PERL/C3/File-Handling/English-timed

Time	Narration
00:01	Welcome to the Spoken Tutorial on File Handling in PERL.
00:06	In this tutorial, we will learn how to: Open a file in read mode Write to a file Open a file in append mode Close the file handle.
00:17	For this tutorial, I am using: Ubuntu Linux 12.04 operating system Perl 5.14.2 and the gedit Text Editor.
00:28	You can use any text editor of your choice.
00:32	To follow this tutorial, you should have working knowledge of Perl programming.
00:37	If not, then go through the relevant Perl spoken tutorials on the spoken tutorial website.
00:43	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.
00:54	The default file handles are: STDIN STDOUT STDERR
01:02	This is the syntax for open function.
01:05	In the syntax, FILEHANDLE is the file handle returned by the open function.
01:11	MODE represents the mode of opening the file. For eg: read, write, etc.
01:18	EXPR is the physical filename used to read or write. In this case, “First.txt” is the filename.
01:27	There is another way to write the open function, as shown here.
01:32	Let us understand how to open an existing file and read the data from it.
01:38	First we will create a text file and store some data in it. Go to the terminal and type: gedit first.txt and press Enter.
01:51	In the first dot txt file, type the following text:
01:55	Save the file and close gedit.
01:59	Now we will look at a Perl program that opens the file 'first.txt' and reads the content.
02:07	Let me open the sample program 'openfile.pl' which I have already saved.
02:13	Type: gedit openfile dot pl ampersand and press Enter.
02:19	In the openfile dot pl file, type the following code as displayed on the screen.
02:25	Let us understand the code now.
02:28	The open function opens a file for reading.
02:33	The first parameter DATA is the filehandle which allows Perl to refer to the file in future.
02:40	The second parameter “<” less than symbol denotes the READ mode.
02:44	If you fail to specify the Mode, by default the file will be opened in “READ” mode.
02:50	The third parameter 'first.txt' is the filename from where the data has to be read.
02:57	What will happen if the file 'first.txt' does not exist?
03:02	The script will die with the appropriate error message, stored in the dollar exclamation ($!)variable.
03:08	The while loop will read line by line and loop through the <DATA> file until all the lines have been read.
03:17	Print dollar underscore ('$_') variable will print the contents of the current line.
03:22	Lastly, close the file with the FILEHANDLE name which we had given in the open statement.
03:29	Closing a file prevents any accidental file changes or overwriting of the content.
03:36	Now, press Ctrl+S to save the file.
03:40	Let us execute the program.
03:42	Switch back to the terminal and type perl openfile dot pl and press Enter.
03:51	The output is displayed as shown.
03:54	This is the same content that we saw earlier in first dot txt file.
03:59	Next we will see how to write data into a file.
04:03	The open statement with greater than (>) symbol defines the WRITE mode.
04:08	Filename represents the name of the file where the data has to be written.
04:13	Let me open the sample program 'writefile.pl' which I have already saved.
04:19	Switch to the terminal.
04:21	Now, type: gedit writefile dot pl ampersand and press Enter.
04:29	In the writefile dot pl file, type the following code as displayed on the screen.
04:34	Let me explain the code now.
04:37	The open function opens a file 'second.txt' in "write" mode.
04:44	“>” - Greater than symbol before the filename denotes the "write" mode.
04:49	The first parameter "FILE1" is the FILEHANDLE.
04:53	The print function prints the given text to FILEHANDLE. i.e 'FILE1'.
04:59	Now, press Ctrl+S to save the file.
05:03	Let us execute the program.
05:05	Switch back to the terminal and type: perl writefile dot pl and press Enter.
05:12	Now, let us check whether the text has been written in 'second.txt' file.
05:18	Type: gedit second.txt and press Enter.
05:23	We can see the text: "Working with files makes data storage and retrieval a simple task!" in our 'second.txt' file.
05:32	Let us close the 'second.txt' file.
05:35	What will happen if we open the same file again in "write" mode? Let us see that.
05:41	In the 'writefile.pl', comment the previous print statement.
05:46	Add the print command shown below.
05:48	Now, press Ctrl+S to save the file. Let us execute the program.
05:54	Switch back to the terminal and type perl writefile dot pl and press Enter.
06:00	Now, let us check the 'second.txt' file once again.
06:04	Type: "gedit second.txt" and press Enter.
06:09	We can see the output: “Greater than symbol (>) overwrites the content of the file!"
06:14	The previous contents of the 'second.txt' file has been overwritten.
06:19	This is because, we had opened the file in the "write" mode again.
06:24	Let us close the 'second.txt' file.
06:27	Next, we will see how to append data to an existing file.
06:32	The open statement with two greater than (>>) symbols denotes the "APPEND" mode.
06:38	Now, I will open the writefile dot pl again in gedit.
06:44	In the open statement, type two greater (>>) than symbols. This will denote that the file is in append mode.
06:52	Comment the previous print statement, as it is already executed.
06:57	Add the line: print FILE1 within double quotes "Two greater than symbols (>>) open the file in append mode", to append to the existing data.
07:07	Now, press Ctrl+S to save the file.
07:11	Let us execute the program.
07:14	Switch back to the terminal and type: perl writefile dot pl and press Enter.
07:20	Now, let us check whether the text has been appended to the 'second.txt' file.
07:26	Type: gedit second.txt and press Enter.
07:31	We can see the text has been appended in our 'second.txt' file.
07:36	Let us close the 'second.txt' file.
07:39	Similarly, there are other modes also.
07:42	Try out these options on your own and understand what happens.
07:49	This brings us to the end of this tutorial. Let us summarize.
07:53	In this tutorial, we learnt to: Open a file in "read" mode Write to a file Open a file in "append" mode and Close the file handle.
08:03	Here is an assignment for you. Change the file attribute to "+>" in the 'writefile.pl' program.
08:11	Save and execute the program.
08:14	Open the 'second.txt' file to see the output.
08:17	Analyze the usage of file attribute "+>".
08:22	The video at the following link summarizes the Spoken Tutorial project. Please download and watch it.
08:29	The Spoken Tutorial Project team: conducts workshops using spoken tutorials and gives certificates on passing online tests.
08:37	For more details, please write to us.
08:41	Spoken Tutorial project is funded by NMEICT, MHRD, Government of India.
08:48	More information on this mission is available at this link.
08:53	This is Nirmala Venkat from IIT Bombay, signing off. Thanks for watching.

Contributors and Content Editors

PoojaMoolya, Sandhya.np14