in reply to Deleting a file after it runs?

On unix, you can also say, 

unlink $0;

[download]
You can say that at any point in the program, and unix will go ahead and remove the file from the directory without losing the object of the file handle. The removal is completed and disk space released once your program closes.

$ cat > try.pl
#!/usr/bin/perl
print $0, $/;
unlink $0;
print "Feeling faint . . .\n";
$ chmod 755 try.pl
$ cp try.pl try.bak
$ ls try.*
try.bak  try.pl
$ ./try.pl
./try.pl
Feeling faint . . .
$ ls try.*
try.bak
$

[download]

After Compline,
Zaxo

Replies are listed 'Best First'.
Re^2: Deleting a file after it runs?
by liverpole (Monsignor) on Oct 06, 2005 at 21:57 UTC 
        The removal is completed and disk space released once your program closes.
    I believe the removal is happening even earlier -- after the program has compiled, and the Perl interpreter has started executing it, but potentially long before the program completes.  Try this to see what I mean: 
    #!/usr/bin/perl -w
                                                                      
+          
# Strict
use strict;
use warnings;
                                                                      
+          
# Main program
print "Check file before unlink\n"; sleep 1;   system("ls $0");
print "Unlinking the file $0\n";    sleep 1;   unlink $0;
print "Check file after unlink\n";  sleep 1;   system("ls $0");
                                                                      
+          
print "File should be now be gone from disk (but still in memory)\n";
print "Try ^Z and check it with 'ls $0' ... (come back with 'fg')\n";
sleep 10;
                                                                      
+          
print "I'm still here, but the original file is long gone.\n";

    [download]
    And when you run it: 
    % chmod +x rmtest
% rmtest
Check file 'rmtest' before unlink
rmtest
Unlinking the file 'rmtest'
Check file 'rmtest' after unlink
ls: rmtest: No such file or directory
File should be now be gone from disk (but still in memory)
Try ^Z and check it with 'ls rmtest' ... (come back with 'fg')
 
Suspended
% ls rmtest
ls: rmtest: No such file or directory
% fg
rmtest
I'm still here, but the original file is long gone.
%
[reply]
[d/l]

      You're both right. Try a bit more in your sample.

      Read more... (1237 Bytes)
      When you run this, even after the file is unlinked, the *DATA filehandle still works. That's because the file is still physically on the disk, even though it has been unlinked from the directory tree. Once the file is finally closed, the filesystem is allowed to reuse that diskspace. To prove this, I created a ram disk (thanks to a recent thread ;-}), and added a check using df to see the filesystem being used.
      Read more... (2 kB)
      In this case I ram it and got output like this: 
      $ cp z.pl /ram; perl /ram/z.pl
Check file before unlink
/ram/z.pl
Filesystem           1K-blocks      Used Available Use% Mounted on
none                       100         4        96   4% /ram
Unlinking the file /ram/z.pl
Check file after unlink
ls: /ram/z.pl: No such file or directory
Filesystem           1K-blocks      Used Available Use% Mounted on
none                       100         4        96   4% /ram
File should be now be gone from disk (but still in memory)
Try ^Z and check it with 'ls /ram/z.pl' ... (come back with 'fg')
I'm still here, but the original file is long gone.
$ df -k /ram
Filesystem           1K-blocks      Used Available Use% Mounted on
none                       100         0       100   0% /ram

      [download]
      Note how the file may have disappeared from /ram, but the disk usage wasn't actually freed up until I was done with the script.

      I re-ran the script without the __END__ marker or the usage of *DATA, and found that the script is closed after compilation as the diskspace is freed immediately. Again, you're both right. :-)

[reply]
[d/l]
[select]
        I still don't think that's right, at least for my system, running Linux.  It might be somehow different because you're using a ramdisk, but when I run this program: 
        #!/usr/bin/perl -w
                                                                      
+          
my $iam = $0;
my $lscmd = "ls -lt $iam";
my $dfcmd = "df --block-size=1 .";
                                                                      
+          
printf "Before remove\n"; system($lscmd); system($dfcmd);    sleep 1;
unlink $0;
printf " After remove\n"; system($lscmd); system($dfcmd);    sleep 1;
printf "Try ^Z to see for yourself (cmd:  $dfcmd) ...\n"; sleep 5;
printf "Now exiting the program\n";

        [download]
        it clearly shows that the space is freed up *before* the program exits.  The free space goes from 44290686976 bytes to 44290691072 (a difference of 4096 bytes) and remains at that value until after the program completes.   
        [liverpole@golux ~/perl]% chmod +x rmtest2
[liverpole@golux ~/perl]% ls -lt rmtest2
-rwxrwxr-x    1 liverpole  liverpole       352 Oct  7 12:25 rmtest2*
[liverpole@golux ~/perl]% rmtest2
Before remove
-rwxrwxr-x    1 liverpole  liverpole       352 Oct  7 12:25 rmtest2
Filesystem            1-blocks      Used Available Use% Mounted on
/dev/hda2            77584592896 29352763392 44290686976  40% /
 After remove
ls: rmtest2: No such file or directory
Filesystem            1-blocks      Used Available Use% Mounted on
/dev/hda2            77584592896 29352759296 44290691072  40% /
Try ^Z to see for yourself (cmd:  df --block-size=1 .) ...
 
Suspended
[liverpole@golux ~/perl]% ls -lt rmtest2
ls: rmtest2: No such file or directory
[liverpole@golux ~/perl]% df --block-size=1 .
Filesystem            1-blocks      Used Available Use% Mounted on
/dev/hda2            77584592896 29352759296 44290691072  40% /
[liverpole@golux ~/perl]% fg
rmtest2
Now exiting the program
[liverpole@golux ~/perl]% df --block-size=1 .
Filesystem            1-blocks      Used Available Use% Mounted on
/dev/hda2            77584592896 29352759296 44290691072  40% /

        [download]
[reply]
[d/l]
[select]

In Section Seekers of Perl Wisdom