Variables: https://www.tutorialspoint.com/awk/awk_built_in_variables.htm
Operators: https://www.tutorialspoint.com/awk/awk_operators.htm
Regular Expression in AWK
1. Match
## ~ ===pronunced as===> 'Tilde'
### N, strings ===read as===> regular expression constant
For numbers:
$ awk '$0 ~ N' filename ### to print out the lines in filename which contains the number N;
$ awk '$0 ~ /strings/' filename ### to print out the lines in file which including strings specified;
2. Not Match: using ' !~ ' to replace ' ~ '
Ref: https://www.tutorialspoint.com/awk/awk_regular_expression_operators.htm
