Explain awk command

Question

Today I was searching for a command online to print next two lines after a pattern and I came across an awk command which I\'m unable to understand.

$ /usr/x         


        

Answer 1

Simply put the command prints a number of lines after a given regular expression expression match not including the matched line.

The number of lines is specified in the block {_=2} and the variable _ is set to 2 if the line matches PATTERN. Every line read after a matching line causes _ to be decremented. You can read _&&_-- as if _ is greater than zero then minus one from it, this happens for every line after a match until _ hits zero. It's quite simple when you replace the variable _ with a more sensible name like n.

A simple demo should make it clear (print the 2 lines that follow any line matching foo):

$ cat file
foo
1
2
3
foo
a
b
c

$ awk 'n && n--;/foo/{n=2}' file
1
2
a
b

So n is only True when it gets set to 2 after matching a line with foo then it decrements n and prints the current line. Due to awk having short circuit evaluation n is only decrement when n is True (n>0) so the only possible values in this case for n are 2,1 or 0.

Awk has the following structure condition{block} and when a condition is evaluated True then block is executed for the current record. If you don't provide a block awk uses the default block {print $0} so n && n--; is a condition without a block that only evaluates to True for n lines after the regular expression match. The semi-colon just delimits the condition n&&n-- for the conditions /foo/ make it explicit that the condition has no block.

To print the two lines following the match including the match you would do:

$ awk '/foo/{n=3} n && n--' file
foo
1
2
foo
a
b

Extra extra: the fact that the full path of /usr/xpg4/bin/awk is used tells me this code is intended for a Solaris machine as the /usr/bin/awk is totally broken and should be avoided at all costs.