NetNews Usenet Archive 1992 #26/NN_1992_26.iso/spool/comp/lang/perl/6871

Text File | 1992-11-07 | 2.5 KB | 64 lines

Newsgroups: comp.lang.perl
Path: sparky!uunet!zaphod.mps.ohio-state.edu!uunet.ca!unilabs!chare
From: [e-mail] (Chris Hare)
Subject: Re: explanation of the camel book code needed
Message-ID: <[e-mail]>
Keywords: sh csh perl ksh running with shells
Organization: UniLabs Research Group
References: <[e-mail]> <nrp.721043320@reading>
Date: Fri, 6 Nov 1992 11:31:02 GMT
Lines: 52

In article <nrp.721043320@reading> [e-mail] writes:
>[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;
>
The code in the book actually is
#!perl
eval '(exit $?0)' && eval 'exec perl -S $0 ${1+"$@"}'
& eval 'exec perl -S $0 $argv:q'
if 0;

>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.
Yup.
>
>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.
Yup.
Now what I need is a concise explanation as to this
   eval '(exit $?0)' && eval 'exec perl -S $0 ${1+"$@"}'
   & eval 'exec perl -S $0 $argv:q'
What is perl actually interpreting?
>--
>[e-mail]  \--------------------------------------------------
>----------------------------\ I'm the kinda guy who likes to have
>his own ... special odour.  And that's why I wear ... TUNA FISH SANDWICH!
>
Who was it that said that anyway??

Thanks!

--
????                --- It Always works for me! ---
Chris Hare, UNIX Scientist                  Preferred : [e-mail]
Just a mild-mannered consultant by day ....