Linux Demo: Using umask

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in this demonstration we're going to work with you masks now you can use the value assigned to you mask to modify the default permissions that are assigned to files and two directories when they're created in the Linux file system so with that in mind we need to look at the default permissions that are assigned to files and directories when they're created in the file system let's first create a file let's just do a touch command and make a new file called my test file and let's also make a new directory let's do make dirt yep now let's run the LS - shell command so we can see the mode the permissions that were automatically assigned to this file in this directory okay first of all let's look at the file we can see that the owner of the file they are tracy user is assigned read/write access to the file but no execute permission which makes sense because it's not an executable file and the owning group also received read/write access to the file and all other authenticated users on the system received read access to the file so if I were to put something in this file I could open it and edit it and anyone who's a member of this group is just me would be able to open it and edit it any other user on the system would still be able to see the contents of the file but not be able to modify it now let's look at the temp directory down here that we just created by default the user that owns the directory me is granted read write and execute permissions to the folder meaning that they can list the file contents they can add and delete files from the folder and they can also enter into the folder the same permissions were assigned to the owning group which again is my group and I'm the only member of it but they receive the same read write and execute permissions to the folder all other authenticated users to the system have read access to the folder and they have execute access to the folder so they can list folder contents and they can go into the folder but they're not allowed to add or remove files from the folder because they did not receive the write permission well understand that the Linux kernel whenever you create a file in the file system actually assigns a mode of six-six-six to files and set 7/7 to directories when they're created however this is not the mode that we are seeing if we had six six six permissions assigned to say my test file we would have read write read write and read right here remember read is assigned a value of 4 right is assigned a value of 2 that's where we get 6 from so there's six there's six but for others we have a value of 4 somewhere along the line the right permission was blocked that's where we lost the 2 that's why it's 6 6 or instead of 6 6 6 likewise with the temp directory there's our first 7 assigned to the owner or 2 + 1 7 and likewise to group 4 2 + 1 7 but noticed for other authenticated users we have 4 + 1 so that's 5 so we have seven seven five instead of seven seven seven once again the right permission which has a value of two was removed this was done by umask now you can view the value of you mask by just entering you mask at the shell prompt and here you can see that we have a value of zero zero two this 0 specifies which permissions are subtracted from the owner this digit right here specifies the permissions that are subtracted from owner this digit specifies the permissions that are subtracted from group and this digit specifies the permissions that are subtracted from other and it doesn't matter if we're dealing with files or directories the same umask values are applied to customize the mode so as you can see because we have 0 here no permissions are used from the owner and no permissions are removed from group but we have a value of 2 here this value of 2 specifies that the write permission be removed and because the 2 is in this place right here it specifies that the write permission be removed from others right here now be aware that not all distributions will use the same you mask most of them that you work with will probably use this one 0 0 - 2 however I know that there are some that use a value of 0 - 2 instead of 0 0 - which would remove the right permission from group as well by default now if for some reason you're not happy with the default modes that are being signed to your folders and files then you can actually modify the value of UMass customize what mode is automatically assigned to the files and folders in your file system when they're created for example over here under directory notice that by default we are giving other authenticated users permission to enter into basically all the directories when they're created in the file system unless we manually go in and customize their permissions after the fact well from a security standpoint that may not be the greatest idea maybe we don't want to give them the ability to go into all these different directories and browse through the file system because remember up here because of the mode that we assign they do have read access to files meaning that they could poke around through all the different files in the file system and actually view their contents they won't be able to change them but they'll be able to see them from a security standpoint that could be concerning so perhaps a better security posture would be to actually remove the execute permission from others when a mode is assigned to a directory that way they cannot go down a level into those directories and open up files within them so what is the value assigned to X well it has a value of 1 therefore if we want to remove the X permission from others with umask what would we set the value of umass to we would have to add another one to it because we still want it to remove group so we have the two but we also want it to remove the execute permission as well so we add the value of execute to the number that is being used as the mask for others so what we would do is oh you mask 0:03 we're adding a 1 to this last digit which will remove both the right and the execute permission from others good enter now if we run the umass command we should see that it has a value of 0 0 3 now whereas before it was 0 0 2 let's go ahead and test to make sure that this worked let's create another new directory make dirt and let's call this directory nuder and let's create a new file touch new file now let's run the LS dash L command and we see that the mode is now different so for the new directory we just created new der owner still gets rewrite execute just like they should and group gets read write execute just like we said that they should but notice what happened over here to others they have read but they do not have execute which will prevent them from actually entering into that directory however if we look at the mold for new file it's really not any different than it was before when we created my test file and that's because the execute permission was not assigned in the first place by the Linux kernel remember the Linux kernel by default will assign a moat of 666 to files when they're created in the filesystem that means they're given read write read write read write execute was never even in the picture so by subtracting it with you mask there was nothing to subtract so it didn't affect our file modes at all but it did affect our directory modes now before we end I need to point out that the value you assigned to you mask from the command line is not persistent meaning that as long as this system stays running this value we assigned will remain in effect but as soon as we reboot the system it'll jump back to its default so if you want to make that value persistent across reboots you'll have to go into a shell configuration file and add the necessary commands that's it for this demonstration and this Devlin we talked about using new masks we first looked at the default modes that are assigned to files and folders when they're created in the file system and then we discussed the role of you mask in subtracting permissions from the default mode assigned to files and folders and then we customized the value of you masks to modify the default mode that's assigned to files and folders
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Channel: The Linux Man
Views: 3,304
Rating: 4.9310346 out of 5
Keywords: Linux
Id: SnogbBNyMdM
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Length: 8min 13sec (493 seconds)
Published: Thu Nov 24 2016
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