Super User is a question and answer site for computer enthusiasts and power users. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

Can You please explain me why: 'cat < file.txt > file.txt ' makes file.txt empty ?

share|improve this question

migrated from stackoverflow.com Dec 21 '09 at 23:07

This question came from our site for professional and enthusiast programmers.

    
Just to add to the bottom two answers: if you wanted to make a loop (endlessly filling file) you could type cat <file.txt >>file.txt as two >> mean appending to instead of deleting the file. – GregC Dec 22 '09 at 6:32
    
Yikes. That's just scary. I've never even thought of using (abusing?) cat and pipes like that! That's why we have the "touch" command. ;-) Makes for a cool example though. – Brian Knoblauch Jan 6 '10 at 20:29

Because it opens and truncates the file before reading the data — it being shell, the redirections are processed by shell before even starting cat.

share|improve this answer

The > redirection happens first and opens file.txt for writing which clears any existing content.

share|improve this answer
    
This answer is somewhat misleading -- @hacker's is more precise. On my systems, the redirections are processed in order of specification. That is, < happens first, and then >, but the latter opens not merely "for writing" but with truncation (O_TRUNC), which is what "clears existing content." @hacker is right, this happens before cat(1) is even executed. – pilcrow Feb 8 '10 at 22:20

You must log in to answer this question.