Comparing the number of unique visitors during different times of the day

Using the NASA log file (NASA_access_log_Aug95), count the unique number of visitors between 10:00 am and 11:59 am.

Compare that number with that one between 4:00 pm (16:00) and 6:00 pm (18:00)

Time is a 4th field. Youll probably want to create an expression with the range something like that:

/1995:1[68]:[0-5][0-9]/

Visitor is on the first field, therefore, after the match, you may want to print the first field and pipe it through the sort command with the -u flag for the unique visitors:

sort -u