Difference between revisions of "PERL/C3/File-Handling/English"

• Open a file in read mode
• Write to a file
• Open a file in append mode
• Close the file handle

• Ubuntu Linux 12.04 operating system
• Perl 5.14.2
• and the gedit Text Editor

You can use any text editor of your choice.

If not, then go through the relevant Perl spoken tutorials on the spoken tutorial website.

The basic operations we can do with files in Perl are:

• open a file
• read from a file
• write to a file and
• close a file

• STDIN
• STDOUT
• STDERR

Opening a File:

Syntax: open (FILEHANDLE, MODE, EXPR);

Example: open (DATA, “<”, “First.txt”);

open (DATA, “<First.txt”);

In the syntax, FILEHANDLE is the file handle returned by the open function.

MODE represents the mode of opening the file. For eg: read, write, etc.

EXPR is the physical filename used to read or write. In this case, “First.txt” is the filename.

There is another way to write the open function, as shown here.

gedit first.txt

Go to the terminal and type gedit first.txt and press Enter.

Welcome to File Handling

Chapter 1

Chapter 2

Chapter 3

Save the file and close gedit.

Type gedit openfile dot pl ampersand and press Enter.

Let us understand the code now.

open(DATA, "<” ,”first.txt") or die "Couldn't open file first.txt, $!";

The first parameter DATA is the filehandle which allows Perl to refer to the file in future.

The second parameter “<” less than symbol denotes the READ mode.

If you fail to specify the “Mode”, by default the file will be opened in “READ” mode.

The third parameter first.txt is the filename from where the data has to be read.

What will happen if the file first.txt does not exist?

The script will die with the appropriate error message stored in the dollar exclamation ($!)variable.

print "$_";

}

• read line by line
• and loop through the <DATA> file
• until all the lines have been read.

Print dollar underscore ($_) variable will print the contents of the current line.

perl openfile dot pl >> press Enter.

perl openfile dot pl and press Enter.

Welcome to File Handling

Chapter 1

Chapter 2

Chapter 3

This is the same content that we saw earlier in first dot txt file.

Opening a file for writing:

Syntax: open (FILEHANDLE, “> filename”)

Example: open (DATA, “ >write.txt”);

Filename represents the name of the file where the data has to be written.

Switch to the terminal.

Let me explain the code now.

open FILE1, ">second.txt" or die "Couldn't open file second.txt, $!";

print FILE1 "Working with files makes data storage and retrieval a simple task! \n";

“>” Greater than symbol before the filename denotes the write mode.

The first parameter FILE1 is the FILEHANDLE.

The print function prints the given text to FILEHANDLE. i.e FILE1

perl writefile dot pl >> press Enter.

perl writefile dot pl and press Enter.

Let us see that.

print FILE1 " > overwrites the content of the file! \n";

Add the below print command.

perl writefile dot pl >> press Enter.

perl writefile dot pl and press Enter.

The previous content of the second.txt file has been overwritten.

This is because we had opened the file in the write mode again.

Opening a file for Appending:

Syntax: open (FILEHANDLE, “>> filename”)

Example: open (DATA, “ >>write.txt”);

This will denote that the file is in append mode.

print FILE1 "Two greater than symbols >> opens the file in append mode"

to append to the existing data.

perl writefile.pl& >> Press Enter

perl writefile dot pl

and press Enter.

File Attributes:

• +< file – READ, WRITE
• +> file – READ, WRITE, CREATE, TRUNCATE,
• +>> file - READ, WRITE, CREATE, APPEND

Try out these options on your own and understand what happens.

Summary

• Open a file in read mode
• Write to a file
• Open a file in append mode and
• Close the file handle

Assignment

1. Change the file attribute to +> in the writefile.pl program.
2. Save and execute the program.
3. Open the second.txt file to see the output.
4. Analyse the usage of file attribute +>

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