Path: utzoo!attcan!uunet!husc6!bloom-beacon!tut.cis.ohio-state.edu!mailrus!ames!killer!vector!rpp386!pigs!haugj
From: haugj@pigs.UUCP (The Beach Bum)
Newsgroups: comp.unix.xenix
Subject: Re: "#!" scripts, kernel patches, and Xenix
Summary: ah! sco speaks up ...
Message-ID: <405@pigs.UUCP>
Date: 13 Sep 88 14:57:30 GMT
References: <161@tessera.UUCP> <858@viscous>
Reply-To: haugj@pigs.UUCP (The Beach Bum)
Organization: The Big "D" Home for Wayward Hackers
Lines: 43

In article <858@viscous> rosso@sco.COM (Ross Oliver) writes:
>In article <161@tessera.UUCP> jtc@tessera.UUCP (J.T. Conklin) writes:
>>Has anyone patched the kernel exec routine to execute interpreter
>>files ("#!" scripts)?
>
>The exec() function executes binaries only.  If exec() fails (which
>it does on a shell script), then it is up to your shell to decide
>what to do.

[ this process has been described in the past.  it was something
  which ken thompson (?) added to the research kernel years ago. ]

there are unix systems where exec() in the kernel knows about shell
scripts.  the magic number #! (one of 021441 or 020443) is trapped by
the kernel, which then scans the rest of the line from the executable
file.

the remainder of the line (which is going to be a binary executable)
is then used in place of the first argument to exec.  an exec call
of the form

	execlp ("myscript", "myscript", "arg", 0);

gets converted internally, after seeing '#! /usr/lbin/perl' say, to

	execlp ("/usr/lbin/perl", "myscript", "myscript", "arg", 0);

[ this is from faulty memory.  the second argument may become the
  interpreter name as well. ]

the first block of the executable is read in and checked for magic
numbers so all of this can be dealt with before the argument list
is constructed for the new task.

it really would be best if sco added this feature.  fork(), exit()
and sbrk() are in the same file (os/sys1.c) [ run nm(1) on your
libraries for more information ] as exec().  you do not want to
disassemble that much code and then expect the result to work ;-)
-- 
=-=-=-=-=-=-=-The Beach Bum at The Big "D" Home for Wayward Hackers-=-=-=-=-=-=
               Very Long Address: John.F.Haugh@rpp386.dallas.tx.us
                         Very Short Address: jfh@rpp386
                           "ANSI C: Just say no" -- Me