bash - how to write finding output to same file using awk command

Question

awk '/^nameserver/ && !modif { printf("nameserver 127.0.0.1
"); modif=1 } {print}' testfile.txt

It is displaying output but I want to write the output to same file. In my example testfile.txt.

Answer 1

Not possible per se. You need a second temporary file because you can't read and overwrite the same file. Something like:

awk '(PROGRAM)' testfile.txt > testfile.tmp && mv testfile.tmp testfile.txt

The mktemp program is useful for generating unique temporary file names.

There are some hacks for avoiding a temporary file, but they rely mostly on caching and read buffers and quickly get unstable for larger files.