Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!ogicse!hakanson
From: hakanson@ogicse.ogi.edu (Marion Hakanson)
Newsgroups: comp.lang.perl
Subject: Re: Perl beginner
Message-ID: <22198@ogicse.ogi.edu>
Date: 31 May 91 21:13:39 GMT
References: <EcEkB=i00VsnE622gr@andrew.cmu.edu> <azWk8Os5gwrTw@idunno.Princeton.EDU> <1991May31.161535.2023@eci386.uucp> <1991May31.181659.28817@jpl-devvax.jpl.nasa.gov>
Organization: Oregon Graduate Institute (formerly OGC), Beaverton, OR
Lines: 64

In article <1991May31.181659.28817@jpl-devvax.jpl.nasa.gov> lwall@jpl-devvax.JPL.NASA.GOV (Larry Wall) writes:
>. . .
>But also see pwd.pl in the Perl library.  It uses the environment variable
>as a guess, but runs pwd if the guess is wrong.  Thenceforth it can keep
>track of the current directory for you, as long as you call its chdir
>routine to do all your chdirs.
>
>My policy heretofore has been to drag my feet on adding routines that
>supply information that you only need once per process.  Besides, it's
>good to force C programmers to use the toolbox occasionally.  :-)

Second thing first: I disagree that you only need such information
(the current working directory) once per process.  In the presence of
symbolic links, the only method I've come up with for finding the
actual full path translation of a given path (relative or starting
with "/") is to chdir() to the destination (or one "/" above, if it's
not a directory) and to then find the "real" current directory.  I
have used this technique on more than one occasion, and without fail I
have wished I didn't have to start up a "pwd" process to get this
information.

Now, to pwd.pl.  It is quite unsatisfactory to me, in that it gives
incorrect results in the presence of symbolic links.  Under these
(very common) conditions, the only way to find the current working
directory is to ask the operating system.  The alternative is to
model the OS in your perl library.  Most versions of Unix these days
have a nice getwd() call -- I sure wish Perl could access it directly
without running another program.

===== pwdtst =====
#!/usr/bin/perl

require('pwd.pl');

&initpwd;

$pwd = `pwd`;
print "PWD=$ENV{'PWD'} pwd=$pwd";

&chdir($ARGV[0]) || die "Can't chdir($ARGV[0]): $!, aborted";

$pwd = `pwd`;
print "PWD=$ENV{'PWD'} pwd=$pwd";
==================
=== test script ==
Script started on Fri May 31 13:55:37 1991
ogicse 51% pwd
/usr/etc
ogicse 52% ls -lg local
lrwxr-xr-x  1 root     System         14 Apr 20 04:51 local -> /usr/local/etc
ogicse 53% ~hakanson/perl/tests/pwdtst local
PWD=/usr/etc pwd=/usr/etc
PWD=/usr/etc/local pwd=/usr/local/etc
ogicse 54% ~hakanson/perl/tests/pwdtst local/..
PWD=/usr/etc pwd=/usr/etc
PWD=/usr/etc pwd=/usr/local
ogicse 55% exit
ogicse 56%
script done on Fri May 31 13:56:51 1991
==================

-- 
Marion Hakanson         Domain: hakanson@cse.ogi.edu
                        UUCP  : ogicse!hakanson