How do I add a line of text to the middle of a file using awk?

Question

This is an great answer. https://stackoverflow.com/a/6739327/15603477 But Still a little bit confused me.

without a variable, awk '/^nameserver/ { printf("nameserver 127.0.0.1\n")} {print}' file2 will get:

# Generated by NetworkManager
domain dhcp.example.com
search dhcp.example.com
nameserver 127.0.0.1
nameserver 10.0.0.1
nameserver 127.0.0.1
nameserver 10.0.0.2
nameserver 127.0.0.1
nameserver 10.0.0.3

After try serval combination, I found out that I had to use awk '/^nameserver/ && !a { printf("nameserver 127.0.0.1\n"); a=1 } {print}' file2

But I am still confused with !a and a=1 function to stop the printf("nameserver 127.0.0.1\n") duplication.

Answer 1

Name the variable already_inserted instead of a and it will become clear. Then it reads like:

if my_pattern matches and not already_inserted { insert "some text"; already_inserted=true; }

awk will run this command for each line.

Answer 2

the use of these kind of variables known as "flag variable", that let the program knows a certain condition has met then decide based on its value for further processing; here that's a simple control flag that whenever it doesn't set and has 0 or empty value (in awk default variables' value is 0 when doing integer comparison or its empty string when doing string comparison) then the string will be printed and the flag a=1 will be set, and so it will stop adding duplicates since that's now non-zero value.

In many programming languages, ! (not) negates the evaluation result of an statement which when it's used like ! a, it negates the evaluation result of the a variable, if a=0, then ! a will retun 1, or will retun 0 if it was a=1.