Show all users and their groups/vice versa

Question

I know I have existing groups and users but I'm not sure about their association. Is there an shell command I can use to list all users or all groups and a command to list all groups/users for a specified user/group?

So something like showusers would list all users, and showgroups -u thisuser would show all the groups that have thisuser in it.

Answer 1

All users:

$ getent passwd

All groups:

$ getent group

All groups with a specific user:

$ getent group | grep username

Answer 2

for user in $(awk -F: '{print $1}' /etc/passwd); do groups $user; done

cat /etc/group | awk -F: '{print $1, $3, $4}' | while read group gid members; do
    members=$members,$(awk -F: "\$4 == $gid {print \",\" \$1}" /etc/passwd);
    echo "$group: $members" | sed 's/,,*/ /g';
done