Shell script, logical structure problem exercises.

awk '/5/{a=1}!a ' file2
Results:
1
2
3
4
Explain:
The first line/5/does not match skip {a=1}, continue to!a, at which point a no value belongs to the false take reverse to true, so the first line of output

The second line/5/does not match skip {a=1}, continue to!a, at this time a no value belongs to false take reverse to true, so output second
.
.
The five line equals 5 match/5/execution {a=1}, continue!a, at this time a=1 has the value belongs to true take reverse false, does not output

Line six/5/does not match skip {a=1}, continue!a, at this time a=1 belongs to true take the false, not output
.
.
.
Until the end is not output

Shell script, logical structure problem exercises.