BASH/C3/Using-File-Descriptors/English-timed

Time	Narration
00:01	Dear friends, welcome to the spoken tutorial on Using File Descriptors.
00:08	In this tutorial, we will learn to:
00:11	Assign an output file descriptor
00:14	Assign an input file descriptor
00:17	Close the file descriptor (fd)
00:19	with the help of some examples.
00:23	To follow this tutorial, you should have knowledge of Shell Scripting in BASH.
00:29	If not, for relevant tutorials, please visit our website which is as shown. http://www.spoken-tutorial.org
00:35	For this tutorial, I am using:
00:38	Ubuntu Linux 12.04 Operating System
00:43	GNU BASH version 4.2
00:46	Please note, GNU Bash version 4 or above is recommended for practice.
00:54	Let us start with an introduction.
00:56	We have already studied about file descriptors in the previous tutorial.
01:02	0, 1 and 2 are the standard file descriptors for stdin, stdout and stderr.
01:15	File descriptors are used for i/o redirection.
01:20	The syntax to assign a file descriptor to an output file is:
01:25	exec [File descriptor] greater than symbol filename
01:31	Let us see an example.
01:33	I have a code file with the name fdassign dot sh.
01:43	The first line is the shebang line.
01:49	The "exec" command replaces the current shell process.
01:56	It will be executed in the place of the current shell without creating a new process.
02:04	We know that 0, 1, and 2 are standard file descriptors.
02:09	For any newly opened file, we have additional file descriptors from 3 to 9.
02:19	Here, 3 is the file descriptor.
02:22	This will write the output to the output dot txt file.
02:30	The string "Welcome to BASH learning" is sent to the file output dot txt.
02:36	This is done via file descriptor 3.
02:42	This is similar to redirecting a string to a file.
02:49	Each new string will be appended to the file.
02:52	For example:
02:54	We will append the current system date to the output dot txt file.
03:00	The syntax is: date SPACE greater-than symbol ampersand sign 3.
03:13	Here, we close the file descriptor.
03:16	After this line, the descriptor cannot write anything to the output dot txt file.
03:23	Let us execute the code and see the output.
03:26	Open the terminal using CTRL+ALT+T keys.
03:34	Type: chmod space plus x space fdassign dot sh
03:41	Type: dot slash fdassign dot sh
03:46	Let us check the output now by typing cat space output dot txt.
03:56	We can see that the string "Welcome to BASH learning" and the current system date is displayed.
04:05	Let us go back to the editor.
04:11	Now I will type echo at the end, after the descriptor is closed.
04:17	Type: echo space within double quotes Hi after quotes space greater than symbol ampersand sign 3
04:31	Click on Save.
04:35	Let us execute the script once again and see what happens.
04:38	On the terminal, press the up-arrow key twice, recall the previous command dot slash fdassign dot sh.
04:50	press Enter.
04:52	We see an error:
04:55	"Bad file descriptor".
04:58	Let us fix this error.
05:00	Come back to the editor.
05:03	I will cut the last line of code and paste it below the date command.
05:11	Click on Save.
05:13	Let us execute the code once again, on the terminal.
05:19	Recall the previous command dot slash fdassign.sh.
05:24	press Enter.
05:26	Now let us open the output dot txt file.
05:29	Type: cat space output dot txt
05:41	We can see the output.
05:43	The string "Hi" is displayed at the end.
05:49	Now we will assign the file descriptor to the input file.
05:54	Let us see an example.
05:56	I have a file named fdread dot sh.
06:03	Let us go through it.
06:07	This is the 'exec' command.
06:13	Here we will read the file output dot txt.
06:19	The line exec 3 lesser than symbol output dot txt will open the file for reading.
06:30	'cat' command will display the content of the file.
06:35	And finally, we close the file descriptor.
06:39	Now, let us execute this shell script.
06:42	On the terminal, let me clear the prompt.
06:47	Type: chmod space plus x space fdread dot sh
06:55	Type dot slash fdread dot sh
07:01	We can see the output on the terminal.
07:05	The content of output dot txt file is displayed.
07:10	This brings us to the end of this tutorial.
07:13	Come back to the slides.
07:16	Let us summarize. In this tutorial, we learned to:
07:19	Assign the output file descriptor
07:22	Assign the input file descriptor
07:26	Close the file descriptor.
07:28	As an assignment:
07:30	Try to append a few lines to a file test dot txt using file descriptors;
07:36	Display the contents of the file using file descriptors.
07:41	Watch the video available at the link shown below.
07:45	It summarizes the Spoken-Tutorial project.
07:48	If you do not have good bandwidth, you can download and watch it.
07:53	The Spoken Tutorial Project team: Conducts workshops using spoken tutorials.
07:58	Gives certificates to those who pass an online test.
08:02	For more details, please write to contact@spoken-tutorial.org
08:10	Spoken Tutorial Project is a part of the Talk to a Teacher project.
08:14	It is supported by the National Mission on Education through ICT, MHRD, Government of India.
08:22	More information on this mission is available at the link shown below http://spoken-tutorial.org/NMEICT-Intro
08:28	The script has been contributed by FOSSEE and Spoken-Tutorial teams.
08:33	This is Ashwini Patil from IIT Bombay, signing off.
08:37	Thank you for joining.

Contributors and Content Editors

Gaurav, PoojaMoolya, Pratik kamble, Sandhya.np14