BASH/C3/Basics-of-Redirection-(error-handling)/English-timed

From Script | Spoken-Tutorial
Jump to: navigation, search
Time Narration
00:01 Dear friends, welcome to the spoken tutorial on Basics of redirection.
00:07 In this tutorial, we will learn:
00:10 input and output in Bash
00:12 Redirection and file descriptors
00:15 standard input standard output
00:18 standard errorwith the help of some examples.
00:22 To follow this tutorial, you should have knowledge of Shell Scripting in BASH.
00:28 If not, for relevant tutorials, please visit our website which is as shown. http://www.spoken-tutorial.org
00:34 For this tutorial, I am using:
00:36 Ubuntu Linux 12.04 Operating System and
00:40 GNU BASH version 4.2
00:43 Please note, GNU Bash version 4 or above is recommended for practice.
00:50 In GNU/Linux we can send output to a file or read input from a file.
00:58 Each Shell command has its own inputs and outputs.
01:03 Input and output is redirected using a special notation interpreted by the Shell.
01:11 Changing the default path of input or output is called as redirection.
01:18 In GNU/Linux everything is a file, including the hardware.
01:24 Common return values are:
01:27 '0' for input i.e. keyboard
01:31 '1' for output i.e. screen
01:34 '2' for error i.e. screen.
01:38 0, 1, 2 are POSIX numbers and also known as file descriptors (FD).
01:46 A re-director uses POSIX numbers to talk with a user or other program.
01:54 Standard input: standard input is the default input method.
02:00 It is used by all commands to read input.
02:04 It is denoted by zero (0).
02:07 Also known as stdin (Standard input).
02:13 The default standard input is the keyboard.
02:17 Less than symbol is input redirection symbol.
02:22 The syntax is- command space less than symbol space filename.
02:30 Let me open a file named redirection dot sh.
02:34 I have typed some code in this file.
02:37 This is the shebang line.
02:41 Type: sort space less than symbol space file dot txt.
02:48 This is an example of input redirection.
02:52 The input is taken from file file dot txt.
02:57 sort command does the sorting of numbers present in file dot txt.
03:04 Click on Save.
03:06 Let us run the file redirection dot sh.
03:10 Open the terminal using Ctrl, Alt and T keys simultaneously on your keyboard.
03:18 Before that, let us see the contents of file dot txt.
03:23 Type: cat space file dot txt.
03:27 Press Enter.
03:30 You can see that the file contains series of numbers.
03:35 Now type: chmod space plus x space redirection dot sh.
03:43 Press Enter.
03:45 Type: dot slash redirection dot sh
03:48 Press Enter.
03:51 We can see the output on the terminal after sorting.
03:56 The numbers are sorted in ascending order.
04:00 Come back to our slides.
04:03 Standard output: standard output is used by all commands to display output.
04:10 The default output is displayed on the screen.
04:14 It is denoted by number one (1).
04:17 Also known as stdout (Standard output).
04:23 ( > )Greater than symbol is the output redirection symbol.
04:28 Syntax is command space greater than symbol space filename.
04:35 Let me go back to our file redirection dot sh.
04:41 Comment the previous line i.e. sort.
04:45 Below it, type: ls space greater than symbol space ls underscore file.txt
04:55 This is an example of output redirection.
04:59 The output of 'ls' will be stored in ls_file dot txt .
05:06 'ls' command lists the information about files in that particular directory.
05:14 Now, save the file and switch to the terminal.
05:19 Let me clear the prompt. First, let us type 'ls' and see the output.
05:28 Now press the up-arrow key thrice.
05:33 Recall the previous command dot slash redirection dot sh.
05:38 And Press Enter.
05:41 Now let us check whether the output is redirected correctly.
05:46 Type: gedit space ls underscore file dot txt and press Enter.
05:56 We can now see the output in this file. So our redirect was successful.
06:03 Come back to our slides.
06:06 Standard error: standard error is the default output error.
06:12 It is used to write all system errors.
06:16 It is denoted by number two (2).
06:20 Also known as stderr (Standard error).
06:25 The default standard error output is visible on the screen or monitor.
06:32 Two greater than symbol (2>) is error redirection symbol.
06:36 Syntax is command space 2 greater than symbol space error dot txt.
06:44 Let me go back to the file redirection dot sh.
06:49 We will comment the previous line. i.e.'ls'.
06:54 Below it, type: rm space backslash tmp backslash 4815 dot txt space 2 greater than symbol space error dot txt.
07:11 The error output is redirected to error dot txt file.
07:17 Now, click on Save and switch to the terminal.
07:22 We will type a command to see the error first.
07:26 Type: rm space slash tmp slash 4815 dot txt
07:36 Press Enter.
07:38 The error displayed is-
07:40 rm: cannot remove slash tmp slash 4815 dot txt: No such file or directory.
07:49 Now we will execute our file.
07:53 Press the up-arrow key
07:55 and recall the previous command dot slash redirection dot sh.
08:01 Press Enter.
08:03 Now let us check whether the error is redirected.
08:07 Type: gedit space error dot txt and press Enter.
08:15 We can now see the error redirected to file error dot txt.
08:22 This brings us to the end of this tutorial.
08:26 Let us summarize.
08:28 In this tutorial, we learned:
08:31 Input and output in Bash
08:35 Redirection and file descriptors
08:38 standard input using '<' (less than) symbol
08:42 standard output using '>' (greater than) symbol
08:47 standard error using '2>' (2 greater than symbol).
08:52 As an assignment-
08:54 write a program in any language like C, C++, Java
08:59 and redirect the output or error to a new file.
09:04 Or, create a text file with some contents like your name, address.
09:11 Redirect the contents to a new file.
09:15 Watch the video available at the link shown below.
09:19 It summarizes the Spoken-Tutorial project.
09:23 If you do not have good bandwidth, you can download and watch it.
09:28 The Spoken Tutorial project team:
09:30 Conducts workshops using spoken tutorials.
09:34 Gives certificates to those who pass an online test.
09:38 For more details, please write to contact@spoken-tutorial.org
09:46 Spoken Tutorial project is a part of the Talk to a Teacher project.
09:50 It is supported by the National Mission on Education through ICT, MHRD, Government of India.
09:58 More information on this mission is available at the link shown below.
10:04 The script has been contributed by FOSSEE and Spoken-Tutorial teams.
10:10 This is Ashwini from IIT Bombay, signing off. Thank you for joining.

Contributors and Content Editors

PoojaMoolya, Pratik kamble, Sandhya.np14