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From: lwall@jpl-devvax.JPL.NASA.GOV (Larry Wall)
Newsgroups: comp.lang.perl
Subject: Re: How to find working directory?
Message-ID: <8974@jpl-devvax.JPL.NASA.GOV>
Date: 2 Aug 90 21:35:38 GMT
References: <139945@sun.Eng.Sun.COM>
Reply-To: lwall@jpl-devvax.JPL.NASA.GOV (Larry Wall)
Organization: Jet Propulsion Laboratory, Pasadena, CA
Lines: 71

In article <139945@sun.Eng.Sun.COM> petolino@joe.Eng.Sun.COM (Joe Petolino) writes:
: I know I can change my working directory with chdir(<string>).  Is there a 
: symmetrical operation which tells me what my working directory is (short
: of invoking pwd in a subshell, that is)?

No.

: Imagine my surprise when I found out that chdir() doesn't set $ENV{'PWD'} !

Ok.

: Do I really have to test each chdir() for success and then conditionally
: assign its argument to $ENV{'PWD'} ?

Yes and/or no, depending on how you work it.

Here's the slow but concise way:

    sub initpwd { chop($ENV{'PWD'} = `pwd`); }
    sub chdir { chdir shift; &initpwd; }

Call initpwd at the top of the show.

Note that the existence of a getwd call wouldn't speed this up all that
much--getwd is a fairly expensive operation.  You really just want to
init PWD at the beginning of the program and then keep track of it.

So here's the fast way

    sub initpwd {
	if ($ENV{'PWD'}) {
	    local($dd,$di) = stat('.');
	    local($pd,$pi) = stat($ENV{'PWD'});
	    return if $di == $pi && $dd == $pd;
	}
	chop($ENV{'PWD'} = `pwd`);
    }

    sub chdir {
	local($newdir) = shift;
	if (chdir $newdir) {
	    if ($newdir =~ m#^/#) {
		$ENV{'PWD'} = $newdir;
	    }
	    else {
		local(@curdir) = split(m#/#,$ENV{'PWD'});
		@curdir = '' unless @curdir;
		foreach $component (split(m#/#, $newdir)) {
		    next if $component eq '.';
		    pop(@curdir),next if $component eq '..';
		    push(@curdir,$component);
		}
		$ENV{'PWD'} = join('/',@curdir) || '/';
	    }
	}
	else {
	    0;
	}
    }

There will be a library package to do this in the next patch.

: The programs I'm invoking after the chdir() expect $PWD to be accurate.

Then the programs you're invoking are highly non-portable.  Only certain
shells keep track of PWD.  Any C program that uses chdir and then invokes
one of these programs will confuse it.

I stop just short of saying the programs are busted.

Larry