Before we get into changing permissions lets at least define what the
most common ones do. This quick help file talks about things like group
and user ownership which is covered in another file so if you don’t know
what they mean at all read chown next and you will.
~# ls -l
total 456
drwx—— 7 root root 4096 Feb 24 05:50 somedir
-rw-rw-rw- 4 root root 323 Feb 18 09:10 somefile
By doing a ls -l (list long) we can see the permissions on two files in
my home dir. All we’re interested in now is the left most column so
lets remove the other stuff
drwx—— somedir
^^^^
||||
|||eXecute
||Write
|Read
Directory
First thing this tells us is that it’s a directory if it’s just a file
it will be – as in somefile above.
Second is Read, I can list the contents of this dir: ls somedir
Third is Write, I can place files into this directory: cp somefile somedir
Fourth is eXecute, I can cd into this directory: cd somedir
-rw-rw-rw- somefile
^^ ^^ ^^
|| || ||
|| || |world Write
|| || world Read
|| |group Write
|| group Read
|user Write
user Read
First permission tells us – which as we found out can mean directory,
since this is blank it is just a file.
Second is user Read, the owner of this file can read it: less somefile
Third is user Write, the owner can modify this file: rm somefile
Fourth is user blank, if it where x the owner could run this file: somefile
Fifth is group Read, anyone in this group can read this file: cat somefile
Sixth is group Write, anyone in this group can modify this file: rm somefile
Seventh is blank, if it where x anyone in the group could run this file
Eighth is world read, anyone can read this file: less somefile
Ninth is world write, anyone can modilfy this file: vi somefile
Tenth is blank, if it were x anyone could run this file
Now regardless of group or world settings only the owner and root can
change permissions on a file or dir The utility that you use to change
permissions on a file or dir is chmod
Chmod has two ways to set permissions on file or directories: Absolute
and Symbolic
Absolute: using numbers to set the permissions
eg: chmod 755 would = -rwxr-xr-x
Symbolic: using letters to set the permissions
eg: chmod a+rx,u+w would = -r-xr-xr-x
Symbolic is easier to understand but it’s also easy to make errors.
Once you understand Absolute you will probably use that all the time.
Absolute settings: The first number changes the user permissions
The second number changes the group permissions
The third number changes the world permissions
1 = eXecute 2 = Write 4 = Read
Now you can do chmod 1,2,4 somefile to change it’s permissions however
that is wasting key strokes. chmod allows you do add the numbers together to
change them so chmod 1,2,4 somefile is the same as chmod 7 somefile
If you only use one number chmod assumes all you want to change is the
user permissions so lets group some together so we can change everything.
-rw-rw-rw- somefile
This is easy to get we want Read Write so add read (4) to write (2)
we get 6 so: chmod 666 somefile is how we would get that.
Now suppose that somefile was a script you were working on once you
are done you probably don’t want to keep typing sh somefile to run it.
So you would make it eXecutable that is the 1 permission.
chmod 111 somefile becomes
—x–x–x somefile.
Opps were did the rw we had just go?
There is nothing wrong with it this way you can still run it like you
wanted, but if you go back and try and change or view it later on you
will get a permission denied error
That is why it’s called absolute. to acheive what we had before plus
the eXecute we want to add read (4), write (2) and eXecute (1) to make 7
chmod 777 somefile becomes -rwxrwxrwx somefile
Symbolic on the other hand is a bit more forgiving then absolute it won’t
change settings you already have if you omit them. So to at the eXecute
bit to somefile from the rw it was before with symbolic it is simply
chmod a+x somefile to get -rwxrwxrwx somefile
The first letter “a” applies to user class that you want to change
(user, group, world) The “+” means add this setting, there is also “-”
which removes it.
User Classes you can use are: a (all), u (user), g (group) and o (world)
or (other)
Permissions you can use are: r read, w write and x eXecute. In addition
you can also use u (user), g (group) and o (other) in the permission
place to set the permissions for the user class to match those of the other
user class you specify using the ‘=’, chmod a=u somefile will make
all the user classes match the one for user.
You can also group together user classes and permissions to make things
a bit quicker
Some quick Examples:
drwx—— somedir -rw-rw-rw- somefile
chmod g+x somefile -> -rw-rwxrw- somefile
chmod o+w somedir -> drwx—-w- somedir
chmod g+o somedir -> d-w–w–w- somedir
chmod go-rw somefile -> -rw—x— somefile
chmod a+u somefile -> -rw-rw-rw- somefile
chmod gou-rwx somedir -> d——— somedir
chmod a+rwx somedir -> drwxrwxrwx somedir
chmod +x somefile -> -rw—x–x somefile
chmod +r somefile -> -rw-r-xr-x somefile
chmod +w somefile -> -rw-r-xr-x somefile
For more information please visit our website at: http://www.cpccci.com and http://www.cpcwebsolutions.com
Tags: changing file permissions in linux, Computer Science news, Engineering News, how to in linux, linux
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