Why reading and writing the same file through I/O redirection results in an empty file in Unix?

Question

If I redirect output of a command to same file it reads from, its contents is erased.

sed \'s/abd/def/g\' a.txt > a.txt

Can anyone expla

Answer 1

stdout and stderr will first prepared and then stdin and then the command execute. so a.txt would be clear for stdout first and then when the comamnd execute no content could be found. try sed -i 's/abd/def/g' a.txt or sed 's/abd/def/g' a.txt 1<> a.txt