origin url Permissions masking with umask, chmod, 777 octal permissions

Ian! D. Allen – idallen@idallen.ca – www.idallen.com

1 umask blocks default permissions

When a process creates a new file system object, such as a file or directory, the permissions of that new object are determined by the default permissions for the object, masked by the permissions set in a process umask.

The process umask controls what permissions are not given to newly created file system objects such as files and directories. It does not affect the permissions of existing objects, only of objects newly created by that process.

The umask also influences the chmod command as well as the permission of newly-created files and directories.

2 Default Permissions: directory 777, file 666

When a process creates a new file system object, such as a file or directory, the object is assigned a set of default permissions that is masked by the umask.

The default Unix permission set for newly created directories is 777 (rwxrwxrwx) masked (blocked) by the permission bits set in the umask of the process. (See below for an explanation of Unix numeric permissions 777.)

The default permissions for newly created files is 666 (rw-rw-rw-) masked by the permission bits set in the umask of the process.

The umask controls what permissions are not given to newly created file system object. Without the umask, all new directories would be created with full 777 permissions, and all new files would be created with full 666 permissions. The umask blocks certain permissions from being given to newly created file system objects.

3 Masking is not subtracting

Every bit set in the umask for the process “masks”, or “takes away”, that permission from the default permissions for newly created file system objects created by that process. The umask value is a mask; it turns off permissions so that they are not assigned to newly created objects.

“Mask” does not mean “subtract”, in the arithmetic sense – there is no borrow or carry involved. The two binary bits 10 masked by the two bits 01 result in the two bits 10. (The mask 01 turns off the rightmost bit; but, it was already off, so no change.) The two bits 10 masked by the two bits 11 result in the two bits 00. (The mask 11 turns off both bits.)

The umask is a mask; it is not a number to be subtracted. It turns off permissions that would normally be granted. Masking is not the same as subtracting, e.g. 666 masked with 001 is still 666 and 666 masked with 003 is 664. The mask turns off permission bits so that they are not assigned to newly created objects. If they are already off, the umask makes no change:

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rw- (6) masked with --x (1) is rw- (6)
  - because the x bit in the mask does not change any permissions
rw- (6) masked with -wx (3) is r-- (4)
  - because only the w bit is changed (turned off) by the mask

4 The umask command affects default permissions

Many modern Linux shells also accept symbolic umask permissions, in addition to the traditional octal numbers. See the manual page for your shell for details. In this course, we use the traditional octal numbers that work everywhere.

The shell command umask 022 sets to 022 (----w--w-) the permissions to be removed (masked) from the default permissions, for new files and directories created by the shell (and by commands run from that shell). It prevents write permission being assigned to group and other on newly created directories and files. A new directory would have permissions 777 (rwxrwxrwx) masked by 022 (----w--w-) resulting in 755 (rwxr-xr-x) permissions. A new file would have permissions 666 (rw-rw-rw-) masked by 022 (----w--w-) resulting in 644 (rw-r--r--) permissions.

The umask only applies to the permissions given to newly created files and directories.

The traditional friendly Unix umask is 022, resulting in default file permissions of 644 and default directory permissions of 755. (Newly-created files and directories are readable by anyone; but, they are only writable by the owner.) A “secure” umask would be 077. (Mask out all group and other permissions; newly-created files and directories are readable/writable/executable only by the single user that created them.)

The umask command cannot affect the permission of already-existing files. To do that, you must use the chmod command:

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$ chmod 711 program
$ chmod go-rwx secrets

Note that the umask and the permissions assigned by chmod are opposites. The chmod command sets permissions to be given to an object; the umask sets permissions not to be given to new objects.

Look for umask in some of the following pages for more examples:

5 umask is set and then passed to child processes

Every process on Unix (including every shell process) has its own umask value, which can be changed. The system standard umask is set for you at login and is inherited by child processes.

Different Linux distributions set different standard (at login) umask values. The values in your particular distribution of Linux may not be the same as other distributions. The values set by the system administrator may differ from the distribution defaults. Do not rely on the umask having any standard value.

Every shell script should set umask at the beginning, so that files and directories created by the script (and by child processes of the script) have known permissions.

6 umask affects chmod

Using the chmod command without specifying whether you want to change User, Group, or Other permissions (e.g. chmod +x foo) causes chmod to use your umask to decide what sets of permissions to change. The umask setting causes chmod to ignore changes for the masked permissions. For example:

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umask 0011 ; chmod +x foo   # only adds User x permissions
umask 0111 ; chmod +x foo   # does nothing (no permissions changed)
umask 0400 ; chmod -r foo   # only removes Group and Other r permissions
umask 0444 ; chmod -r foo   # does nothing (no permissions changed)
umask 0727 ; chmod +rwx foo # adds only Group rx permissions

The umask value tells chmod which permissions chmod is allowed to affect. The masked-out permissions are not affected. If you want chmod to ignore the current umask, specify exactly which permission sets to affect:

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umask 0077 ; chmod g+x foo   # ignores umask; adds Group x permissions
umask 0700 ; chmod u-r foo   # ignores umask; removes User r permissions

Always specify the precise User/Group/Other permission string when using chmod, since you don’t know what the current umask might be.

7 Using numeric 022-style octal permissions

Unix permissions for user, group, and other have traditionally been expressed using a set of three (octal) digits, where each digit represents the octal number you get by expressing the three rwx permissions in binary form. Convert the enabled permission bits in rwx into binary, using 1 for enabled and 0 for not enabled, then convert the binary number to an octal digit. Three sets of three permissions becomes three (octal) digits, e.g. rwxr-x-wx becomes 111|101|011 which is753.

Permissions (mode) can be represented in two ways: symbolic (three letters) or numeric (one octal digit). The single octal digit represents the three symbolic letters using a numeric weighting scheme shown below. The permission is treated as a binary number, with zeroes taking the place of the dashes (not enabled) and ones taking the place of the allowed permissions.

Numeric weighting for each of the three rwx permissions (three binary digits to one octal digit):

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x (execute) --x  becomes 001 binary and has binary weight 2^0 = 1
w (write)   -w-  becomes 010 binary and has binary weight 2^1 = 2
r (read)    r--  becomes 100 binary and has binary weight 2^2 = 4

Each of the three sets of symbolic permissions (user/owner, group, other) can be summarized by a single octal digit by adding up the three numeric rwx values using the three weights (4,2,1) given above:

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rwx ==> 111 binary ==> digit 7 because r is 4, w is 2, and x is 1 so 4+2+1=7
r-x ==> 101 binary ==> digit 5 because r is 4 and x is 1 so          4+0+1=5
-wx ==> 011 binary ==> digit 3 because w is 2 and x is 1 so          0+2+1=3
--- ==> 000 binary ==> digit 0 because no permissions are set so     0+0+0=0

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

The full set of nine permission characters can then be grouped and summarized as three octal digits:

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rwxr-x-wx  is  rwx|r-x|-wx  is 111|101|011 ==> the three digits 753
---r----x  is  ---|r--|--x  is 000|100|001 ==> the three digits 041
---------  is  ---|---|---  is 000|000|000 ==> the three digits 000
rwxrwxrwx  is  rwx|rwx|rwx  is 111|111|111 ==> the three digits 777

Make sure you always write exactly nine characters when writing symbolic permissions. Exactly nine. Do not include the leading “inode type” character when listing the nine characters of symbolic permissions.

Thus chmod 741 file means “set the mode to 741 (rwxr----x)”. That is 7 (7=111="rwx") for owner, 4 (4=100="r--") for group, and 1 (1=001="--x") for others. In most modern Unix systems, you can do the same thing using symbolic permissions as chmod u=rwx,g=r,o=x file.

The shell command umask 027 means “mask (remove) permissions 027 from newly created files and directories”:

  • Octal 027 = "----w-rwx" which can be split into three parts: 0=000="---" for owner, 2=010="-w-" for group, and 7=111="rwx" for others.
  • A new directory created under this umask 027 (e.g. by mkdir) would have directory default permissions 777 masked by 027 = 750 (rwxr-x---).
  • A new file created under this umask 027 (e.g. created by output redirection or by a file copy) would have file default permissions 666 masked by 027 = 640 (rw-r-----).
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Author: 
| Ian! D. Allen, BA, MMath  -  idallen@idallen.ca  -  Ottawa, Ontario, Canada
| Home Page: http://idallen.com/   Contact Improv: http://contactimprov.ca/
| College professor (Free/Libre GNU+Linux) at: http://teaching.idallen.com/
| Defend digital freedom:  http://eff.org/  and have fun:  http://fools.ca/

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