Difference between revisions of "BASH/C3/Basics-of-Redirection-(error-handling)/English"

• Input and output in Bash
• Redirection and file descriptors
• Standard input
• Standard output
• Standard error

With the help of some examples

If not, for relevant tutorials please visit our website which is as shown,(http://www.spoken-tutorial.org)

System requirements

• Ubuntu Linux 12.04 Operating System and
• GNU BASH version 4.2

Please note, GNU Bash version 4 or above is recommended for practice.

Input and Output

• In GNU/Linux we can send output to a file or read input from a file.
• Each Shell command has its own inputs and outputs
• Input and output is redirected using a special notation interpreted by the Shell

Redirection and file descriptors

• Changing the default path of input or output is called as redirection
• In GNU/Linux everything is a file, including the hardware
• Common return values are:
  • 0 for Input i.e. Keyboard
  • 1 for Output i.e. Screen
  • 2 for Error i.e. Screen

Redirection and file descriptors

• 0, 1, 2 are POSIX numbers and also known as file descriptors (FD).
• A redirector uses POSIX numbers to talk with a user or other program.

Standard Input

• Standard input is the default input method
• It is used by all commands to read input.
• It is denoted by zero (0).
• Also known as stdinb(Standard input).

Input Redirection

command < filename

• The default standard input is the keyboard.
• (<) Less than symbol is input redirection symbol
• The syntax is Command space lesser symbol space filename

#!/usr/bin/env bash

This is the shebang line.

• Type sort space less than symbol space file.txt
  This is an example of input redirection.
• The input is taken from file file.txt
• sort command does the sorting of numbers present in file.txt

Open the terminal using Ctrl, Alt and T simultaneously on your keyboard.

Type cat space file.txt.

You can see that the file contains series of numbers.

chmod +x redirection.sh>>Press Enter

dot slash redirection.sh

Press Enter.

Type dot slash redirection.sh

Press Enter.

0

2

3

7

8

9

The numbers are sorted in asscending order.

<<PAUSE>>

Standard Output

• Standard output is used by all commands to display output.
• The default output is displayed on the screen.
• It is denoted by number one(1).
• Also known as stdout (Standard output).

Output Redirection

command > filename

• ( > )Greater than symbol is the output redirection symbol
• Syntax is Command space greater symbol space filename

This is an example of output redirection.

ls command lists the information about files in that particular directory.

Recall the previous command dot slash redirection.sh.

Press Enter.

gedit ls_file.txt

We can now see the output in this file. So our redirect was successful.

<<PAUSE>>

Standard Error

• Standard error is the default output error
• It is used to write all system errors
• It is denoted by number two (2).
• It is also known as stderr (Standard error).

Error Redirection

command 2> errors.txt

• The default standard error output is visible on the screen or monitor.
• Two greater than symbol (2>) is error redirection symbol
• Syntax is command space 2 greater than space error.txt

i.e.ls

The error output is redirected to error.txt file.

Click on Save and switch to the terminal.

rm /tmp/4815.txt

Type rm /tmp/4815.txt

Press Enter.

rm: cannot remove `/tmp/4815.txt': No such file or directory

Recall the previous command dot slash redirection.sh

Press Enter.

gedit error.txt

Summary

Let us summarise.

In this tutorial we learnt

• Input and output in Bash
• Redirection and file descriptors
• Standard input using <(less than) symbol
• Standard output >(greater than) symbol
• Standard error using 2> (2 greater than symbol)

Write a program in any language like C, C++, Java.

And redirect the output or error to a new file.

OR

Create a text file with some content like your name, address.

Redirect the content to a new file.

http://spoken-tutorial.org /What\_is\_a\_Spoken\_Tutorial

About the Spoken Tutorial Project

It summarises the Spoken Tutorial project.

If you do not have good bandwidth, you can download and watch it.

Spoken Tutorial Workshops

• Conducts workshops using spoken tutorials
• Gives certificates to those who pass an online test

For more details, please write to

contact@spoken-tutorial.org

Acknowledgement

It is supported by the National Mission on Education through ICT, MHRD, Government of India.

More information on this Mission is available at:

http://spoken-tutorial.org\NMEICT-Intro

This is Ashwini from IIT Bombay.

Thank you for joining.