how to understand this nawk command

Question

How to understand the following awk command?

nawk 'a-- >= 0; /datamart_extractrelations_static/ {a = 30}' app.log

Given a awk program has a format of:

pattern { action }

What is 'a-- >= 0' do? when does this operation get executed?

Answer 1

The command is the same as:

nawk 'a-- >= 0 {print}; /datamart_extractrelations_static/ {a = 30}' app.log

If a-- >= 0 awk will print the current line since print is the default action in awk which will be executed if no action was specified.

As a result, the command will print the 30ths line after a line that contains the pattern datamart_extractrelations_static

To better understand this, you can simply try the following command:

awk '1' input

Since 1 will always evaluate to true and the default action is print awk prints every line of input unchanged.

Answer 2

Someone didn't quite get the idiom right for printing 30 lines after datamart_extractrelations_static is found in the input. See Printing with sed or awk a line following a matching pattern and look specifically at idiom e there "Print the N records after some pattern" for the correct code. The code you posted is missing the leading test on a (who names a count variable a???) to make sure the var doesn't keep decrementing below zero for a huge file and eventually wrap around and appear positive again.