Working with files in perl

Opening a file for Read/Write/Append

You can open a file using the function open(). It takes three arguments: a file handle which is just a symbolic name of your choice for the file, a mode symbol which specifies the file operation mode in which the file should be opened, and third being the file to be opened itself.

Mode operators are as given below.

< : Read - Default if a mode is not specified at all.
> : Write -File is Created if not existing else Overwrites.
>> : Append - creates file if not existing.

If you want to combine read and write, prepend a "+" to the modes.

+< : Read/Write,Don't create file,Overwrites existing.
+> : Read/Write,Create file if not existing, else Overwrite.
+>> : Read/Append,Create file if not existing, no Overwrite

Let us take an example.

open ( MY_FILE_HANDLE,"<","/home/safeer/docs/accounts.txt");

This will open the file "/home/safeer/docs/accounts.txt" for reading and will be accessible through the file handle MY_FILE_HANDLE. We can use this handle to read data from the actual file.

The second and third arguments for open() can be combined into a single string as "< /home/safeer/docs/accounts.txt". In the case of mode, it is OK even if you skip the "<" symbol - since read is the default mode. So,

open ( MY_FILE_HANDLE,"<","/home/safeer/docs/accounts.txt");
open ( MY_FILE_HANDLE,"< /home/safeer/docs/accounts.txt");
open ( MY_FILE_HANDLE,"/home/safeer/docs/accounts.txt");

are all the same.

If you want to open this file for writing, use

open ( MY_FILE_HANDLE,">","/home/safeer/docs/accounts.txt"); OR
open ( MY_FILE_HANDLE,"> /home/safeer/docs/accounts.txt");

Reading data from a file

Open the file in read mode

open ( MY_FILE_HANDLE,"< /home/safeer/docs/accounts.txt");

You can assign the the entire file content to an array variable where each element of the array will correspond to a single line in the file.

@acounts_data = <MY_FILE_HANDLE> ;

But this can be memory intensive if the file you are opening is very large. A better way is to read file line by line and process it as follows.

while ( $line = <MY_FILE_HANDLE> ){
#Process $line here
}

In every iteration $line will contain a single line of the file. Note that the last character of the $line will be newline (\n), so you may want to chomp $line before processing it. The following code will be equivalent to printing the file contents.

while ( $line = <MY_FILE_HANDLE> ){
chomp ($line);
print"$line\n";
}

Write/Append to a file:

We use print command to write/append (depends on the mode in which the file is opened) to a file.

Syntax: print $FILEHANDLE $STRING;

To start writing, first open file in write mode and then print using the file handle obtained from open().

open ( MY_FILE_HANDLE,"> /home/safeer/docs/newaccounts.txt");
$my_account = "safeer:/home/safeer/:usr/bin/bash" ;
print MY_FILE_HANDLE "$my_account\n";

Alternatively, write directly without using variable:

print MY_FILE_HANDLE "safeer:/home/safeer/:usr/bin/bash\n" ;

If the file was opened in append mode ( ">>" ) the string would have been appended to the end of file.

open ( MY_FILE_HANDLE,">> /home/safeer/docs/accounts.txt");
print MY_FILE_HANDLE "safeer:/home/safeer/:usr/bin/bash\n" ;

Closing the file

any file that is opened should be closed once the purpose of open is over.
close (MY_FILE_HANDLE);