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[timothy]

Hi folks

Hier zwei Möglichkeiten aus dem "Perl Cookbook":

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7.8. Modifying a File in Place with Temporary File
Problem

You need to update a file in place, and you can use a temporary file.
Solution

Read from the original file, write changes to a temporary file, and then rename the temporary back to the original:

open(OLD, "< $old")         or die "can't open $old: $!";
open(NEW, "> $new")         or die "can't open $new: $!";
while (<OLD>) {
    # change $_, then...
    print NEW $_            or die "can't write $new: $!";
}
close(OLD)                  or die "can't close $old: $!";
close(NEW)                  or die "can't close $new: $!";
rename($old, "$old.orig")   or die "can't rename $old to $old.orig: $!";
rename($new, $old)          or die "can't rename $new to $old: $!";

This is the best way to update a file "in place."
Discussion

This technique uses little memory compared to the approach that doesn't use a temporary file. It has the added advantages of giving you a backup file and being easier and safer to program.

You can make the same changes to the file using this technique that you can with the version that uses no temporary file. For instance, to insert lines at line 20:

while (<OLD>) {
    if ($. == 20) {
        print NEW "Extra line 1\n";
        print NEW "Extra line 2\n";
    }
    print NEW $_;
}

Or delete lines 20 through 30:

while (<OLD>) {
    next if 20 .. 30;
    print NEW $_;
}

Note that rename won't work across filesystems, so you should create your temporary file in the same directory as the file being modified.

The truly paranoid programmer would lock the file during the update.

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7.10. Modifying a File in Place Without a Temporary File
Problem

You need to insert, delete, or change one or more lines in a file, and you don't want to (or can't) use a temporary file.
Solution

Open the file in update mode ("+<"), read the whole file into an array of lines, change the array, then rewrite the file and truncate it to its current seek pointer.

open(FH, "+< FILE")                 or die "Opening: $!";
@ARRAY = <FH>;

change ARRAY here

seek(FH,0,0)                        or die "Seeking: $!";
print FH @ARRAY                     or die "Printing: $!";
truncate(FH,tell(FH))               or die "Truncating: $!";
close(FH)                           or die "Closing: $!";

Discussion

As explained in the Introduction, the operating system treats files as unstructured streams of bytes. This makes it impossible to insert, modify, or change bits of the file in place. (Except for the special case of fixed-record-length files, discussed in Recipe 8.13.) You can use a temporary file to hold the changed output, or you can read the entire file into memory, change it, and write it back out again.

Reading everything into memory works for small files, but it doesn't scale well. Trying it on your 800 MB web server log files will either deplete your virtual memory or thrash your machine's VM system. For small files, though, this works:

open(F, "+< $infile")       or die "can't read $infile: $!";
$out = '';
while (<F>) {
    s/DATE/localtime/eg;
    $out .= $_;
}
seek(F, 0, 0)               or die "can't seek to start of $infile: $!";
print F $out                or die "can't print to $infile: $!";
truncate(F, tell(F))        or die "can't truncate $infile: $!";
close(F)                    or die "can't close $infile: $!";

For other examples of the things you can do in-place, look at the recipes in Chapter 8.

This approach is for the truly determined. It's harder to write, takes more memory (potentially a lot more), doesn't keep a backup file, and may confuse other processes trying to read from the file you're updating. For most purposes, therefore, we suggest it's probably not worth it.

Remember to lock if you're paranoid.
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Vielleicht hilft das ja etwas weiter.

Bye
Timothy