Difference between revisions of "BASH/C3/More-on-Redirection/English"

• Redirection of both standard error and output
• Appending redirected output
• With the help of some examples

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System requirements

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

Please note, GNU Bash version 4 or above is recommended to practice this tutorial.

Redirection of both stderr and stdout

• This can be done in multiple methods.

We will cover two of the most important methods of redirections in this tutorial.

Redirection of both stderr and stdout

Syntax:

command &>filename

&>

ampersand followed by greater-than sign

The syntax is

Command space ampersand greater than space filename

I have typed some code in this file.

#!/usr/bin/env bash

Note that '/user' directory does not exist.

Hence the command 'ls' will throw an error.

And ampersand followed by greater than will redirect stdout and stderr to out_file.txt

Open the terminal using CTRL+ALT+T keys

simultaneously on your keyboard

chmod +x redirect.sh>>Press Enter

dot slash redirect.sh

Press Enter.

Type dot slash redirect.sh

Press Enter.

cat out_file.txt

Type cat space out_file.txt

[Highlight]

ls: cannot access /user: No such file or directory

/usr:

arm-linux-gnueabi

arm-linux-gnueabihf

bin

games

i686-linux-gnu

include

lib

local

sbin

share

src

The error for directory /user is recorded in this file.

It says that there was no '/user' directory found.

The directory content for /usr is displayed.

Please note that the content for '/usr' directory may vary on your system.

rm out_file.txt

Redirection of both stderr and stdout

Syntax:

command > filename 2>&1

command >/dev/null 2>&1

2>&1

2 greater than ampersand 1 after the filename

The syntax iscommand space greater than

filename space 2 greater than ampersand 1We can also redirect to slash dev slash null (/dev/null) file.

/dev/null

• It is a null file or a place where we can dump anything.
• It includes the output and error messages
• It is also called as bit bucket.

/dev/null 2>&1

So, we have

ls /usr /user >/dev/null 2>&1

So I will change this part of the copied code.> (greater than) means truncate or write.

/dev/null is the null file

2>&1 (2 greater than ampersand 1)

Number “2” will redirect standard error to standard output, denoted by number “1”.

Go to the terminal.

dot slash redirect.sh

and press Enter.

cat out_file.txt

Appen redirected output

Synatx:

command >> filename

• We can capture as well as append standard output or error to a file.
• The output or the error will be appended at the end of the file.
• If the file does not exist, it will create a new file.

The syntax is:command space greater than greater than space followed by filename

Let me open the file redirect.sh

date space greater than greater than space out_file.txt

date

We can check this command on the terminal by typing 'date'.

Come back to the terminal.

Type: date

You can see that the system date i.e. the current date is displayed.

The output of date command will be appended to the out_file.txt file.

We are using this file to capture standard output and error of 'ls' command.

Switch to the terminal.

Recall the previous command

./redirect.sh

and press Enter.

Type cat space out underscore out_file.txt

Wed Jul 2 12:06:32 IST 2014

Summary

Let us summarise.

In this tutorial we learnt

• Redirection of both standard error and output
• And to append the redirected output

Assignment

1. Create 'X_file.txt' file with some content.
2. Redirect the content of both 'out_file.txt' and 'X_file.txt' to a new file.

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

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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.