List sudoers

To check all the sudo users in current system, you need to run

# This command will print all the users listed in the sudo group in /etc/group
# sudo:x:27:guzhou means that user guzhou belongs to sudo group.
guzhou@guzhou-ThinkPad-P53:~$ getent group sudo
sudo:x:27:guzhou

# Only list sudo users in your system.
guzhou@guzhou-ThinkPad-P53:~$ getent group sudo | cut -d: -f4
guzhou

# Check the sudo group.
guzhou@guzhou-ThinkPad-P53:~$ cat /etc/group
root:x:0:
daemon:x:1:
bin:x:2:
sys:x:3:
adm:x:4:syslog,guzhou
tty:x:5:
disk:x:6:
lp:x:7:
mail:x:8:
news:x:9:
uucp:x:10:
man:x:12:
proxy:x:13:
kmem:x:15:
dialout:x:20:
fax:x:21:
voice:x:22:
cdrom:x:24:guzhou
floppy:x:25:
tape:x:26:
sudo:x:27:guzhou
audio:x:29:pulse
dip:x:30:guzhou
www-data:x:33:
backup:x:34:
operator:x:37:
list:x:38:
irc:x:39:
src:x:40:
gnats:x:41:
shadow:x:42:
utmp:x:43:
video:x:44:
sasl:x:45:
plugdev:x:46:guzhou
staff:x:50:
games:x:60:
users:x:100:
nogroup:x:65534:
systemd-journal:x:101:
systemd-network:x:102:
systemd-resolve:x:103:
input:x:104:
crontab:x:105:
syslog:x:106:
messagebus:x:107:
netdev:x:108:
mlocate:x:109:
ssl-cert:x:110:
uuidd:x:111:
avahi-autoipd:x:112:
bluetooth:x:113:
rtkit:x:114:
ssh:x:115:
lpadmin:x:116:guzhou
whoopsie:x:117:
scanner:x:118:saned
saned:x:119:
pulse:x:120:
pulse-access:x:121:
avahi:x:122:
colord:x:123:
geoclue:x:124:
gdm:x:125:
guzhou:x:1000:
sambashare:x:126:guzhou
nvidia-persistenced:x:127:
nm-openvpn:x:128:
rdma:x:129:
docker:x:130:guzhou
_chrony:x:131:
vboxusers:x:132:

Command "su" and "sudo"

su is the Linux command to substitute (change) user. In Ubuntu, we cannot change to root account using su command since there is no password set for root account. In a word, the root account is blocked. However, we can use "sudo su" to switch to root.

guzhou@guzhou-ThinkPad-P53:~$ sudo su
[sudo] password for guzhou: 
# Now you are in root account as you can see "#".
# Switch from root to user guzhou.
root@guzhou-ThinkPad-P53:/home/guzhou# su - guzhou
guzhou@guzhou-ThinkPad-P53:~$ sudo su

# You can also switch back using "exit"
root@guzhou-ThinkPad-P53:/home/guzhou# exit
exit
guzhou@guzhou-ThinkPad-P53:~$