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2-File-Permissions-and-Sudo.md

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File Permissions & sudo Command:

File Permissions set for 3 sets of users:

  1. User (current/owner)
  2. Group
  3. Others

Each type of user has:

  1. Read(r)
  2. Write(w)
  3. Execute(x) permissions
  • Execute for a folder means we can go into/access the folder.
  • Execute for a file means we can run the program/execute the script/etc.

Denial of a permission is indicated by a -. Ex:

  • drwxr-xr-- => means that it is a directory(d), user has all 3 permissions(rwx), group has only read and execute permission(r-x), and others have only read permission(r--).

Checking Permissions:

  • ls -l => longlisting of files in a directory will show us the permissions it has.

Format Displayed:

  1. Is Directory? and File permissions (user, group, others)
  2. Owner of the file/dir (A user on the sytem)

Changing permissions:

Use chmod which changes or modifies permissions.

  • chmod o-rx fileOrDirectoryPath = Changes permissions for 'others' by removing read and execute permissions '-rx' (for others) on the mentioned File.

Above might not work sometimes. (Ex: chmod: abc.txt: Operation not permitted)

.. AND THAT'S BECAUSE WE DON'T HAVE THE AUTHORITY TO CHANGE OWNERSHIP.

IN THIS CASE, WE WILL HAVE TO RUN THE COMMAND AS A 'SUPERUSER' (Therefore, prefix command with sudo which will run it as from the superuser - requires the password to superuser login.)

Therefore:(Solution)

  • sudo chmod o-rx fileOrDirectoryPath = Changes permissions for 'others' by removing read and execute permissions '-rx' (for others) on the mentioned File.

Another Usage:

  • sudo chmod a=rx fileOrDirectoryPath = Changes permissions for 'all(u,g,o)' by having only read and execute permissions(no write access).

Combining it for user, group and others. Ex: chmod u=rwx,g=rx,o=r myfile

chmod 'octal' mode:

Ex: chmod 754 myfile

Here the digits 7, 5, and 4 each individually represent the permissions for the user, group, and others, in that order. Each digit is a combination of the numbers 4, 2, 1, and 0:

  • 4 stands for "read",
  • 2 stands for "write",
  • 1 stands for "execute", and
  • 0 stands for "no permission."

So 7 is the combination of permissions 4+2+1 (read, write, and execute), 5 is 4+0+1 (read, no write, and execute), and 4 is 4+0+0 (read, no write, and no execute).

NOTE:

  • + = Add permissions (Ex: +rx => add read and execute permissions)
  • - = Remove permissions (Ex: -wx => remove write and execute permissions)
  • = = Set permissions to only what's mentioned(on RHS of =) and remove un-mentioned permissions

NOTE:

  • u = user
  • g = group
  • o = others
  • a = all (u,g,o)

We may even combine them. Ex: 'ug' => user and group

NOTE:

  • r = read
  • w = write
  • x = execute.

Changing Owner (of a file or directory):

Refers to a user on the system with a login name. (Use chown)

Every file/dir is owned by some user on the system. The default user on the system id ID'ed as root.

  • chown userName fileOrDirectoryPath = Changes the Ownership of mentioned file(s)/dir(s) to the mentioned User.

ABOVE MIGHT NOT WORK SOMETIMES. Ex: chown: changing ownership of abc.txt: Operation not permitted

.. AND THAT'S BECAUSE WE DON'T HAVE THE AUTHORITY TO CHANGE OWNERSHIP.

IN THIS CASE, WE WILL HAVE TO RUN THE COMMAND AS A 'SUPERUSER' (Therefore, prefix command with sudo which will run it as from the superuser - requires the password to superuser login.)

Therefore:(Solution)

  • sudo chown userName fileOrDirectoryPath = Changes the Ownership of mentioned file(s)/dir(s) to the mentioned User.

Changing Owner & Group (of a file or directory):

Refers to a user on the system with a login name. (Again, use chown)

  • chown userName:groupName fileOrDirectoryPath = Changes the Ownership of mentioned file(s)/dir(s) to the mentioned User and the mentioned Group.

IF NOT PERMITTED THEN USE sudo:

  • sudo chown userName:groupName fileOrDirectoryPath = Changes the Ownership of mentioned file(s)/dir(s) to the mentioned User and the mentioned Group.

NOTE: CHANGING USER OR GROUP OWNERSHIP => THEY (USER, GROUP) MUST EXIST!

NOTE: We can also use chgrp command to change group:

  • chgrp groupName fileOrdirectoryPath = changes group of the file(s)/dir(s) to the mentioned group

If not permitted, use sudo:

  • sudo chgrp groupName fileOrdirectoryPath = changes group of the file(s)/dir(s) to the mentioned group

sudo prefix can be used to run any command for which we don't have permissions (And not just for file commands, but any command)