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Old 02-22-2016, 12:29 PM   #1
yolonaut
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Permission on a file


Hi eveyone,

excuse me for my bad English.

I know that in Linux systems every file has got its own rights in a format like this:

rwx rwx rwx

The first block is for the creator, second for the group, and the third for all the others.

I know that every user is part of a group. So, if I am member of 2 groups, and I set the second block to --x, does that mean that every member of group 1 and every member of group 2 would be able to execute my file or is it limited to my primary group (the one I got in first).

Also I got an exercise regarding this topic:

Quote:
I created a shell script and I want only me and all members of one of my secondary groups to be able to execute the script. Which commands and in which order do I have to enter to get there (and the group's name is 'mygroup').
I would say chmod 110 script.sh would do the job. But I'm pretty sure that I have to work with 'mygroup' in some sort of command and I do not know why it says 'secondary group'.


Hope you guys can help out getting this sorted.
Thanks a lot!
 
Old 02-22-2016, 01:12 PM   #2
hydrurga
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Quote:
I know that in Linux systems every file has got its own rights in a format like this:

rwx rwx rwx

The first block is for the creator, second for the group, and the third for all the others.
That's almost correct, except that the first block is for the owner, not the creator. A file may have been created by one user but then ownership subsequently transferred to another user.

To help you understand your next point, take a sample file listing produced by ls -l:

Code:
-rwxr-xr-- 1 hydrurga seals 0 Feb 22 17:59 example
hydrurga, the owner, has "rwx" access to the file "example" (we know it's a file because of the initial '-').

Anyone in the group "seals" has "r-x" access

All the others have only "r" access

You can in fact set access rights for multiple groups using something called "extended attributes", but that is probably beyond the scope of what you're learning at the moment.

So, if you're not the owner, you have to be a member of the "seals" group to have "r-x" access - it doesn't matter what other groups you may belong to.

For the exercise you were given, have a think about how you can change the file in two ways: (i) set its group so that only 'mygroup' members have the relevant access rights; (ii) how to set those access rights, as well as those for the owner and the others.

It doesn't matter whether the group is "secondary" - all that matters is that only one group is to get group access.

I hope that this makes sense.

Last edited by hydrurga; 02-22-2016 at 01:16 PM.
 
Old 02-22-2016, 02:04 PM   #3
pan64
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you may check man chmod, although it is not really trivial.
http://www.comptechdoc.org/os/linux/..._ugfilesp.html
https://www.linux.com/learn/tutorial...le-permissions
 
Old 02-22-2016, 02:15 PM   #4
rknichols
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The only difference between your primary group and secondary groups is that files you create will by default get your primary group ID. A non-default case would be files you create in a directory that has its set-GID bit set -- those files inherit the GID of the directory. You can use chgrp to change a file's group to any group of which you are a member.
 
1 members found this post helpful.
Old 02-23-2016, 01:16 AM   #5
chrism01
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As above, see chmod for permissions & chown or chgrp for ownerships (chgrp only changes grp ownership; chown can change owner and/or grp ownerships)
 
  


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