Question file testing

[whn]

I am having some trouble in file testing. Please see my sample code first:

[Linux|bash] 133 => cat /etc/issue
Red Hat Enterprise Linux Server release 5.3 (Tikanga)
Kernel \r on an \m

[Linux|bash] 134 => pwd
/test
[Linux|bash] 135 => ls -l
total 16
-r-------- 1 root root  28 Apr 28 18:23 log.txt
-rwxr-xr-x 1 root root 140 Apr 28 18:21 tt.pl*
[Linux|bash] 136 => cat tt.pl
#! /usr/local/bin/perl

my $f = "log.txt";
if(-w $f) {
        print "File '$f' is writable.\n";
}
else {
        print "File '$f' is NOT writable.\n";
}
[Linux|bash] 137 => ./tt.pl
File 'log.txt' is writable.

I don't understand why '-w $f' returns TRUE?

Thanks for the help.

 

[Annihilannic]

Who are you logged in as? If you are root then any file is writable, regardless of the permissions.

perldoc -f -X says for -w: "File is writable by effective uid/gid."

If you want to check the file's permissions rather than whether the current UID/GID can write the file, use the stat function.

Annihilannic.

 

[whn]

Thank you, Annihilannic.

That's what I thought (In the example above, I logged in as root) and I also tend to think that could be a potential problem.

Please take a look at following example.

1) I created two files in my home dir. I then mount my home to a remote host.

[Linux|bash] 21 => pwd
/corp/whn
[Linux|bash] 22 => whoami
whn
[Linux|bash] 23 => ls -l test?.txt
[b]-r-------- 1 whn ccasepp 26 Apr 29 11:24 test1.txt[/b]
-rw-rw-rw- 1 whn ccasepp 26 Apr 29 11:49 test2.txt

2) I then logged on as root.

[Linux|bash] 44 => pwd
/test
[Linux|bash] 45 => whoami
root
[Linux|bash] 46 => ./t2.pl /corp/whn/test1.txt
[b]File '/corp/whn/test1.txt' is writable.
Cannot open file '/corp/whn/test1.txt' for writing!![/b]

[Linux|bash] 47 => ./t2.pl /corp/whn/test2.txt
File '/corp/whn/test2.txt' is writable.
The contents of file /corp/whn/test2.txt.
============================
Can this file be written?
Append a line.

And lastly, here is my sample code of t2.pl

[Linux|bash] 48 => cat t2.pl
#! /usr/local/bin/perl

my $f = $ARGV[0];
if(!-e $f) {
  print "File '$f' does not exist.\n";
  exit;
}

if(-w $f) {
  print "File '$f' is writable.\n";
  if(open(WFH, ">>$f")) {
    print WFH "Append a line.\n";
    close(WFH);
    my $content = `cat $f`;
    print "The contents of file $f.\n";
    print "============================\n";
    print "$content";
  }
  else {
    print "Cannot open file '$f' for writing!!\n";
  }
}
else {
  print "File '$f' is NOT writable.\n";
}

What I expected was --
When /corp/whn/test1.txt is passed in, '-w $f' would return FALSE.

Again, thank you very much for your help.

 

[stevexff]

Do you mean you logged on as root on the remote system, the one that has the filesystem mounted as remote?

Steve

[small]"Every program can be reduced by one instruction, and every program has at least one bug. Therefore, any program can be reduced to one instruction which doesn't work." (Object:erlDesignPatterns)[/small]

 

[whn]

Yes, that's the case.

 

[Annihilannic]

Seems buggy to me! The shell isn't similarly fooled if you do a [[ -w filename ]] on an NFS-mounted filesystem.

Annihilannic.

 

[whn]

It's a perl bug, right, Annihilannic?

I also tested using csh. It is not fooled as you already indicated.

This is a surprise, really.

But thank you very much for your time and help.

 

[Annihilannic]

Right.

Annihilannic.