Enumerate

Current User

whoami	Displays the user name of the user running the command.
sudo -l	May be configured to allow users to run some commands with root privileges.
id	Shows a user's privileges and group membership.
groups	Displays the current user's groups.
finger	Provides detailed information for the current user (if installed).
logname	Shows the username of the user who initiated the session.
env	Lists all environment variables associated with the current user's shell session.
echo $USER	Prints the username.
echo $HOME	Displays the user's home directory path.
echo $SHELL	Shows the user's default login shell.
history	Displays previously executed commands from the last terminal session.

All Users

cat /etc/passwd	Lists user accounts.
grep <pattern> /etc/passwd	Search /etc/password for pattern.
id <username>	Shows user and group IDs for a user.
finger <username>	Provides user details (may not be installed by default).
users	Displays logged-in users.
who	Display currently logged-in users.
w	Display who is logged in and what they are doing.
last	Lists last logged-in users (/var/log/wtmp).
lastb	List last bad login attempts (/var/log/btmp).
lastlog	Shows the last login time for users.
cat /etc/sudoers	Display sudo configuration.
groups <username>	Lists groups for a user.
compgen -u	Lists usernames (bash built-in).
getent passwd	Entries from passwd database similar to /etc/passwd, but includes network-based user databases.
passwd -s <username>	Info about a user's password status (locked, expired, etc.) (requires root privileges).

Extract just usernames from the /etc/passwd file:

awk -F ':' '{print $1}' /etc/passwd

Groups

cat /etc/group	Lists groups.
cat /etc/sudoers	Sudo configuration.
groups	Current user's groups.
groups <username>	Lists groups for a user.
compgen -g	Lists group names (bash built-in).
id	Shows the current user's group IDs.
id <username>	Shows the user's group IDs for a specific user.
grep <pattern> /etc/group	Search /etc/groups by patterns.
getent group	Group entries from databases similar to /etc/group, but includes network-based group databases.

Extract just group names from the /etc/group file:

awk -F ':' '{print $1}' /etc/group