Confusion about sed and '>'

Question

The following command works fine:

sed s/input/raw_input/ p.py >p2.py && mv p2.py p.py

However, the following command turns p.py into an empty file:

sed s/input/raw_input/ p.py >p.py

The following also turns p.py into an empty file:

sed s/input/raw_input/ p.py | cat > p.py

Why don't the last two commands behave as expected? In particular why is p.py empty?

Answer 1

This is because the redirection operator > is called before any command and thus the following happens in this order:

1. p.py is opened for writing, truncating whatever was there before
2. sed is called to perform commands on p.py which is now empty
3. The output (which is nothing) is written to p.py (so it still contains nothing)

You have two options depending on how portable you want your code to be:

not portable

Use GNU sed -i option to do an in-place edit

portable

redirect the output to a temp file then move the temp file over the original after sed is complete