Show all groups a user belongs to
groups <user>
Show primary groups for user
id <user>
Perform a command as the super user
sudo <command>
Switch to specific user
su <user>
Switch to root or specific user and load user profile (CTRL + d to exit user mode)
su -
su - <user>
Create a new user
adduser <username>
Change password for active user
passwd
Change file mode and access control
chmod <permissions> <file>
Permission types
r = read
w = write
x = execute
Numerical values
r = 4
w = 2
x = 1
Permission groups
u = user
g = group
o = other
a = all (sets u, g and o)
Set user to rwx, group to rw and other to r
chmod u=rwx,g=rw,o=r <file>
Set u to rwx, group and other to r
chmod u=rwx,go=r <file>
Add write permissions to user group
chmod g+w <file>
Remove read and write permission from other group
chmod o-rw <file>
Add execute for user, remove write for group, remove rwx for other
chmod u+x,g-w,o-rwx
Set group to read and write for the directory and all sub files and folders
chmod -R g=rw <dir>
Set group automatically when creating new files within <dir> via the g+s SETGID (SET Group ID) flag
chmod g+s <dir>
Remove setting group automatically for new files within <dir> by removing set group flag
chmod g-s <dir>
List available groups
less /etc/group
Modify a user (usermod) and add (-a) them to a group (-G)
usermod -a -G <group> <username>
Change file group associated with the file permissions
chgrp <group> <file>
Change file owner
chown <user> <file>
Find files with permission set to 777 and change them to default 644
find . -type f -perm 777 -print0 | xargs -0 chmod 644
Find directories with permission set to 777 and change them to default 755
find . -type d -perm 777 -print0 | xargs -0 chmod 755
Unlock all locked files found under the current directory
chflags -R nouchg *