how to write finding output to same file using awk command

Question

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.

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.

Answer 2

Since GNU Awk 4.1.0, there is the "inplace" extension, so you can do:

$ gawk -i inplace '{ gsub(/foo/, "bar") }; { print }' file1 file2 file3 

To keep a backup copy of original files, try this:

$ gawk -i inplace -v INPLACE_SUFFIX=.bak '{ gsub(/foo/, "bar") } > { print }' file1 file2 file3 

This can be used to simulate the GNU sed -i feature.

See: Enabling In-Place File Editing