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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.
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
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.
07.17 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
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
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