Re: Swapping Files

Let me see if I understand:

before doing anything, files A and B are both already present
your program is supposed to modify the contents of A in some manner
after file A has been updated and closed, you want the existing content of B to have the file name "A", and you want the newly updated content of the existing "A" file to have the file name "B"

Another way to phrase that last step, based on your second point, is that the existing "B" file (which had been considered "temp" data) becomes the new "A" file (no longer temp data?), whereas the existing "A" file, after being update by the script, becomes the new "B" file (to be considered henceforth as "temp" data?) That does seem strange, but... whatever.

Usually, the only way to swap file names is by a three-step process:

  rename "A", "A.prime";
  rename "B", "A";
  rename "A.prime", "B";
[download]

(though you'll want to come up wiht a "third-name" string that is certifiably guaranteed to be unique and novel, so you don't inadvertently obliterate some other file in the process.

Comment on Re: Swapping Files Download Code

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Re^2: Swapping Files by deadpickle (Pilgrim) on Oct 08, 2007 at 21:06 UTC
Her is the code, I keep getting the error on the rename command. Anyone know why? #!/usr/bin/perl #~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +~ #Reads in uavposition and outputs the uavpf.txt file # #~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +~ use strict; use warnings; $\="\n"; my $img = '"http://updraft.unl.edu/~uas/uas/uas.png"'; for(;;){ #Open Placefile open OUT,'>',"/home/uas/public_html/uas/uaspf.txt.bak" or die +"Cannot Open File uavpf.txt.bak!"; #print placefile elements print OUT "Title: UAV GPS Position"; print OUT "Threshold: 999"; print OUT "RefreshSeconds: 2"; print OUT "Iconfile: 1, 22, 21, 10, 14, $img\n"; #read in UAV GPS lines #Open UAV GPS file open IN,'<',"/home/uas/Scripts/COORDS/uasposition" or die "Can +not Open File uavposition!"; while (my @gps = <IN>){ chomp @gps; my $ngps = @gps; my @uav = split(" ",$gps[$ngps-1]); my $lat1 = sprintf("%.5f", $uav[0]); my $lon1 = sprintf("%.5f", $uav[1]); my $head1 = sprintf("%.2f",$uav[2]); print OUT "Icon: $lat1, $lon1, $head1, 1, 1, Latitude: + $lat1 Longitude: $lon1 Heading: $head1 "; } close (IN); close (OUT); rename("uaspf.txt", "uaspf.txt.bak") or die"Unable to replace" +; sleep 2; } [download]	[reply] [d/l]
Re^3: Swapping Files by BrowserUk (Patriarch) on Oct 09, 2007 at 04:27 UTC
You have the order of the names in the rename call reversed. The old name comes first, then new name second. Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice. "Too many [] have been sedated by an oppressive environment of political correctness and risk aversion."	[reply]
Re^3: Swapping Files by toolic (Bishop) on Oct 09, 2007 at 02:14 UTC
I do not know why, but here are some things to look at. Does printing the value of `$!` give you any clues? `rename("uaspf.txt", "uaspf.txt.bak") or die"Unable to replace:$!\n";` Alos, take a look at rename because there are portability issues with this function.	[reply] [d/l] [select]