[SOLVED] How to calculate permissions for chmod command?

Issue

After a lot of reading, I still don't understand how to use chmod. My main question is what do the numbers mean? I am new to Unix and am doing some emergency fixes on our web server and am being asked to fix the permissions for some files. I don't understand what I need to do to make the files accessible.

Can someone provide some help here?

EDIT: Thanks for the great answers everyone. Unfortunately, I was in a panic and didn't read them in time and we ended up losing our client.. Hard to choose any single answer but I chose the one that I understood best.

Solution

You have 3 permission types:

• User (permissions applying to the file's owner)
• Group (permissions applying to the file's group)
• Other (permissions applying to everyone else)

For each of these types, you can allow 3 things:

• Ability to read (note that you need read permissions on a directory to list it)
• Ability to write
• Ability to execute

If you say:

chmod 755 some_file

It gets broken down like this:

User  Group  Other
7     5      5      (octal value,  base-8)
111   101    101    (binary value, base-2 or binary)
RWX   RWX    RWX

where: R - Read, W - Write, X - eXecute

So that command would mean that the owner gets all permissions, but group members, and others can only read and execute.

There's another input format with chmod that's handy, and doesn't require you to do conversion to binary in your head. As an example, to add execute permission for user:

chmod u+x some_file

To remove those permissions, you would say

chmod u-x some_file

You can replace 'u' with 'u', 'g', or 'o' (user, group, other respectively), and 'x' with 'r', 'w', or 'x' (you get the idea).

You have to be careful with this. If you did:

chmod -R 777 some_directory

And that file contained sensitive configuration data, then you just gave 'other' (i.e. the outside world) full read permissions (meaning the web server may serve up this information).

It would also be worthwhile for you to have a look at the chown command as well.

Answered By - user3738848