What are the three sets of permission for a file?

This chapter contains details about basic file security through file ownership and file permissions.

user owner and group owner

The users and groups of a system can be locally managed in /etc/passwd and /etc/group, or they can be in a NIS, LDAP, or Samba domain. These users and groups can own files. Actually, every file has a user owner and a group owner, as can be seen in the following screenshot.

User paul owns three files; file1 has paul as user owner and has the group paul as group owner, data.odt is group owned by the group snooker, file2 by the group tennis.

The last file is called stuff.txt and is owned by the root user and the root group.

You can use the following command to list all local user accounts.

You can change the group owner of a file using the chgrp command.

The user owner of a file can be changed with chown command.

You can also use chown to change both the user owner and the group owner.

When you use ls -l, for each file you can see ten characters before the user and group owner. The first character tells us the type of file. Regular files get a -, directories get a d, symbolic links are shown with an l, pipes get a p, character devices a c, block devices a b, and sockets an s.

Table 1.1. Unix special files

Below a screenshot of a character device (the console) and a block device (the hard disk).

And here you can see a directory, a regular file and a symbolic link.

The nine characters following the file type denote the permissions in three triplets. A permission can be r for read access, w for write access, and x for execute. You need the r permission to list (ls) the contents of a directory. You need the x permission to enter (cd) a directory. You need the w permission to create files in or remove files from a directory.

Table 1.2. standard Unix file permissions

We already know that the output of ls -l starts with ten characters for each file. This screenshot shows a regular file (because the first character is a - ).

Below is a table describing the function of all ten characters.

Table 1.3. Unix file permissions position

When you are the user owner of a file, then the user owner permissions apply to you. The rest of the permissions have no influence on your access to the file.

When you belong to the group that is the group owner of a file, then the group owner permissions apply to you. The rest of the permissions have no influence on your access to the file.

When you are not the user owner of a file and you do not belong to the group owner, then the others permissions apply to you. The rest of the permissions have no influence on your access to the file.

Some example combinations on files and directories are seen in this screenshot. The name of the file explains the permissions.

To summarise, the first rwx triplet represents the permissions for the user owner. The second triplet corresponds to the group owner; it specifies permissions for all members of that group. The third triplet defines permissions for all other users that are not the user owner and are not a member of the group owner.

setting permissions (chmod)

Permissions can be changed with chmod. The first example gives the user owner execute permissions.

This example removes the group owners read permission.

This example removes the others read permission.

This example gives all of them the write permission.

You don't even have to type the a.

You can also set explicit permissions.

Feel free to make any kind of combination.

Even fishy combinations are accepted by chmod.

setting octal permissions

Most Unix administrators will use the old school octal system to talk about and set permissions. Look at the triplet bitwise, equating r to 4, w to 2, and x to 1.

Table 1.4. Octal permissions

This makes 777 equal to rwxrwxrwx and by the same logic, 654 mean rw-r-xr-- . The chmod command will accept these numbers.

When creating a file or directory, a set of default permissions are applied. These default permissions are determined by the umask. The umask specifies permissions that you do not want set on by default. You can display the umask with the umask command.

As you can also see, the file is also not executable by default. This is a general security feature among Unixes; newly created files are never executable by default. You have to explicitly do a chmod +x to make a file executable. This also means that the 1 bit in the umask has no meaning--a umask of 0022 is the same as 0033.

When creating directories with mkdir you can use the -m option to set the mode. This screenshot explains.

To preserve permissions and time stamps from source files, use cp -p.

practice: standard file permissions

1. As normal user, create a directory ~/permissions. Create a file owned by yourself in there.

2. Copy a file owned by root from /etc/ to your permissions dir, who owns this file now ?

3. As root, create a file in the users ~/permissions directory.

4. As normal user, look at who owns this file created by root.

5. Change the ownership of all files in ~/permissions to yourself.

6. Make sure you have all rights to these files, and others can only read.

7. With chmod, is 770 the same as rwxrwx--- ?

8. With chmod, is 664 the same as r-xr-xr-- ?

9. With chmod, is 400 the same as r-------- ?

10. With chmod, is 734 the same as rwxr-xr-- ?

11a. Display the umask in octal and in symbolic form.

11b. Set the umask to 077, but use the symbolic format to set it. Verify that this works.

12. Create a file as root, give only read to others. Can a normal user read this file ? Test writing to this file with vi.

13a. Create a file as normal user, give only read to others. Can another normal user read this file ? Test writing to this file with vi.

13b. Can root read this file ? Can root write to this file with vi ?

14. Create a directory that belongs to a group, where every member of that group can read and write to files, and create files. Make sure that people can only delete their own files.

solution: standard file permissions

1. As normal user, create a directory ~/permissions. Create a file owned by yourself in there.

2. Copy a file owned by root from /etc/ to your permissions dir, who owns this file now ?

The copy is owned by you.

3. As root, create a file in the users ~/permissions directory.

4. As normal user, look at who owns this file created by root.

The file created by root is owned by root.

5. Change the ownership of all files in ~/permissions to yourself.

You cannot become owner of the file that belongs to root.

6. Make sure you have all rights to these files, and others can only read.

7. With chmod, is 770 the same as rwxrwx--- ?

yes

8. With chmod, is 664 the same as r-xr-xr-- ?

No

9. With chmod, is 400 the same as r-------- ?

yes

10. With chmod, is 734 the same as rwxr-xr-- ?

no

11a. Display the umask in octal and in symbolic form.

11b. Set the umask to 077, but use the symbolic format to set it. Verify that this works.

12. Create a file as root, give only read to others. Can a normal user read this file ? Test writing to this file with vi.

13a. Create a file as normal user, give only read to others. Can another normal user read this file ? Test writing to this file with vi.

Yes, others can read this file

13b. Can root read this file ? Can root write to this file with vi ?

Yes, root can read and write to this file. Permissions do not apply to root.

14. Create a directory that belongs to a group, where every member of that group can read and write to files, and create files. Make sure that people can only delete their own files.

You can not yet do the last part of this exercise...