22.Input_Output_Redirection.md

Redirection

As Standard Input, Standard Output, and Standard Error are data streams, we can redirect these streams.

Redirecting Standard Output

We can redirect standard output by using > and >> symbols.

• > will perform overwriting of existing data
• >> will perform appending to existing data

Example: To redirect the standard output of cat command from terminal to output.txt:

$ cat 1> output.txt
sample data
ctrl+d

Sample data won't be displayed to the terminal and will be written to output.txt.

Note: The redirection symbol > is always associated with 1 (Standard Output) by default. Hence, we are not required to specify 1 explicitly.

Appending to a File

$ cat >> output.txt
sample data
ctrl+d

Instead of overwriting, if we want to perform appending, we should use >>.

Redirecting Standard Error

We can redirect error messages from the terminal to our own file by using > and >> symbols.

$ cal 34 w3892384208342 2>> error.txt

Now the error message won't be displayed to the console and will be written to error.txt. For error redirection, 2 is mandatory.

Error Redirection

We can implement error redirection by using > and >> symbols.

Standard Error Stream is associated with digit 2.

$ cal pqr347 2>> error.txt #===>Error Redirection

$ cal abc721 >> error.txt #===>Output redirection

Redirecting Standard Input

We can redirect standard input from keyboard to our required file. We can perform input redirection by using the < symbol.

akshay@myubuntu:~/Documents/Linux$ cat 0<demo.txt
add some content here
add omre extra cotent here
this is extra line added after permission updated

added last line by akshay 

$ cat 0< a.txt 1>>output.txt 2>>error.txt

The < symbol is always associated with 0 by default. Hence we can remove it:

$ cat < a.txt >>output.txt 2>>error.txt

Note: To redirect both standard output and standard error to the same destination, we can use the following shortcut:

$ cat < a.txt &> output.txt

&> means both standard output and standard error.

Redirecting Standard Output from One Terminal to Another Terminal

In Unix, everything is treated as a file, even our terminal. We can find terminal-related files by using the tty command.

Terminal 2:

$ tty
/dev/pts/1

Terminal 1:

$ ls -l 1> /dev/pts/1

Terminal 1's long listing output will be displayed on Terminal 2.

Bits

1. In how many ways can a command get input?

   • Answer: 2 ways
     1. From Standard Input
     2. From command line arguments

2. Which of the following contains Data Streams?

   • A. Standard Input
   • B. Standard Output
   • C. Standard Error
   • D. Command Line arguments
   • Answer: A, B, C

3. By default, Standard Input is connected to:

   • A. Terminal
   • B. Keyboard
   • C. A File
   • Answer: B

4. True or False: By default, Standard Output is connected to Terminal.

   • Answer: True

5. True or False: By default, Standard Error is connected to Terminal.

   • Answer: True

6. What number represents the Standard Input Stream?

   • Answer: 0

7. What number represents the Standard Output Stream?

   • Answer: 1

8. What number represents Standard Error?

   • Answer: 2

9. How can we redirect Standard Output of the ls command to a file called output.txt?

   • A ls 2> output.txt
   • B ls 0< output.txt
   • C ls 1< output.txt
   • D ls 1> output.txt
   • Answer: D

   Note:

   • ls 2> output.txt redirects standard error from terminal to output.txt
   • ls 0< output.txt redirects standard input from keyboard to output.txt
   • ls 1< output.txt is meaningless

10. How can we redirect the Standard Output of the ls command to output.txt, while at the same time redirecting Standard Error to error.txt?

    • Answer:

      ls 1> output.txt 2> error.txt

      or

      ls > output.txt 2> error.txt

11. Explain the difference between <, >, >> in Redirection?

    • Answer:
      • < symbol is meant for input redirection
      • > symbol is meant for output redirection where the existing data will be overwritten
      • >> symbol is meant for output redirection where the data will be appended instead of overwriting