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awk '/^nameserver/ && !modif { printf("nameserver 127.0.0.1\n"); modif=1 } {print}' testfile.txt
It is displaying output but I want to write the output to same file. In my example testfile.txt.
Not possible per se. You need a second temporary file because you can’t read and overwrite the same file. Something like:
The
mktempprogram 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.