NetNews Usenet Archive 1992 #26/NN_1992_26.iso/spool/comp/lang/perl/6869

Internet Message Format | 1992-11-07 | 2.0 KB

Path: sparky!uunet!europa.asd.contel.com!emory!sol.ctr.columbia.edu!spool.mu.edu!agate!doc.ic.ac.uk!uknet!reading!nrp
From: [e-mail] ("Scented fads" Neilski)
Newsgroups: comp.lang.perl
Subject: Re: explanation of the camel book code needed
Keywords: sh csh perl ksh running with shells
Message-ID: <nrp.721043320@reading>
Date: 6 Nov 92 09:48:40 GMT
References: <[e-mail]>
Sender: [e-mail]
Reply-To: [e-mail]
Organization: University of Reading
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Nntp-Posting-Host: angeles

[e-mail] (Chris Hare) writes:

>On pg 216 of the camel book is a description of how to make perl run from
>shells which do not support the #! construct.  I need a conscise, word by
>word, letter by letter explanation, as I'm somewhat lost o how this is
>interpreted by the shells, and what it all means.

I don't have the camel book (a serious shortcoming, I know) but I assume
that you're referring to something like the output produced by a2p and s2p:

#!/gnu/bin/perl
eval 'exec /gnu/bin/perl -S $0 ${1+"$@"}'
    if $running_under_some_shell;

OK.  The first line is obviously the #! line.  The next two lines are split
because shells interpret one line at a time, whereas perl goes to
semicolons.  The shell comes along and sees the eval line, and executes it.
This gets the perl program run with all the right arguments, and since it
contains an exec the shell goes no further, but the eval makes it return
the exit code produced by the perl program.

Now perl comes along and finds the two-line statement, which says "eval
this if the variable $running_under_some_shell is true".  Since we're at
the beginning of the program $running_under_some_shell has not been defined
and so the eval is not performed, and perl goes on with the rest of the
program.

Simple! (?)

-- neilski

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