Recipe 9.8. Setting Permissions Defaults with umask

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9.8.1 Problem

You want to understand why files are created with a certain set of default permissions, and how to configure them yourself.

9.8.2 Solution

The umask (user file-creation mode mask) controls this behavior. To see what yours is, use:

$ umask 0022

or:

$ umask -S u=rwx,g=rx,o=rx

To change it temporarily, for the duration of your login session, use:

$ umask 0002

You can set the umask permanently by inserting the line umask 0022 or whatever value you want in your ~/.bashrc file. Table 9-3 shows common umask values.

Table 9-3. Common umask values
umask	User	Group	Other
0002	All	All	Read and Execute
0022	All	Read and Execute	Read and Execute
0007	All	All	None
0077	All	None	None

9.8.3 Discussion

Table 9-4 shows that you can come up with a umask for any occasion.

Table 9-4. All umask values
umask	File permissions	Directory permissions
7	None	None
6	None	Execute
5	Write	Write
4	Write	Write and execute
3	Read	Read
2	Read	Read and execute
1	Read and write	Read and write
0	Read and Write	Read-write-execute

umasks "subtract" permissions though I warn you, do not say this to a programmer, because you will be subjected to a tedious lecture on how it's not subtraction, it's that "the umask is bitwise and-ed to whatever modes are specified when the file is created." However, I do not have a problem with calling it subtraction.

When a program, such as a text editor or a compiler, creates a file, it is hardcoded to set the file permissions at either 0666 or 0777. If it knows the file it is creating is executable, it sets the permissions to 0777. The most common value is 0666, which is why you have to chmod +x scripts. Neither 0666 nor 0777 are appropriate permissions most of the time, so umask screens out the bits you don't want. For example, a umask of 0002 means regular files will become 0664, and executable files will become 0775.

9.8.4 See Also

Chapter 4 of LPI Linux Certification in a Nutshell, for exhaustive detail on umask, right down to the binary level