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The umask

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The umask (an abbreviation for "user file-creation mode mask") is a four-digit octal number used to determine permissions for newly created files. This is a very interesting and underutilized in Unix concept. One, moderately successful attempt to extend this concept by associating umask with directories was AppArmor. Unfortunately it was uses only in Suse and Ubuntu. Red Hat was already married to much more complex SELinux (see How to disable SELinux ;-) and that prevented this valuable concept from getting to mainstream...

Every Unix process has its own umask, inherited from its parent process. The idea of the umask is to remove the permissions you do not want to give by default to newly created files and directories. By default, most Unix versions specify an octal mode of 666 (any user can read or write the file) when they create new files. Likewise, directories are created with a mode of 777.

In classic Unix there is only one umask, global for the system. 

To calculate actual permissions granted, the complement of the umask value (the bits that are not set in the umask ) is combined with the default permissions using bitwise AND. In other words the mode specified in the open call is masked using the value specified by the umask ó thus its name.

You can set the umask on per system and per user basis using /etc/profile, .profile files, correspondingly. For example:

umask 027

When the umask is set in this manner, it should be one of the first commands in the profile. Anything executed prior to the umask command will have its prior, possibly unsafe, value.

In modern Unixes you can specify a system-wide default umask value in the /etc/defaults/login file. This umask is then given to every user that executes the login program. This method is a much better (and more reliable) system-wide way of setting the value for every user than setting the umask in the shell's startup files. The method is relatively straight forward to execute. If you follow our instructions you will get a lot out of the umask .

The umask Command

An interface to the umask function is a built-in shell command umask. There is also a umask( ) system call for programs that wish dynamically change their umask.

A simple way to calculate the result of application of umask is to subtract umask value from default permissions as if they are normal decimal values. For example the number 2 in the umask results in actual permission 4 (turns off write permission), while 7 results in zero (turns off read, write, and execute permissions). For example, a umask value of 002 can be used for groups of users who are working on common projects. If you create a file with your umask set to 002, anyone in the file's group will be able to read or modify the file. Everybody else will only be allowed to read it.

 0666                  Default file-creation mode
(0002)                 Umask
______________________________________________________
 0664                  Resultant mode

The most common umask values are 022, 027, and 077. A umask value of 022 applied to default permission 666 render permissions 644 which lets the owner both read and write to the created files, but everybody else can only read them. A umask value of 077 is more restrictive and allows access to the file for only file's owner.

umask User access Group access Other
0000 All All All
0002 All All Read, Execute
0007 All All None
0022 All Read, Execute Read, Execute
0027 All Read, Execute None
0077 All None None

If you use the Korn shell or bash, then you can set your umask symbolically. The same syntax as the chmod command is used. For example, the following two commands would be equivalent:

 umask u=rwx,g=x,o=
 umask 067

Common umask Values

On most POSIX compatible systems, the default umask is 022. This value is inherited from the init process, as all processes are descendants of init .

Note: In the Linux, the fat, hfs, hpfs, ntfs, and udf filesystem drivers support a umask mount option, which controls how the on-disk information is mapped to Unix permissions. This is not the same as the per-process umask described above, although the permissions are calculated in a similar way. Some of these file system drivers also support separate umasks for files and directories, using mount options such as fmask.

If you want to enhance security you can set up user accounts with a umask of 077, so a user's files will, by default, be unreadable by anyone else on the system unless the user makes a conscious choice to make them readable.

See AppArmor for the generalization which permits dynamic setting of the umask for each process.

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