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Old 04-28-2007, 05:47 PM   #1
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Understanding permissions

Regarding file and folder permissions, I'm having some trouble understanding the difference between them. For files it seems pretty straightforward (is it?), but what exactly does read, write, and execute mean for folders?

I've seen a few contradictory explanations in the last little while, so I'm hoping somebody here could clarify.

My other question relates to dependencies between RWX permissions. For files or folders, is there any requirement that a something with X permission must have R, or any sort of dependency like that, or are they all truly independent?

I'd imagine with folders there might be some element of that...but I'm really not sure.

Your explanations on these issues are greatly appreciated.
Old 04-28-2007, 06:20 PM   #2
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Should be lots of good info on the net, but here are the very basics:
For any given file or folder, there are 3 'people-categories' for whom there may be permissions: USER, GROUP, and OTHER. USER means the owner of the file. Group means what 'group' of users who might access the machine, and OTHER means anyone on planet earth (Any user, any Group) who is using the machine.
For each of those 3 caterories there are 3 BASIC permissions:READ, WRITE, and EXECUTE.
READ means those people allowed, may READ the file. WRITE means those people allowed, may WRITE TO or CHANGE the file. And EXECUTE means those people allowed can EXECUTE or RUN the file (if it's a script or a binary, for example).
So, for any given file or folder, you must set WHO is allowed to do WHAT with a given file.

Search LQ here for things like PERMISSIONS, SUID, ACCESS, and stuff like that. There are other threads around with lots of info.
NOTE: These were the BASICS. There is also SUID, GUID, and STICKY which are three 'extra' special permissions, but that's a long story, which you will get the hang of if you read a bit.
Also, in a console, type 'man chmod' or 'man chgrp' for further info. For the most part, each permission is independant of each other. For example, a file can be USER(root)=RWX, GROUP(humans)=RW, and OTHER(anyone)=RX. (These would be rather silly settings mind you ) but they would mean that the ROOT user could read, write, or execute the file. Anybody in the group 'humans' could read or change the file, but not execute it. And anyone at all could read or execute the file. As I said, stupid permissions, but it's only an example.

Last edited by GrapefruiTgirl; 04-28-2007 at 06:25 PM.
Old 04-28-2007, 10:30 PM   #3
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A bit more detail:
Read, write, and execute are totally independent.

Permissions for directories are slightly different. for example, to look inside a directory--even only to read--you have to have execute privileges on the directory. If you don't have read privileges on a directory, you cannot ls to see the files, but you CAN read the files (if you have permission for them).

The best way to get you head around this is to do some experiments.
Old 07-18-2007, 05:21 PM   #4
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Originally Posted by GrapefruiTgirl
For example, a file can be USER(root)=RWX, GROUP(humans)=RW, and OTHER(anyone)=RX. They would mean that the ROOT user could read, write, or execute the file.
User "root" is special; it can normally read and write any file, regardless of permissions.
Anybody in the group 'humans' could read or change the file, but not execute it. And anyone at all could read or execute the file.
Not true, 'humans' cannot execute the file. That is anyone having 'humans' as their primary or an auxiliary group. The OTHER permissions only apply to people other than the owning USER or GROUP.


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