Users and permissions

Users and groups

Add a new user

$ adduser username

Create a new system group

$ sudo groupadd --system webapps
$ sudo useradd --system --gid webapps --home-dir /home/webapp1 webapp1

Add user to group

$ sudo usermod -aG group user

Sudo

Add user to sudoers group

$ usermod -aG sudo username

Switch accounts to new user

$ su - username

Remove a user

$ deluser newuser

Show group membership

$ id
$ groups

File permissions

chmod

0    No permission                                              ---
1    Execute permission                                         --x
2    Write permission                                           -w-
3    Execute and write permission: 1 (execute) + 2 (write) = 3  -wx
4    Read permission                                            r--
5    Read and execute permission: 4 (read) + 1 (execute) = 5    r-x
6    Read and write permission: 4 (read) + 2 (write) = 6        rw-
7    All permissions: 4 (read) + 2 (write) + 1 (execute) = 7    rwx

Change file permission

chmod [ugo][+-=][rwxs] file

Set files to inherit group membership as set. Source. More reading.

chmod g+s dir

File owner and group

Change ownership of a directory/file

sudo chown -R user/scratch

Change group of file

chown :friends file
chgrp friends file

Change user and group

chown user:friends file

Show/set default file permissions

umask -S # Show symbolically
umask 0027 #u=rwx,g=rx
umask 0077 #u=rwx