Number of logins on Linux using Shell script and AWK - bash

Question

How can I get the number of logins of each day from the beginning of the wtmp file using AWK?
I thought about using an associative array but I don't know how to implement it in AWK..
myscript.sh
#!/bin/bash
awk 'BEGIN{numberoflogins=0}
#code goes here'
The output of the last command:
[fnorbert#localhost Documents]$ last
fnorbert tty2 /dev/tty2 Mon Apr 24 13:25 still logged in
reboot system boot 4.8.6-300.fc25.x Mon Apr 24 16:25 still running
reboot system boot 4.8.6-300.fc25.x Mon Apr 24 13:42 still running
fnorbert tty2 /dev/tty2 Fri Apr 21 16:14 - 21:56 (05:42)
reboot system boot 4.8.6-300.fc25.x Fri Apr 21 19:13 - 21:56 (02:43)
fnorbert tty2 /dev/tty2 Tue Apr 4 08:31 - 10:02 (01:30)
reboot system boot 4.8.6-300.fc25.x Tue Apr 4 10:30 - 10:02 (00:-27)
fnorbert tty2 /dev/tty2 Tue Apr 4 08:14 - 08:26 (00:11)
reboot system boot 4.8.6-300.fc25.x Tue Apr 4 10:13 - 08:26 (-1:-47)
wtmp begins Mon Mar 6 09:39:43 2017
The shell script's output should be:
Apr 4: 4
Apr 21: 2
Apr 24: 3
, using associative array if it's possible

Answer 1

In awk, arrays can be indexed by strings or numbers, so you can use it like an associative array.
However, what you're asking will be hard to do with awk reliably because the delimiters are whitespace, therefore empty fields will throw off the columns, and if you use FIELDWIDTHS you'll also get thrown off by columns longer than their assigned width.
If all you're looking for is just the number of logins per day you might want to use a combination of sed and awk (and sort):
last | \
sed -E 's/^.*(Mon|Tue|Wed|Thu|Fri|Sat|Sun) (Jan|Feb|Mar|Apr|May|Jun|Jul|Aug|Sep|Oct|Nov|Dec) ([ 0-9]{2}).*$/\2 \3/p;d' | \
awk '{arr[$0]++} END { for (a in arr) print a": " arr[a]}' | \
sort -M
The sed -E uses extended regular expressions, and the pattern just prints the date of each line that is emitted by last (This matches on the day of week, but only prints the Month and Date)
We could have used uniq -c to get the counts, but using awk we can do an associative array as you hinted.
Finally using sort -M we're sorting on the abbreviated date formats like Apr 24, Mar 16, etc.

Answer 2

Try the following awk script(assuming that the month is the same, points to current month):
myscript.awk:
#!/bin/awk -f
{
a[NR]=$0; # saving each line into an array indexed by line number
}
END {
for (i=NR-1;i>1;i--) { # iterating lines in reverse order(except the first/last line)
if (match(a[i],/[A-Z][a-z]{2} ([A-Z][a-z]{2}) *([0-9]{1,2}) [0-9]{2}:[0-9]{2}/, b))
m=b[1]; # saving month name
c[b[2]]++; # accumulating the number of occurrences
}
for (i in c) print m,i": "c[i]
}
Usage:
last | awk -f myscript.awk
The output:
Apr 4: 4
Apr 21: 2
Apr 24: 3