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A list of who's currently logged in:

NOTE: getent will return local users assuming you do not have sssd (or some similar service running which pulls LDAP users in too) and your /etc/nsswitch.conf is restricted to files, i.e. it's not including things like NIS or NIS+. For pure local user's only you can resort to awk -F: '{print $1}' /etc/passwd.

A list of who's currently logged in:

Using join instead of grep

You could also forgo using grep and use join instead, since this type of problem is more in join's wheelhouse. We'll need to use join -v 2 which means that we want to exclude matches and only show the uniques from our second argument, getent ....

join man page

-a FILENUM
        also print unpairable lines from file FILENUM, where FILENUM is 
        1 or 2, corresponding to FILE1 or FILE2

-v FILENUM
        like -a FILENUM, but suppress joined output lines


$ join -v 2 <(who | awk '{print $1}' | sort -u) \
    <(getent passwd | awk -F: '{print $1}' | sort) | wc -l
53

NOTE: The only caveat with using join is that both lists need to be sorted, so we have to add on a | sort to getent ....

Listing out groups of users

You can get a list of all your local users with this command:

$ getent passwd | awk -F: '{print $1}'

A list of who's currently logged in:

$ who

A list of user's that are currently not loggd in:

$ grep -vf <(who | awk '{print $1}' | sort -u) \
    <(getent passwd | awk -F: '{print $1}')

This last one takes the list of users who are logged in and shows the list of all user's minus the logged in users, using grep -vf.

Getting counts

To get counts, simply take a wc -l on to the end of commands.

not logged in

$ grep -vf <(who | awk '{print $1}' | sort -u) \
    <(getent passwd | awk -F: '{print $1}') | wc -l
53

logged in

$ who | awk '{print $1}' | sort -u | wc -l
1

Listing out groups of users

You can get a list of all your local users with this command:

$ getent passwd | awk -F: '{print $1}'

A list of who's currently logged in:

$ who

A list of user's that are currently not loggd in:

$ grep -Fxvf <(who | awk '{print $1}' | sort -u) \
    <(getent passwd | awk -F: '{print $1}')

This last one takes the list of users who are logged in and shows the list of all user's minus the logged in users, using grep -vf.

Getting counts

To get counts, simply take a wc -l on to the end of commands.

not logged in

$ grep -Fxvf <(who | awk '{print $1}' | sort -u) \
    <(getent passwd | awk -F: '{print $1}') | wc -l
53

logged in

$ who | awk '{print $1}' | sort -u | wc -l
1

grep flags

-F, --fixed-strings
        Interpret PATTERN as a list of fixed strings, separated by newlines, 
        any of which is to be matched.

-x, --line-regexp
        Select only those matches that exactly match the whole line.

-v, --invert-match
        Invert the sense of matching, to select non-matching lines.

-f FILE, --file=FILE
        Obtain patterns from FILE, one per line.  The empty file contains 
        zero patterns, and therefore matches nothing.