@gmail.com> wrote:
>

Q1) How can one write a script which is maximally compatible with different systems? >
>
I am thinking to write in /the language of/ the Bourne shell, if feasible, so that it could be run by either the Bourne shell or Bash, etc? (Ideally, the shebang line would be #!/bin/sh.) >

>
Yes.  POSIX shell, more specifically.  That is the easy part.  The
difficult part is that your script will likely call various external
commands and those have a lot of variation as well.
>

Q2) How does one go about handling arguments in preferably a simple but universal way? >

>
That's too vague... >
>

I read up on getopts >

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If you want to handle option flags, getopts is the way to go. >
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but from tests it seems to require that switches precede arguments rather than allowing them to be specified after, so that doesn't seem very good, either. >

>
But that's the way Unix commands work.  You cannot specify flags
after the first non-flag argument.
>
$ touch foo -l
$ ls foo -l
-l   foo
$ ls -l foo -l
-rw-r--r--  1 naddy  naddy  0 Apr 15 15:28 -l
-rw-r--r--  1 naddy  naddy  0 Apr 15 15:28 foo
>
Apparently GNU implementations deviate from this, which makes for a bad surprise and is incompatible with other implementations as well as historical practice. >

 To handle this deviation, always put an end‐of‐flags marker  ("--", as specified by POSIX) before the first non‐flag argument,  then even the GNU implementations will well‐behave, i. e. behave  as specified by POSIX:  Compare (using GNU ls) with Christians well‐behaving "ls":    touch -- foo -l
   $ ls foo -l
   -rw------- 1 helmut helmut 0 Apr 15 15:28 foo
 which deviates from POSIX,    $ ls -l foo -l
   -rw------- 1 helmut helmut 0 Apr 15 15:28 foo
 which deviates from POSIX,    $ ls -- foo -l
   -l
   foo
 which behaves as specified by POSIX,    $ ls -l -- foo -l
   -rw------- 1 helmut helmut 0 Apr 15 15:28 -l
   -rw------- 1 helmut helmut 0 Apr 15 15:28 foo
 which behaves as specified by POSIX.