unix-permissions-solutions.md

Unix Permissions Solutions

Here are some solutions to the Unix permissions exercise. Note that this is not an exhaustive list of solutions. There are often several ways to achieve the same result with different Unix commands.

• Create a file named file.txt in alice's home directory that is readable by alice but not by you.

  Solution 1

  # Switch to alice.
  $> sudo su - alice
  # Create the file (it will belong to alice and the alice group).
  $> touch file.txt
  # Switch back to your user.
  $> exit
  # Remove read permissions for other users (including yours).
  $> sudo chmod o-r /home/alice/file.txt

  Solution 2

  # Create the file as root (it will belong to root and the root group).
  $> sudo touch /home/alice/file.txt
  # Give the file to alice.
  $> sudo chown alice /home/alice/file.txt
  # Remove read permissions for other users (including yours).
  $> sudo chmod o-r /home/alice/file.txt

  Setting the permissions in octal mode

  $> sudo chmod 640 /home/alice/file.txt

• Create a directory named for_alice in the system's temporary directory (/tmp). The alice user must be able to traverse this directory, but not list its contents or create new files in it.

  Solution

  # Create the directory.
  $> sudo mkdir /tmp/for_alice
  # Give the directory to your user and alice's group.
  $> sudo chown jde:alice /tmp/for_alice
  # Specify that the group can only traverse the directory and (optionally) that
  # other users have no permissions.
  $> sudo chmod g=x,o-rwx /tmp/for_alice

  Setting the permissions in octal mode

  $> sudo chmod 710 /tmp/for_alice

• The directory must contain a readable.txt file that alice can read from, but not write to.

  Solution with access by other users

  # Move into the directory.
  $> cd /tmp/for_alice
  # Simply create the file. It will be readable by everyone by default.
  $> echo "Hello, I'm readable" > readable.txt

  Without access by other users

  # Give the file to alice's group (which should have read access by default).
  $> sudo chown jde:alice readable.txt
  # Remove all permissions for other users.
  $> sudo chmod o-rwx readable.txt

  Setting the permissions in octal mode

  $> sudo chmod 640 readable.txt

• The directory must contain a writable.txt file that alice can read from and write to.

  Solution with access by other users

  $> cd /tmp/for_alice
  # Create the file.
  $> echo "Hello, I'm writable" > writable.txt
  # Add the write permission to other users.
  $> sudo chmod o+w writable.txt

  Without access by other users

  # Give the file to alice's group.
  $> sudo chown jde:alice writable.txt
  # Add the write permission to the group and (optionally) remove all
  # permissions for other users.
  $> sudo chmod g+w,o-rwx writable.txt

  Setting the permissions in octal mode

  $> sudo chmod 660 writable.txt