CODEWITHSUNDEEP
perlfileappendtruncate
Mr. MonoChrome
Mr. MonoChrome

Reputation: 1383

File truncated, when opened in Perl

Im new to perl, so sorry if this is obvious, but i looked up how to open a file, and use the flags, but for the life of me they dont seem to work right I narrowed it down to these lines of code.

    if ($flag eq "T"){
        open xFile, ">" , "$lUsername\\$openFile";
    }
    else
    {
        open xFile, ">>", "$lUsername\\$openFile";
    }

Both of these methods seem to delete the contents of my file. I also checked if the flag is formatted correctly and it is, i know for a fact ive gone down both conditions.

EDIT: codepaste of a larger portion of my code http://codepaste.net/n52sma

Upvotes: 0

Views: 1744

Answers (4)

David W.
David W.

Reputation: 107080

New to Perl? I hope you're using use strict and use warnings.

As other's have stated, you should be using a test to make sure your file is open. However, that's not really the problem here. In fact, I used your code, and it seems to work fine for me. Maybe you should try printing some debugging messages to see if this is doing what you think it's doing:

use strict;
use warnings;
use autodie;   #Will stop your program if the "open" doesn't work.

my $lUsername = "ABaker";
my $openFile  = "somefile.txt";
if ($flag eq "T") {
    print qq(DEBUG: Flag = "$flag": Deleting file "$lUsername/$openFile");
    open xFile, ">" , "$lUsername/$openFile";
}
else {
    print qq(DEBUG: Flag = "$flag": Appending file "$lUsername/$openFile");
    open xFile, ">>", "$lUsername/$openFile";
}

You want to use strict and warnings in order to make sure you're not having issues with variable names. The use strict forces you to declare your variables first. For example, are you setting $Flag, but then using $flag? Maybe $flag is set the first time through, but you're setting $Flag the second time through.

Anyway, the DEBUG: statements will give you a better idea of what your error could be.

By the way, in Perl, you're checking if $flag is set to T and not t. If you want to test against both t and T, test whether uc $flag eq 'T' and not just $flag eq 'T'.

@Ukemi

I reformated to comply with use strict, i also made print statements to make sure i was trunctating when i want to, and not when i dont. It still is deleting the file. Although now sometimes its simply not writing, im going to give a larger portion of my code in a link, id really appreciate it if you gave it a once over.

Are you seeing it say Truncating, but the file is empty? Are you sure the file already existed? There's a reason why I put the flag and everything in my debug statements. The more you print, the more you know. Try the following section of code:

$file = "lUsername/$openFile"  #Use forward slashes vs. back slashes.
if ($flag eq "T") {
    print qq(Flag = "$flag". Truncating file "$file"\n);
    open $File , '>', $file
        or die qq(Unable to open file "$file" for writing: $!\n);
}
else {
    print qq(Flag = "$flag". Appending to file "$file"\n);
    if (not -e $file) {
        print qq(File "$file" does not exist. Will create it\n");
    }
    open $File , '>>', $file
        or die qq(Unable to open file "$file" for appending: $!\n);
}
  • Note I'm printing out the flag and the name of the file in quotes. This will allow me to see if there are any hidden characters in my file name.
  • I'm using the qq(...) method to quote strings, so I can use the quotation marks in my print statements.
  • Also note I'm checking for the existence of the file when I truncate. This way, I make sure the file actually exists.

This should point out any possible errors in your logic. The other thing you can do is to stop your program when you finish writing out the file and verify that the file was written out as expected.

print "Write to file now:\n";
my $writeToFile = <>;
printf $File "$writeToFile";
close $File;
print "DEBUG: Temporary stop. Examine file\n";
<STDIN>;     #DEBUG:

Now, if you see it saying it's appending to the file, and the file exists, and you still see the file being overwritten, we'll know the problem lies in your actual open xFile, ">>" $file statement.

Upvotes: 3

ikegami
ikegami

Reputation: 386371

>> does not clobber or truncate. Either you ended up in the "then" clause when you expected to be in the "else" clause, or the problem is elsewhere.

To check what $flag contains:

use Data::Dumper;
local $Data::Dumper::Useqq = 1;
print(Dumper($flag));

Upvotes: 1

dgw
dgw

Reputation: 13656

You should use the three-argument-version of open, lexical filehandles and check wether there might have been an error:

# Writing to file (clobbering it if it exists)
open my $file , '>', $filename 
    or die "Unable to write to file '$filename': $!";

# Appending to file
open my $file , '>>', $filename 
    or die "Unable to append to file '$filename': $!";

Upvotes: 3

Madhan
Madhan

Reputation: 1321

For your reference I have mentioned some basic file handling techniques below.

open FILE, "filename.txt" or die $!;

The command above will associate the FILE filehandle with the file filename.txt. You can use the filehandle to read from the file. If the file doesn't exist - or you cannot read it for any other reason - then the script will die with the appropriate error message stored in the $! variable.

open FILEHANDLE, MODE, EXPR

The available modes are the following:

read    < #this mode will read the file     
write   > # this mode will create the new file. If the file already exists it will truncate and overwrite.
append  >> #this will append the contents if the file already exists,else it will create new one.

if you have confusion on this, you can use the module called File::Slurp; I have mentioned the sample codes using File::Slurp module.

use strict;
use File::Slurp;

my $read_mode=read_file("test.txt"); #to read file contents
write_file("test2.txt",$read_mode); #to write file
my @all_files=read_dir("/home/desktop",keep_dot_dot=>0); #read a dir
write_file("test2.txt",{append=>1},"@all_files"); #Append mode

Upvotes: -3

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