How do unix umasks work?

Often file permission problems on unix systems are caused by users who don't understand their umask and set it properly

What is a umask?

A umask is a bitmask by means of which a user can affect the default permissions of files they create on unix systems. You should find out what file permissions are if you are unsure. Its important to fully appreciate that a umask is not a default permission set it is a default permission mask so bits in the mask are the bits which you want not to be in the permissions of the file. A umask of 777 will thus mean that processes create files with permissions of zero whereas one of 0 will create files with full permissions thussly:

% umask 777                                                
% touch foo                                                
% umask 0                                                  
% touch bar                                                
% ls -l foo bar                                            
-rw-rw-rw-    1 sean   sean             0 Jul  2 11:15 bar
----------    1 sean   sean             0 Jul  2 11:15 foo

Now you might well wonder why (if my umask is zero) the execute bits aren't also being set on "bar"? Lets see what "touch" is doing under the covers...

% strace touch furb 2>&1 | grep furb 
execve("/bin/touch", ["touch", "furb"], [/* 137 vars */]) = 0
open("furb", O_WRONLY|O_NONBLOCK|O_CREAT|O_NOCTTY|O_LARGEFILE, 0666) = 3
utime("furb", NULL)                     = 0

As you can see, the "touch" process only tries to create the file with permissions of 0666. The permissions requested at open(2) are then masked by the umask and the bits requested in the open(2) call that are not in the mask end up in the permissions of the file created. Processes (such as compilers) which know they are creating executable files will generally use open(2) with the execute bits set as well. Because umask is a mask it can't raise the permissions higher than the process asks for in the open(2) call. The same is true of any other process which opens files.

How do I change my umask?

Now, if you want to change your umask, the command you want is the shell builtin called umask not /usr/bin/umask. From man umask:

/usr/bin/umask
The umask utility sets the file mode creation  mask  of  the
current  shell  execution environment to the value specified
by the mask operand.  This mask affects the initial value of
the  file permission bits of subsequently created files.  If
umask is called in a subshell or separate utility  execution
environment, such as one of the following:
    (umask 002)
    nohup umask ...
    find . -exec umask ...
it does not affect  the  file  mode  creation  mask  of  the
caller's environment.

If the mask operand is  not  specified,  the  umask  utility
writes  the  value of the invoking process's file mode crea-
tion mask to standard output.

Let's just see if that's true, shall we?

% umask                 #check my current umask
022
% /usr/bin/umask 0      #try to change it using /usr/bin/umask
% umask                 #Did it work?  No!
022
% umask 0               #try to change it using the builtin umask
% umask                 #Did it work?  Yes!                              
000

So it is true. Why? The reason is that the shell does a fork(2) to create a copy of itself to run the /usr/bin/umask process, so then even if that process sets its own umask, those values happen in the child process and don't end up in the calling process when the child quits. That's why you need the shell builtin if you want to change your umask. If you don't understand, just always type umask and never /usr/bin/umask.

Now you know why you need to use chmod +x to set the "execute" bits on the things you want to be executable. Its particularly important to get file permissions set correctly before checking them in to CVS because all future checkouts of a file have the file permissions which a file had when it was first checked in unless someone tinkers in the repository.


Unless otherwise specified the contents of this page are copyright © 2015 Sean Hunter. This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.