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

You need to use the -i option to edit the file in place:

sed -i .bck 's/abd/def/g' a.txt

EDIT: as noted by neil, the redirection first opens the file for writing thus clears it.

EDIT2: it might be interesting for some reader

On OSX, if you want to use -i with an empty extension to prevent the creation of a backup file, you need to use the -eswitch as well otherwise it fails to parse the arguments correctly:

sed -i -e 's/abc/def/g' a.txt