1.j List, set, and change standard ugo/rwx permissions

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Linux divides the file permissions into 3 groups with read, write and execute denoted by r,w, and x respectively.

Each file and directory has three user based permission groups:

• user – The Owner permissions apply only the owner of the file or directory, they will not impact the actions of other users.
• group – The Group permissions apply only to the group that has been assigned to the file or directory, they will not effect the actions of other users.
• others – The All Users permissions apply to all other users on the system, this is the permission group that you want to watch the most.

And each user group has 3 permissions:

• Read - This permission give you the authority to open and read a file. Read permission on a directory gives you the ability to lists its content.

• Write - The write permission gives you the authority to modify the contents of a file. The write permission on a directory gives you the authority to add, remove and rename files stored in the directory as well as the directory itself.

• Execute - In Windows, an executable program usually has an extension ".exe" and which you can easily run. In Unix/Linux, you cannot run a program unless the execute permission is set. If the execute permission is not set, you might still be able to see/modify the program code (provided read & write permissions are set), but not run it. Fon a directory the execute permission allows you to cd into the directory.

  r = read permission
  w = write permission
  x = execute permission
  - = no permission
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On the example below we observe the following permission:

# ls -l my_file.txt
-rwxr-xr-x. 1 root root 0 Dec  9 15:34 my_file.txt
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• User root - Can read, write and execute
• Group root - Can read and execute
• Others - Can read and execute

📝 NOTES:

• Default permission is '666' for files and '777' for directories
• umask value is subtracted from the default permission (666)
• umask is not persistent. To make it persistent, add it to /etc/profile (login shells), /etc/bashrc (non-login shells) or ~/.bash_profile
• First digit of umask (if specifying with 4 digits) is for special permission (sticky bit, etc.)

Example of umask configuration in /etc/profile so values are different for system accounts vs real users:

# By default, we want umask to get set. This sets it for login shell
# Current threshold for system reserved uid/gids is 200
# You could check uidgid reservation validity in
# /usr/share/doc/setup-*/uidgid file
if [ $UID -gt 199 ] && [ "`/usr/bin/id -gn`" = "`/usr/bin/id -un`" ]; then
    umask 002
else
    umask 022
fi
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📝 NOTE: It looks like special permission (except SGID) may not be part of the v8 exam

A '.' can represent special permissions (SELinux related).

-rw-rw-rw-.
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A '+' indicates ACLs are applied.

-rw-rw-rw-+
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