Redirection in Linux

Redirection

  
whenever we execute a command, it shows its output on the terminal window only. But, if you want this output to be saved in a file then concept of redirection comes in picture.

• STDOUT

> (Greater than) is used for redirecting output to a file

so whenever we use >, the output is saved to a new file. But if you want to redirect its output to an existing file then you have to use >> else your previous data will be lost.



TO verify this you can see below screenshot.



 Now appended content screenshot

• STDIN 

   

   < (less than) is used for redirecting input to file.

  
  if you want to do some operation on the file. like counting number of words, lines.. etc in a file.
  
  
  
  Note the difference, when you are using redirection, your file name is not shown because during redirection file is sent anonymously.
   

you can also easily combine STDIN and STDOUT. e.g. you want to process a file and want to save the output into a new file then this combination is useful.

Note: you can also use 0> for STDIN and >1 for STDOUT.

• STDERR

This stream has value 2 and it will be used as 2>.

ls video.mp4 2> error.txt

If video.mp4 is not present in your file directory then you will get an error- 

ls: cannot access 'video.mp4': No such file or directory

and this error will be saved to a new file error.txt.

If you want to append the errors in an existing file then you must use 2>>.

if you want to save output and error both then you can do this like

ls -l video.mp4 file.txt > myoutput  2>&1 

video.mp4 is not in your system and file.txt is inside your home directory. so whenever you will run above code. it will make a new file myoutput and save the error as well as output in file named myoutput.


You can read about pipe here.