standard file permissions

(Written by Paul Cobbaut, https://github.com/paulcobbaut/, with contributions by: Alex M. Schapelle, https://github.com/zero-pytagoras/, Bert Van Vreckem, https://github.com/bertvv/)

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

file ownership

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 example.

student@linux:~/owners$ ls -lh
total 636K
-rw-r--r--. 1 student snooker  1.1K Apr  8 18:47 data.odt
-rw-r--r--. 1 student student  626K Apr  8 18:46 file1
-rw-r--r--. 1 student tennis    185 Apr  8 18:46 file2
-rw-rw-r--. 1 root root           0 Apr  8 18:47 stuff.txt

User student owns three files: file1 has student as user owner and has the group student 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.

chgrp

You can change the group owner of a file using the chgrp command. You must have root privileges to do this.

root@linux:/home/student/owners# ls -l file2
-rw-r--r--. 1 root tennis 185 Apr  8 18:46 file2
root@linux:/home/student/owners# chgrp snooker file2
root@linux:/home/student/owners# ls -l file2
-rw-r--r--. 1 root snooker 185 Apr  8 18:46 file2
root@linux:/home/student/owners#

chown

The user owner of a file can be changed with chown command. You must have root privileges to do this. In the following example, the user owner of file2 is changed from root to student.

root@linux:/home/student# ls -l FileForStudent 
-rw-r--r-- 1 root student 0 2008-08-06 14:11 FileForStudent
root@linux:/home/student# chown student FileForStudent 
root@linux:/home/student# ls -l FileForStudent 
-rw-r--r-- 1 student student 0 2008-08-06 14:11 FileForStudent

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

root@linux:/home/student# ls -l FileForStudent 
-rw-r--r-- 1 student student 0 2008-08-06 14:11 FileForStudent
root@linux:/home/student# chown root:project42 FileForStudent 
root@linux:/home/student# ls -l FileForStudent 
-rw-r--r-- 1 root project42 0 2008-08-06 14:11 FileForStudent

list of special files

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.

first character	file type
-	normal file
d	directory
l	symbolic link
p	named pipe
b	block device
c	character device
s	socket

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

student@linux:~$ ls -l /dev/console /dev/sda
crw--w---- 1 root tty  5, 1 Mar  8 08:32 /dev/console
brw-rw---- 1 root disk 8, 0 Mar  8 08:32 /dev/sda

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

student@linux:~$ ls -ld /etc /etc/hosts /etc/os-release
drwxr-xr-x 81 root root 4096 Mar  8 08:32 /etc
-rw-r--r--  1 root root  186 Feb 26 14:58 /etc/hosts
lrwxrwxrwx  1 root root   21 Dec  9 21:08 /etc/os-release -> ../usr/lib/os-release

permissions

rwx

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.

permission	on a file	on a directory
read	read file contents (cat)	read directory contents (ls)
write	change file contents	create/delete files (touch,rm)
execute	execute the file	enter the directory (cd)

three sets of rwx

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

student@linux:~/test$ ls -l proc42.sh
-rwxr-xr--  1 student proj  984 Feb  6 12:01 proc42.sh

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

position	characters	function
1	-	file type
2-4	rwx	permissions for the user owner
5-7	r-x	permissions for the group owner
8-10	r--	permissions for others

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.

permission examples

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

student@linux:~/perms$ ls -lh
total 12K
drwxr-xr-x 2 student student 4.0K 2007-02-07 22:26 AllEnter_UserCreateDelete
-rwxrwxrwx 1 student student    0 2007-02-07 22:21 EveryoneFullControl.txt
-r--r----- 1 student student    0 2007-02-07 22:21 OnlyOwnersRead.txt
-rwxrwx--- 1 student student    0 2007-02-07 22:21 OwnersAll_RestNothing.txt
dr-xr-x--- 2 student student 4.0K 2007-02-07 22:25 UserAndGroupEnter
dr-x------ 2 student student 4.0K 2007-02-07 22:25 OnlyUserEnter

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. The root user ignores all restrictions and can do anything with any file.

setting permissions with symbolic notation

Permissions can be changed with chmod MODE FILE.... You need to be the owner of the file to do this. The first example gives (+) the user owner (u) execute (x) permissions.

student@linux:~/perms$ ls -l permissions.txt 
-rw-r--r-- 1 student student 0 2007-02-07 22:34 permissions.txt
student@linux:~/perms$ chmod u+x permissions.txt 
student@linux:~/perms$ ls -l permissions.txt 
-rwxr--r-- 1 student student 0 2007-02-07 22:34 permissions.txt

This example removes (-) the group owner's (g) read (r) permission.

student@linux:~/perms$ chmod g-r permissions.txt 
student@linux:~/perms$ ls -l permissions.txt 
-rwx---r-- 1 student student 0 2007-02-07 22:34 permissions.txt

This example removes (-) the other's (o) read (r) permission.

student@linux:~/perms$ chmod o-r permissions.txt 
student@linux:~/perms$ ls -l permissions.txt 
-rwx------ 1 student student 0 2007-02-07 22:34 permissions.txt

This example gives (+) all (a) of them the write (w) permission.

student@linux:~/perms$ chmod a+w permissions.txt 
student@linux:~/perms$ ls -l permissions.txt 
-rwx-w--w- 1 student student 0 2007-02-07 22:34 permissions.txt

You don't even have to type the a.

student@linux:~/perms$ chmod +x permissions.txt 
student@linux:~/perms$ ls -l permissions.txt 
-rwx-wx-wx 1 student student 0 2007-02-07 22:34 permissions.txt

You can also set explicit permissions with =.

student@linux:~/perms$ chmod u=rw permissions.txt 
student@linux:~/perms$ ls -l permissions.txt 
-rw--wx-wx 1 student student 0 2007-02-07 22:34 permissions.txt

Feel free to make any kind of combination, separating them with a comma. Remark that spaces are not allowed!

student@linux:~/perms$ chmod u=rw,g=rw,o=r permissions.txt 
student@linux:~/perms$ ls -l permissions.txt 
-rw-rw-r-- 1 student student 0 2007-02-07 22:34 permissions.txt

Even fishy combinations are accepted by chmod.

student@linux:~/perms$ chmod u=rwx,ug+rw,o=r permissions.txt 
student@linux:~/perms$ ls -l permissions.txt 
-rwxrw-r-- 1 student student 0 2007-02-07 22:34 permissions.txt

Summarized, in order to change permissions with chmod using symbolic notation:

• first specify who the permissions are for: u for the user owner, g for the group owner, o for others, and a for all. a is the default and can be omitted.
• then specify the operation: + to add permissions, - to remove permissions, and = to set permissions.
• finally specify the permission(s): r for read, w for write, and x for execute.
• multiple operations can be combined with a comma (no spaces!)

setting permissions with octal notation

Most Unix administrators will use the "old school" octal system to talk about and set permissions. Consider the triplet to be a binary number with 0 indicating the permission is not set and 1 indicating the permission is set. You then have \(2^3=8\) possible combinations, hence the name octal. You can then convert the binary number to an octal number, equating r to 4, w to 2, and x to 1.

permission	binary	octal
---	000	0
--x	001	1
-w-	010	2
-wx	011	3
r--	100	4
r-x	101	5
rw-	110	6
rwx	111	7

Since we have three triplets, we can use three octal digits to represent the permissions. This makes 777 equal to rwxrwxrwx and by the same logic, 654 mean rw-r-xr--. The chmod command will accept these numbers.

student@linux:~/perms$ chmod 777 permissions.txt 
student@linux:~/perms$ ls -l permissions.txt 
-rwxrwxrwx 1 student student 0 2007-02-07 22:34 permissions.txt
student@linux:~/perms$ chmod 664 permissions.txt 
student@linux:~/perms$ ls -l permissions.txt 
-rw-rw-r-- 1 student student 0 2007-02-07 22:34 permissions.txt
student@linux:~/perms$ chmod 750 permissions.txt 
student@linux:~/perms$ ls -l permissions.txt 
-rwxr-x--- 1 student student 0 2007-02-07 22:34 permissions.txt

Remark that in practice, some combinations will never occur:

• The permissions of a user will never be smaller than the permissions of the group owner or others. Consequently, the digits will always be in descending order.
• Setting the write or execute permission without read access is useless. Consequently, you will never use 1, 2, or 3 in an octal permission code
• A directory will always have the read and execute permission set or unset together. It is useless to allow a user to read the directory contents, but not let them cd into that directory. Allowing cd without read access is also useless. The permission code for a directory will therefore always be odd.

Here's a little tip: you can print the permissions of a file in either octal or symbolic notation with the stat command (check the man page of stat to see how this works).

[student@linux ~]$ stat -c '%A %a' /etc/passwd
-rw-r--r-- 644
[student@linux ~]$ stat -c '%A %a' /etc/shadow
---------- 0
[student@linux ~]$ stat -c '%A %a' /bin/ls
-rwxr-xr-x 755

umask

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

[student@linux ~]$ umask
0002
[student@linux ~]$ touch test
[student@linux ~]$ ls -l test
-rw-rw-r--  1 student student    0 Jul 24 06:03 test
[student@linux ~]$

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 value of 0022 has the same effect as 0033.

In practice, you will only use umask values:

• 0: don't take away any permissions
• 2: take away write permissions
• 7: take away all permissions

You can set the umask value to a new value with the umask command. The umask value is a four-digit octal number. The first digit is for special permissions (and is always zero), the second for the user permissions (is in practice always 0, since there is no use in taking away the user's permissions), the third for the group owner (sometimes 0, but usually 2 or 7), and the last for others (usually 2 or 7, 0 is very uncommon and can be considered to be a security risk).

The umask value is subtracted from 777 to get the default permissions and in the case of a file, the execute bit is removed.

[student@linux ~]$ umask 0002
[student@linux ~]$ touch file0002
[student@linux ~]$ mkdir dir0002
[student@linux ~]$ ls -ld *0002
drwxrwxr-x. 2 student student 6 Mar  8 10:48 dir0002
-rw-rw-r--. 1 student student 0 Mar  8 10:47 file0002
[student@linux ~]$ umask 0027
[student@linux ~]$ touch file0027
[student@linux ~]$ mkdir dir0027
[student@linux ~]$ ls -ld *0027
drwxr-x---. 2 student student 6 Mar  8 10:48 dir0027
-rw-r-----. 1 student student 0 Mar  8 10:48 file0027
[student@linux ~]$ umask 0077
[student@linux ~]$ touch file0077
[student@linux ~]$ mkdir dir0077
[student@linux ~]$ ls -ld *0077
drwx------. 2 student student 6 Mar  8 10:51 dir0077
-rw-------. 1 student student 0 Mar  8 10:51 file0077

mkdir -m

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

student@linux~$ mkdir -m 700 MyDir
student@linux~$ mkdir -m 777 Public
student@linux~$ ls -dl MyDir/ Public/
drwx------ 2 student student 4096 2011-10-16 19:16 MyDir/
drwxrwxrwx 2 student student 4096 2011-10-16 19:16 Public/

cp -p

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

student@linux:~/perms$ cp file* cp
student@linux:~/perms$ cp -p file* cpp
student@linux:~/perms$ ll *
-rwx------ 1 student student    0 2008-08-25 13:26 file33
-rwxr-x--- 1 student student    0 2008-08-25 13:26 file42

cp:
total 0
-rwx------ 1 student student 0 2008-08-25 13:34 file33
-rwxr-x--- 1 student student 0 2008-08-25 13:34 file42

cpp:
total 0
-rwx------ 1 student student 0 2008-08-25 13:26 file33
-rwxr-x--- 1 student student 0 2008-08-25 13:26 file42

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. Delete the file created by root. Is this possible?

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--?

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

12. Set the umask to 0077, but use the symbolic format to set it. Verify that this works.

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

14. Create a file as a normal user, take away all permissions for the group owner and others. Can you still read the file? Can root read the file? Can root write to the file?

15. Create a directory that belongs to group users, 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.

   [student@linux ~]$ mkdir permissions
   [student@linux ~]$ touch permissions/myfile.txt
   [student@linux ~]$ ls -l permissions/
   total 0
   -rw-r--r--. 1 student student 0 Mar  8 10:59 myfile.txt
   

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

   [student@linux ~]$ ls -l /etc/hosts
   -rw-r--r--. 1 root root 174 Feb 26 15:05 /etc/hosts
   [student@linux ~]$ cp /etc/hosts ~/permissions/
   [student@linux ~]$ ls -l permissions/hosts
   -rw-r--r--. 1 student student 174 Mar  8 11:00 permissions/hosts
   

   The copy is owned by you.

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

   [student@linux ~]$ sudo touch permissions/rootfile.txt
   [sudo] password for student:
   

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

   [student@linux ~]$ ls -l permissions/*.txt
   -rw-r--r--. 1 student student 0 Mar  8 10:59 permissions/myfile.txt
   -rw-r--r--. 1 root    root    0 Mar  8 11:02 permissions/rootfile.txt
   

   The file created by root is owned by root.

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

   [student@linux ~]$ chown student ~/permissions/*
   chown: changing ownership of '/home/student/permissions/rootfile.txt': Operation not permitted
   

   You cannot become owner of the file that belongs to root. Root must change the ownership.

6. Delete the file created by root. Is this possible?

   [student@linux ~]$ rm ~/permissions/rootfile.txt
   rm: remove write-protected regular empty file '/home/student/permissions/rootfile.txt'? y
   [student@linux ~]$ ls -l permissions/*.txt
   -rw-r--r--. 1 student student 0 Mar  8 10:59 permissions/myfile.txt
   

   You can delete the file since you have write permission on the directory!

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

   yes

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

   no, rw-rw-r-- is 664 and r-xr-xr-- is 774

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

   yes

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

    no, rwxr-xr-- is 754 and rwx-wxr-- is 734

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

    umask and umask -S

12. Set the umask to 0077, but use the symbolic format to set it. Verify that this works.

    [student@linux ~]$ umask -S u=rwx,go=
    u=rwx,g=,o=
    [student@linux ~]$ umask
    0077
    

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

    [student@linux ~]$ sudo vi permissions/rootfile.txt
    [student@linux ~]$ sudo chmod 644 permissions/rootfile.txt
    [student@linux ~]$ ls -l permissions/*.txt
    -rw-r--r--. 1 student student 0 Mar  8 10:59 permissions/myfile.txt
    -rw-r--r--. 1 root    root    6 Mar  8 13:53 permissions/rootfile.txt
    [student@linux ~]$ cat permissions/rootfile.txt
    hello
    [student@linux ~]$ echo " world" >> permissions/rootfile.txt
    -bash: permissions/rootfile.txt: Permission denied
    

    Yes, a normal user can read the file, but not write to it.

14. Create a file as a normal user, take away all permissions for the group and others. Can you still read the file? Can root read the file? Can root write to the file?

    [student@linux ~]$ vi permissions/privatefile.txt
    ... (editing the file) ...
    [student@linux ~]$ cat permissions/privatefile.txt
    hello
    [student@linux ~]$ chmod 600 permissions/privatefile.txt
    [student@linux ~]$ ls -l permissions/privatefile.txt
    -rw-------. 1 student student 0 Mar  8 16:06 permissions/privatefile.txt
    [student@linux ~]$ cat permissions/privatefile.txt
    hello
    

    Of course, the owner can still read (and write to) the file.

    [student@linux ~]$ sudo vi permissions/privatefile.txt
    [sudo] password for student:
    ... (editing the file) ...
    [student@linux ~]$ cat permissions/privatefile.txt
    hello world
    

    Root can read and write to the file. In fact, root ignores all file permissions and can do anything with any file.

15. Create a directory shared/ that belongs to group users, where every member of that group can read and write to files, and create files.

    [student@linux ~]$ mkdir shared
    [student@linux ~]$ sudo chgrp users shared
    [student@linux ~]$ chmod 775 shared/
    [student@linux ~]$ ls -ld shared/
    drwxrwxr-x. 2 student users 6 Mar  8 18:26 shared/