Perl File Handling: open, read, write and close files

This article describes the facilities provided for Perl file handling.

Opening files

Opening a file in perl in straightforward: 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.

What if you wanted to modify the file instead of just reading from it? Then you'd have to specify the appropriate mode using the three-argument form of open. open FILEHANDLE, MODE, EXPR The available modes are the following:

mode	operand	create	truncate
read	`<`
write	`>`	✓	✓
append	`>>`	✓

Each of the above modes can also be prefixed with the + character to allow for simultaneous reading and writing.

mode	operand	create	truncate
read/write	`+<`
read/write	`+>`	✓	✓
read/append	`+>>`	✓

Notice, how both +< and +> open the file in read/write mode but the latter also creates the file if it doesn't exist or truncates (deletes) an existing file. So, if you wanted to open a file for writing, creating it if it doesn't exist and truncating it first if does, you'd do the following: open FILE, ">", "filename.txt" or die $! This operation might fail if for example you don't have the appropriate permissions. In this case $! will be set appropriately.

The mode and the filename in the three-argument form can be combined, so the above can also be written as: open FILE, ">filename.txt" or die $!; As you might have guessed already if you just want read access you can skip the mode just as we did in the very first example above.

Reading files

If you want to read a text file line-by-line then you can do it as such: my @lines = <FILE>; The <FILE> operator - where FILE is a previously opened filehandle - returns all the unread lines of the text file in list context or a single line in scalar context. Hence, if you had a particularly large file and you wanted to conserve memory you could process it line by line:

while (<FILE>) {
  print $_;
}

The $_ variable is automatically set for you to the contents of the current line. If you wish you may name your line variable instead: while (my $line = <FILE>) { ... will set the $line variable to the contents of the current line. The newline character at the end of the line is not removed automatically. If you wish to remove it you can use the chomp command. After all lines have been read the <FILE> operator will return a false value hence causing the loop to terminate.

There may cases where you need to read a file only a few characters at a time instead of line-by-line. This may be the case for binary data. To do just that you can use the read command.

open FILE, "picture.jpg" or die $!;
binmode FILE;
my ($buf, $data, $n);
while (($n = read FILE, $data, 4) != 0) {
  print "$n bytes read\n";
  $buf .= $data;
}
close(FILE);

There is a lot going on here so let's take it step by step. In the first line of the above code fragment a file is opened. As you can guess from the filename it is a binary file. Binary files need to treated differently than text files on some operating systems (eg, Windows). The reason is that on these platforms a newline "character" is actually represented within text files by the two character sequence \cM\cJ (that's control-M, control-J). When reading the text file Perl will convert the \cM\cJ sequence into a single \n newline characted. The converse also holds when writing files. Clearly, when reading binary data this behavior is undesired and calling binmode on the filehandle will make sure that this conversion is avoided.

The read command takes either 3 or 4 arguments. The 3-argument form is: read FILEHANDLE, SCALAR, LENGTH while the 4-argument form is: read FILEHANDLE, SCALAR, LENGTH, OFFSET In the first case LENGTH characters of data are read in the variable specified by SCALAR from FILEHANDLE. The return value of read is the number of characters actually read, 0 at the end of the file or undef in the case of an error. Returning to our example above the third line of code will read at most 4 characters of data into the $data variable. The number of characters read will be stored in $n. Successive read operations on the same filehandle will set the current file position to be just before the first unread character. Thus the code above will read the contents of the file picture.jpg and store them in $buf, printing the number of characters read at every iteration.

If OFFSET is specified then the characters read will be placed at that position within the SCALAR. Taking advantage of this we could rewrite the loop above as such:

my ($data, $n, $offset);
while (($n = read FILE, $data, 4, $offset) != 0) {
  print "$n bytes read\n";
  $offset += $n;
}

Even though the example above demonstrates binary reading the read command works just as well on text files - just make sure to use (for binary) or not use (for text) binmode accordingly.

Writing files

Now that you know how to open and read files learning how to write to them is straighforward. Take a look at the following code:

open FILE, ">file.txt" or die $!;
print FILE $str;
close FILE;

Not much is new here. The only thing to observe is the two-argument use of print, the first argument being the FILEHANDLE to write to and the second an expression to be written. The expression can be anything: a scalar, a list, a hash, etc. Appending to a file can be accomplished in exactly the same manner - apart from specifying the appropriate (>>) mode of course.

At this point you are probably thinking that a description of the write is what follows. Actually, as the manual page for write puts it:

Note that write is not the opposite of read. Unfortunately.

Instead write is used to write formatted records to file, a subject outside the scope of this article.

Closing files

Once you are done reading and writing you should close any open filehandles.

open FILE1, "file.txt" or die $!;
open FILE2, "picture.jpg" or die $!;
...
close FILE2;
close FILE1;

If you forget to close a filehandle Perl will do it for you before your script exists but it is good practice to close yourself what you have opened.

The close command may also fail returning false, eg, if you try to close a closed filehandle. If you want to catch these errors you can check the return value of close and the approriate error message stored in $! as is done in the following example: close FILE or die $!

Summary of perl file handling

The open, close, print and read commands will allow you to perform most common file operations. However, much more is possible. Apart from opening files you may open pipes to other commands using the | mode and read from them or write to them using the techniques described. This and more in an article to come.

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Comments


David	Posted at 6:38am on Tuesday, June 26th, 2007
Cool tutorials but the perl installation file. havidworld@yahoo.co.nz
fountain_spray	Posted at 12:50pm on Wednesday, June 27th, 2007
Not describing Format for Write is a cop-out! Where do I turn for THAT tutorial?
Richard Wicks	Posted at 4:11pm on Thursday, August 23rd, 2007
Well, great, I can read binary files now. How the heck do I write them?
Tobias Maier	Posted at 7:13am on Thursday, August 30th, 2007
You can use 'pack' to write binary files. E.g.: The following snippet writes all numbers between 0 and zero as unsigned characters. open(BIN, '>', 'binary_file') or die "Couldn't open file for writing: $!n"; foreach my $i (0 ... 255) { print BIN pack('C', $i); } close BIN;
Jo	Posted at 9:09am on Monday, September 24th, 2007
I need to be able to read the first 20 lines of a txt file. How about that. @ARGV?
Richard	Posted at 2:57pm on Tuesday, September 25th, 2007
open(FILE, "
Richard	Posted at 2:59pm on Tuesday, September 25th, 2007
open(FILE, "
Richard	Posted at 3:00pm on Tuesday, September 25th, 2007
Sorry, I don't know why it won't take my entire comment :(
Sasi	Posted at 9:25am on Monday, October 1st, 2007
I have a hash variable with 10K records and I want to write 1000 records at one shot into the file. Is there a way in perl?
Clement	Posted at 1:42pm on Monday, October 1st, 2007
Thanks for this site, it is really good. If I want to read several files such as c_1, c_2, c_3 etc in the same directory, how do I write the perl script to do this for me. Please, you could send me a response to agclems@yahoo.com Many thanks for your help. Regards, Clement.
Salvor	Posted at 12:47am on Tuesday, October 23rd, 2007
I think it's better to write here and not send you a private response. Anyelse can read it, so: my $i=1; while (-e "c_$i"){ open (FILE, "c_$i") \|\| die ("What the hell??? Can't open c_$i!!!"); my @lines = ; close FILE; doincrediblethingswhiththe(@lines); $i++; }
Salvor	Posted at 12:50am on Tuesday, October 23rd, 2007
Sorry, the forum eat some of my comment. 4th line: my @lines = <FILE>; well, you know, read all file into an array, as explained before in this page. Regards.
Norton	Posted at 7:13am on Wednesday, November 21st, 2007
I would really like to see how can you store the contents of the file in an array, where lines are appended as array elements. Could you please add it to this tutorial? Cheers, Norton
Johan	Posted at 4:17am on Friday, December 7th, 2007
Norton: to add a file to an array, @array = ; should do the trick.
harsh	Posted at 8:55am on Thursday, December 13th, 2007
read multiple files from same directory opendir DIR, "C:/perl/bin/1" \|\| die "Cannot opendir 1 $!"; while ( $filename = readdir(DIR) ) { if($filename=~/.*.htm/){ open IN, "C:/perl/bin/1/$filename"; open OUT,">>C:/perl/bin/1/all.txt"; while(){ chomp ($_); and so on} by using this you can read multiple files from directory 1 and write all the files in a new file. regards harsh
Jared	Posted at 9:19am on Wednesday, December 19th, 2007
This: foreach my $i (0 ... 255) { print BIN pack('C', $i); } Could also be written as: print BIN map(pack($_),@{[0..255]}); :)
cris	Posted at 10:37am on Wednesday, December 26th, 2007
how do you open a image and print it?
Nikhil	Posted at 2:52pm on Saturday, December 29th, 2007
How do you search for a pattern and then delete all the lines above and below it? eg abbbbbbbbbcioiheiodnweiniwqoed Serial Number iuawheiuhoiehoqwiheeewq I want to search for Serial Number and delete all three lines.. I want to repeat this all over the file wherever the Serial Number occurs..
kumar	Posted at 12:06pm on Thursday, January 3rd, 2008
I need a script for the below task.I have a txt file with xxxxx numbers , one number each line. I should run a perl script so that my script should read this txt file line 1 and then line 2 , then line 3 etc till the predefined match is found.I wanna use this line 1 value in the command executed below this script.this command output should be updated to a new file always. Obiliged if any answers for my query.
SK Rahman	Posted at 4:49am on Friday, January 4th, 2008
if i run the following script i am getting a binary file but when i open with hex editor i am able see that the actual numbers are "01 02 03 04 05 06 07 08 09" what i wanted output like this "12 34 56 78" please help me to write a script for this... thanks in advance open(BIN, '>', 'binary_file') or die "Couldn't open file for writing: $!n"; foreach my $i (1 ... 9) { binmode BIN; print BIN pack('C', $i); } close BIN;
ZwribMaster	Posted at 6:39pm on Thursday, January 17th, 2008
Hi, I want to try to open a file, and if it fails, I want to write the filename to a log file but continue on with my program to open the other files in my list. Is there an alternative to using open the file \|\| or die?
ZwribMaster	Posted at 6:52pm on Thursday, January 17th, 2008
Hi Nikhil, You can read the file contents into an array, and then use a foreach loop to scan each line of the array. For ex: open(PAGE,"yourfile.txt") \|\| die "I can't open yourfile.txt"; #store the contents of the file into the File array @File = ; close(PAGE); $Ctr = 0; #then loop through the file... foreach (@File){ if ($File[$Ctr] =~ m/SerialNumber/){ #remove the 3 lines from the array $File[$Ctr] = ""; $File[$Ctr+1] = ""; $File[$Ctr-1] = ""; } $Ctr = $Ctr+1; #increment through the rest of array } #end foreach #write the array contents to the file... open(PAGE,">yourfile.txt") \|\| die "I can't open yourfile.txt"; #open the file to read print PAGE @File; close(PAGE);
Dave	Posted at 8:51am on Sunday, January 20th, 2008
I'm trying to load a txt file into an array and filter out specific data. I used ZwribMaster post as a template and it has helped a lot. I have one issue. Instead of removing the lines from the array, it is just replacing them with a blank space. Here is sample of what I'm running: #Open and Load Input file to array open INPUT_FILE, "data.txt" or die $!; print OUTPUT_FILE "@Data"; close OUTPUT_FILE; Any help would be appreciated,, Thanks, Dave C
DaveC	Posted at 11:24am on Sunday, January 20th, 2008
It's seems my script was cut off in my last post. #Open and Load Input file to array open INPUT_FILE, "data.txt" or die $!; print OUTPUT_FILE "@Data"; close OUTPUT_FILE;
Sai Ram	Posted at 9:52pm on Monday, January 21st, 2008
How to make use oF APIs in perl..please help me out...i am lost becoz of this.. :-( I need this immediately...
Sylvia	Posted at 8:05am on Tuesday, January 29th, 2008
Hi there! I would like to print the same content to 4 output files at the same time, is there anyway to do this, or do I have to do it on four separate lines? eg print FILE_1 "example"; print FILE_2 "example"; print FILE_3 "example"; print FILE_4 "example"; can I do something like this? print {FILE_1 FILE_2 FILE_3 FILE_4} "example"; thanks for your help
Kiran	Posted at 2:24am on Friday, February 1st, 2008
Hi Friends!! I need to write a script which will logon to sftp server & then check for files in a particular directory if the file is present it should append that file is present ina text file if not it should append file not present how this can be achieved i am stuck up at logging on to sftp server please help thanks in advance
Kiran	Posted at 11:23pm on Wednesday, February 13th, 2008
Hi, I have 2 files, say file1 & file2. I have some, variable in file1 say $work. I want to read this in file2 & use it in file2. Could anyone help me in this Thanks Kiran
Rich	Posted at 8:39pm on Saturday, February 23rd, 2008
Kiran, regarding your question on the thirteenth, simply open both files, and copy the $work to file2.
Kiran	Posted at 2:46am on Monday, February 25th, 2008
Guys can any body help me on question posted on 1st feb 08 " need to write a script which will logon to sftp server & then check for files in a particular directory if the file is present it should append that file is present ina text file if not it should append file not present how this can be achieved i am stuck up at logging on to sftp server please help thanks in advance"
Torsten	Posted at 3:17am on Friday, March 14th, 2008
Hi, I have the following problem: I want to open a file and read its content into a variable. After that a standard input should be read into another variable. But after this input, the script gets stuck. If I swap both, the script works. Here a part of my script: open FILE, "c:/tmp/mail_receivers.txt" or die "Couldn't open: $!"; local $/; $receiver = ; close FILE; print ("nPlease enter mail subject."); $subject = ; print ("nSubject: $subject"); print("$receiver"); Can anybody help me? Thanks, Torsten
Bassir	Posted at 11:23pm on Sunday, March 16th, 2008
i have too many problems with doing an algorithm
Manojg	Posted at 11:23am on Wednesday, March 19th, 2008
Hi, I tried to open a file by using open FILE, ">", "filename.txt" or die $! The script did not give any error and there was no any file opened. Where does it open the file? in memory? How do I open a file in disk? Thanks.
Shruthi	Posted at 1:12am on Monday, March 24th, 2008
How do i write perl script for reading a file and how i can see output from this. If any body knows please send it back as comments in this same page.
Suresh,S	Posted at 1:44am on Monday, March 24th, 2008
created a excel sheet as Book1.xls just tried this coding for opening Book1.xls unable to open the excel file the output is coming in Dos prompt itself why? #!/usr/local/bin/perl open (MYFILE, 'Book1.xls'); while () { chomp; print "$_n"; } close (MYFILE); Just reply the solution Immediately
Jacob	Posted at 11:24am on Wednesday, April 2nd, 2008
Hi all.. Im trying to make this littel hash cracker in perl (JTR dosent take saltet hash), and it works fine, but now i want it to goto next hash, when it finds a correct password, insted of trying the intire paasword list.. (aka for more speed) The script looks like this: #!/usr/bin/perl use Digest::MD5 qw(md5_hex); use Data::Dumper; open(FH0, " < digits"); #long list of passwords while(){ chomp; ($password,$b) = split(/:/); open(FH1, "< alle1"); #long list of hash while() { chomp; ($last,$hash,$salt,$mail) = (split(/:/))[0,1,2,3]; { open (FH2, '>>data.txt'); print FH2 "$mail:$passwordn" if(md5_hex($salt.$password) eq $hash); # print "BINGO CHAMPn" if(md5_hex($salt.$password) eq $hash); # print "$passwordn"; } } } close(FH0); close(FH1); close(FH2); Can someone help me whit that?
Pramod Badgujar	Posted at 10:17pm on Tuesday, April 8th, 2008
How to replace the content of line with the another text?
Pramod Badgujar	Posted at 10:22pm on Tuesday, April 8th, 2008
How to replace the content of line with the another text? Please give me answer of the above question.
emma	Posted at 9:10am on Friday, April 11th, 2008
Hi all, I am new to Perl scripting and need your help. Here is the problem I am having . I have a file with some strings each delimited by a "\|'character. I have to open the file , each each line and replace the '\|' with a tab or space and then copy them to to a file. Eg: file ABC Contents are as follows - asdfd\|\|\|sdhh\|\|sdghhgd\|\|\| afsgasfd\|\|asdsadsad\|adff\|\|\|\|\|\|fasffasf\| etc Desired output asdfd sdkjf dsfhdh sffhjd djfhjhds dhfjhfjh (ie no "\|" characters.) I appreciate your help. Please some one help me with the perl code to do this task . I am sure it is easy job for perl experts.
Bharath	Posted at 6:42pm on Monday, April 21st, 2008
Hi, I want to open an image and see the binary values of that image.Is there a way to open an jpeg or png or bmp file and prints the contents of that file in zeroes and ones using perl. Thanks in advance, Bharath
reddy	Posted at 3:00pm on Tuesday, April 29th, 2008
Does anybody answer here or just post Q's?
The bumbling bee	Posted at 12:44pm on Saturday, May 10th, 2008
Hi Emma, Can the following piece of code do? $fileName = "ABC"; open(FILE,"
The bumbling bee	Posted at 12:48pm on Saturday, May 10th, 2008
The bumbling bee Hi Emma, How do I avoid the "eating" of comments after the "<" glyph?
The bumbling bee	Posted at 12:52pm on Saturday, May 10th, 2008
Ok, trying one more time! $fileName = "ABC"; open(FILE,"< $fileName") or die "Could not open file $fileName"; while ($line = < FILE> ) { $line =~ s/\|+$//; ## 1 (see below) $line =~ s/\|+/ /g; ## 2 (see below) print $line; } close(FILE); ## BRIEF EXPLANATION: ## ------------------ ## The command "s/+++/*/" substitute "+++" with "". ## The "\|+" is regular expression matching one or more ## consecutive "\|" chars. ## ## ## EXPLANATION OF "1": ## The regular expression "\|+$" requires one or more ## consecutive "\|" chars at the end of the line since ## "$" match end-of-line. This is replaced by nothing. ## ## EXPLANATION OF "*2": ## The regular expression "\|+$" requires one or more ## consecutive "\|" as before. This is replaced by a ## single space. ## The option "g" (global) in the end indicates, that ## several sequences may be replaced if such exist. ##
beta32c	Posted at 12:22am on Thursday, May 15th, 2008
Can we open files which are more than 2gb in size. Im not able to open my files for processing which are more than 2gb.. any ways.. reply back.
kumar	Posted at 8:50am on Tuesday, May 20th, 2008
I have a file which the format is ".dat". I would like to read the information from this file and convert all the information to txt format. Is the perl able to do this.
Joe	Posted at 6:51am on Friday, May 30th, 2008
what drives me nuts about many perl people offering suggestions is they offer squat. Try using this,try using that. For newbies this is no help at all. Explain it or don't bother offering suggestions. I keep seeing tons of people posting things like try this in your code. Suppose the person asking doesn't even know how to begin let alone put the code in the proper place in the script. Suppose the person has spent hours trying to figure it out and just wants some help and needs the entire script. Get real posters. I'm fed up with those who don't post the complete solution.
Newbies - start here	Posted at 9:56am on Friday, June 13th, 2008
A lot of you are asking questions that are hard to answer if you don't know hardly anything about perl. Teach yourself the basics here: http://www.pageresource.com/cgirec/index2.htm. Or search google for beginning perl tutorial. There are many books as well. To try to help some of you, here is a full working script that will read a file in, process the file and save it to a new file. The files will be in the same folder as the perl script. This will overwrite your output file each time you run it. #!/usr/bin/perl -w use strict; my $infile = 'in.txt'; # name of the file you are reading - must be in same folder as script my $outfile = 'out.txt'; # name of file that will be created for output open (FILEHANDLE, '' will create the file if necessary # WARNING: this will overwrite your $outfile if it exists - copy to new file if you want to save what you have in your output file print OUT $outdata; # write the data - will overwrite whatever you had in the output file previously close OUT; # close the file exit; # you are done
Anonymous	Posted at 10:14am on Friday, June 13th, 2008
OK. One last try with the HTML characters escaped. (In fact, I escaped the characters, like <> using my script below) A lot of you are asking questions that are hard to answer if you don't know hardly anything about perl. Teach yourself the basics here: http://www.pageresource.com/cgirec/index2.htm. Or search google for beginning perl tutorial. There are many books as well. To try to help some of you, here is a full working script that will read a file in, process the file and save it to a new file. The files will be in the same folder as the perl script. This will overwrite your output file each time you run it. #!/usr/bin/perl -w use strict; my $infile = 'in.txt'; # name of the file you are reading - must be in same folder as script my $outfile = 'out.txt'; # name of file that will be created for output open (FILEHANDLE, '<', $infile); # this makes the file ready for reading from FILEHANDLE # '<' means read only my @inlines; # this is an array. An array holds a list of items. Each list item will be 1 line from the input file @inlines = <FILEHANDLE>; # this reads the input file into the array - each array item holds 1 line of the file my @outlines; # we will put the output lines here #we're done reading the file - close it close FILEHANDLE; foreach my $line (@inlines) { # this cycles through the input file info 1 line at a time chomp($line); # remove newline character at end of line # I will show a few examples of what you can do to process each line # just uncomment the code (remove the # in front of the code) to use what you want # # change all '\|' characters to tabs # $line =~ s/\\|/\t/g; # # separate each word - one word per line - changes whitespace (spaces, tabs, etc) to new lines # $line =~ s/\s+/\n/g; # # replace the line with new content # $line = "Goodbye my friend." if ($line =~ /^Hello everyone!$/); # if the line said 'Hello everyone!', now it will say 'Goodbye my friend.' # # replace a word with another # my $search_word = 'aks'; # my $replace_word = 'ask'; # $line =~ s/$search_word/$replace_word/gi; # g replaces all occurances, i makes it case insesitive - matches lower case and upper case # url encode the line to make it safe for posting, so that < characters won't mess it up (such as for the comments at http://www.perlfect.com/articles/perlfile.shtml) # $line =~ s/([^A-Za-z0-9])/sprintf("%%%02X", ord($1))/seg; #escape HTML Characters "<", ">", "&" etc. $line =~ s/([^\w\s])/sprintf ("&#%d;", ord ($1))/ge; # put the line in the output array push(@outlines,$line); } # join the output lines into one variable with a newline character between each line my $outdata = join("\n",@outlines); # save $outdata to the $outfile open (OUT,">", $outfile); # this will open the $outfile for writing, and '>' will create the file if necessary # WARNING: this will overwrite your $outfile if it exists - copy to new file if you want to save what you have in your output file print OUT $outdata; # write the data - will overwrite whatever you had in the output file previously close OUT; # close the file exit; # you are done
hiru..	Posted at 2:38am on Wednesday, June 18th, 2008
i want to write an array into a textfile. i m using active perl in windows xp....
noor	Posted at 11:57pm on Wednesday, June 18th, 2008
how to delete a portion or couple lines from the file basically text file
Anonymous	Posted at 5:05am on Thursday, July 3rd, 2008
i have one already open file . please tell how to delete the 4 th line . again i needto addd differnt data to fil
Barney	Posted at 2:58pm on Friday, July 4th, 2008
I want to manually process many text files as follows: myscript.pl file1.txt myscript.pl differentfilename.txt I am able to read the file into an array as follows: my @INFILE = ; But I don't know how to get the filenames to uniquely assign output files (ie. file1_out.txt, differentfilename_out.txt) Thanks in advance.

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