How can I use a file in a command and redirect output to the same file without truncating it?

Question

Basically I want to take as input text from a file, remove a line from that file, and send the output back to the same file. Something along these lines if that makes it any

Answer 1

You can do that using process-substitution.

It's a bit of a hack though as bash opens all pipes asynchronously and we have to work around that using sleep so YMMV.

In your example:

grep -v 'seg[0-9]\{1,\}\.[0-9]\{1\}' file_name > >(sleep 1 && cat > file_name)

• >(sleep 1 && cat > file_name) creates a temporary file that receives the output from grep
• sleep 1 delays for a second to give grep time to parse the input file
• finally cat > file_name writes the output