commas. A space terminates the maskExpression(s). The permissions are applied to different user classes: The operator specifies how the permission modes of the mask should be adjusted. The permission-symbols indicate which file permission settings are to be allowed or prohibited by the mask. For example: umask u-w Prohibit write permission from being set for the user. The rest of the flags in the mask are unchanged. Example of multiple changes: umask u-w,g=r,o+r This would set the mask so that it would: prohibit the write permission from being set for the user, while leaving the rest of the flags unchanged; allow the read permission to be enabled for the group, while prohibiting write and execute permission for the group; allow the read permission to be enabled for others, while leaving the rest of the other flags unchanged. Command line examples Here are more examples of using the command to change the mask: Example showing effect of : $ umask -S # Show the (frequently initial) setting u=rwx,g=rx,o=rx $ gcc hello.c # compile and create executable file a.out $ ls -l a.out -rwxr-xr-x 1 me developer 6010 Jul 10 17:10 a.out $ # the umask prohibited Write permission for Group and Others $ ls > listOfMyFiles # output file created by redirection does not attempt to set eXecute $ ls -l listOfMyFiles -rw-r--r-- 1 me developer 6010 Jul 10 17:14 listOfMyFiles $ # the umask prohibited Write permission for Group and Others $ ############################################################ $ umask u-w # remove user write permission from umask $ umask -S u=rx,g=rx,o=rx $ ls > protectedListOfFiles $ ls -l protectedListOfFiles -r--r--r-- 1 me developer 6010 Jul 10 17:15 protectedListOfFiles $ rm protectedListOfFiles override r--r--r-- me/developer for protectedListOfFiles? $ # warning that protectedListOfFiles is not writable, answering Y will remove the file $ ##################################################################################### $ umask g-r,o-r # removed group read and other read from mask $ umask -S u=rx,g=x,o=x $ ls > secretListOfFiles $ ls -l secretListOfFiles -r-------- 1 me developer 6010 Jul 10 17:16 secretListOfFiles Mask effect The mask is applied whenever a file is created. If the mask has a bit set to "1", that means the corresponding file permission will always be disabled when files are subsequently created. A bit set to "0" in the mask means that the corresponding permission will be determined by the requesting process and the OS when files are subsequently created. In other words, the mask acts as