Linux cat example

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

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