Re: Writing to files
by plaid (Chaplain) on Apr 08, 2000 at 02:18 UTC
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Two things:
1. It's good to understand a little bit of theory as to why
you can't write before the beginning of a file. Files
are stored on disk with a fixed beginning, whether that be
at the beginning of a block or a pointer in a tree..
however the particular filesystem stores files.
So, in order to prepend
to a file, what actually has to be done is contents of the
file have to physically be moved forward, so that the
prepended string is located at the beginning of the file.
Basically, the way to do this is to read the entire
file, write out the new string, and then write out the
original contents of the file, as said above.
2. The title of the section referenced above, "How do I
change one line in a file/delete a line in a file/insert
a line in the middle of a file/append to the beginning of
a file?", doesn't make sense. Append means to add at the
end, it should be prepend.
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Re: Writing to files
by turnstep (Parson) on Apr 07, 2000 at 22:52 UTC
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The short answer is, no, not as easily as you might
think. Unlike say, word processing, you can't just go
to the start of the file, hit the 'insert' key,
and add a string: file editing is 'overwrite' mode
only. There are two basic ways to do it, and both
require you to read in all the data from the file
at some point.
- Open a new temporary file, add your string, then
open the old file. While reading from the old file,
write everything to the new file. Close both when
done, then turn the new file into the old file through
renaming.
This is the "general solution" mentioned
in the above link. It has the advantage of not using
much memory, as you only have to store one line at a
time in memory, instead of the whole file. It has the
disadvantages of using a temporary file, making the
renaming system calls, not working well with file
locking, and not being able to work with the data as
easily, although you can always read in the
whole file (@myfile = <ODLFILE>) at the cost of the
memory advantage mentioned above.
- A second way is to not use a temporary file, but just
open the file for reading and writing, slurping in
the whole file, make changes/add strings, rewind the
file via seek, then write the changed data back into
the file. One truncate and one close later, and
you are done.
This method has the disadvantages of
reading the whole file into memory, and perhaps being
harder for those not familiar with seek and truncate.
The advantages are not using a temporary file, not
having to rename, and a better ability to make
non line-by-line changes. It's also the best way to
do it if you are file locking.
I prefer the second way myself, but whatever floats
your boat - as long as it gets the job done!
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Re: Writing to files
by btrott (Parson) on Apr 07, 2000 at 22:28 UTC
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RE: Writing to files
by Anonymous Monk on Apr 08, 2000 at 07:03 UTC
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I've got a program online that does insertion into a file
(http://www.pobox.com/~japhy/tmp/perl/add_lines). But the
bigger question is, WHY DO YOU NEED TO DO THIS? Observe:
open FILE, ">>data" or die "can't append to data: $!";
print FILE "NEW RECORD\n";
close FILE;
# now, to store the file's contents in an array, MOST
# RECENT DATA FIRST, use unshift instead of push!
open FILE, "data" or die "you know the drill... $!";
unshift @contents, $_ while <FILE>;
close FILE;
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Why do you need to do what? Your question is not clear. Yes, you
can unshift into an array, but that has nothing to do with writing the file...
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Not to put words in his mouth, but the point I think japhy
(right?) was trying to make was that, if the reason you're
writing to the beginning of the file is that you think that
there's where you should store the most "recent"
information, then there's really no point.
Just keep your
file in chronological order (oldest at beginning, newest
at end) and, when you're reading the file into memory,
use unshift rather than push so that the newest data
ends up at the beginning of the array.
Which definitely makes sense, and makes the problem quite
a lot less complicated, if that's the reason the op
wanted to write to the beginning of the file.
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Re: Writing to files
by jbert (Priest) on Apr 10, 2000 at 19:11 UTC
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All the above good advice applies, but one liners are cool:
perl -i -p -e "BEGIN{print 'A new line!\n';}" your files here
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I had better luck with this:
perl -i -p -e 'print "first line\n" if $. == 1;' data
It depends on the particular magic of $., of course.
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Unfortunately, this doesn't work, because the BEGIN occurs
before STDOUT is redirected to the output file (the file
you're writing to). Plus, in your case, you used single
quotes to surround your "\n", so it didn't get interpolated.
Here's what I get:
% perl -i -p -e "BEGIN{print 'A new line!\n';}" foo
A new line!\n%
I tried this myself, the other day, but ran into the
same problem. | [reply] [d/l] |