Clunch protection

[Arthur David Olson]

brl-tgr!gwyn:

> #!/bin/sh
> on line 1 will let the kernel exec the script using the correct shell
> no matter what shell the user is running at the time.

1.  The "#!/bin/sh" line lacks a space or tab between the "#!" and the
    "/bin/sh".  Quoting the second page of the exec(2) page in the UNIX
    Programmer's Manual for the Fourth Berkeley Software Distribution
    (dated 4/1/81--April Fools' Day), "The space (or tab) following the '#!'
    is mandatory."  While some implementations may not require the space, it
    behooves those of us interested in portability to use the documentation
    when trying to create portable scripts, rather than finding out what we
    can "get away with" on a particular system.

2.  The "#!" construct is not recognized on all UNIX systems.  Some may respond
    that anyone who'd continue running a system that fails to recognize "#!"
    "ougtht to" upgrade.  Alas, in the real world, there are legitimate
    economic, political, and legal reasons for staying with older versions of
    UNIX; those using older versions are ill-served by folks who would lecture
    them to upgrade rather than try to keep their needs in mind.

3.  Even if the "#!/bin/sh" line is present at the start of a shell script,
    "/bin/sh" will NOT be used to interpret the script if the script is
    executed using the command
	csh script
    (This is, at any rate, true on 4.1bsd and 4.2bsd).  "Clunch protection"
    lines are designed to guard against this possibility.  Some may respond
    that anyone who'd use the above command is a clunch; a moment's
    reflection. . .
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