grep: if input file is also the output, then why clearing the input file?

Question

Scenario:

$ cat t0.txt
xxx
$ grep xxx t0.txt > t0.txt
grep: t0.txt: input file is also the output
exit status 2
$ cat t0.txt
<nothing>

Question: if input file is also the output AND the exist status is 2 (an error occurred), then why clearing the input file?

Answer 1

shell redirections happens first; so at first shell empty your output file t0.txt then it passes to the grep and then grep complains input & output are the same; how grep knows they are same(?) because it checks the inode number of the input &output file (see the source code here/lines 1377~1403).

unfortunately you lost your t0.txt content as the result.