Ownership

drwxrwxr-x 2        root                        staff                     4096 Sep 17 23:30 bin

                         (owner user)        (owner group)

Change ownership:

sudo chown root alex_test.txt # change owner user to root

sudo chgrp root alex_test.txt # change owner group to root

Recursive change of all files in <Folder>:

chown -R root Somefolder # Somefolder - name of folder, where every user owner will be changed to root

Change of owner AND group:

sudo chown alex:admin alex_test.txt #  change owner user to alex, owner group to admin

Permissions

drwxr-xr-x 2 alex alex 4096 Oct  8 15:52 Documents

1st letter: 'd'=directory,'l'=link,'-'=regular file;

The rest of the letters are divided in 3 groups of 3 letters: 'r'=read, 'w'=write, 'x'=eXecute.

Group 1: what permissions the owner has;

Group 2: what permissions the owner group has;

Group 3: what permissions everybody else has.

Octal numbers in permissions

Read = 4

Write = 2

Execute = 1

"rwx" = 7

"r-x" = 5

 "rw-" = 6

"r--" = 4

Change permissions

chmod ugo-x alex_test.txt # change permissions to everyone  ('ugo'=ownerUser, Group, Other), '-x' = deny execution

chmod u+x alex_test.txt # '+r'=allow execute of file by file owner

chmod 664 alex_test.txt # change permissions in octal mode ('664' = 'rw-rw-r--')

Changing default permissions

umask  022 # the default permissions, where 777 - 022 = 755 in octal = rwxr-xr-x for folders and rw-r--r-- for files (execute is disabled for files by default)

SUID

Set User ID bit - on executables.

If this bit is set, then the executable file will run with permissions of the owner of the file (usually root).

chmod u+s execfile # add SUID to exec file

GUID

Group User ID

If executable file, then it runs with permissions of the owner group ID. On folders: gives group ownership to any file inside this folder.

chmod g+s execfile # add GUID to exec file or folder

Sticky Bit

Used on folders, particularly on the /tmp (everybody writes there, need to protect from somebody deleting your files). It means that only the owner of the file, or the owner group member can delete the file.