You can see the existing permissions by using the ls command, and you can change them by using the chmod command--like this:
If you type: ls -alg you get the entries like the following:
-rw-r--r-- 1 someuser student 273 Apr 9 18:45 file1 drwxr-x--- 1 someuser student 591 Apr 9 18:45 directory1 ^\ /\ /\ / t u g o y s r t p e o h e r u e p r
The first character tells you if the name at the end of the line is a file or a directory. A "-" indicates a file; a "d" indicates a directory.
The next 3 characters tell you the "user's" permissions on the file (your permissions assuming you own the file; "someuser" in the example). The three characters are for read, write, and execute. In this example, you have read/write privileges on file1 and read/write/execute privileges on directory1.
For files, read/write/execute characters give those permissions. For directories, read privilege means that you can type ls to see what is in it. Write privileges means that you can create new files in that directory or remove files from it (EVEN IF THE FILE PERMISSIONS DON'T ALLOW YOU TO ACCESS THE FILE ITSELF). Execute privileges means that you have access to files in the directory if you have permission to read the files, such as when using cat or more.
You should have read/write/execute on all your directories. If you want others to be able to look at certain files in one of your directories, then give them read permission for those files, and give them execute permission for the directory. If you want them to be able to list the files (using ls) in that directory and to be able to read them if they have read privileges on them, then they need read/execute privileges on the directory.
In the example above, the next 3 characters are the permissions for group. Users in group "student" have read permission in file1, so they can read that file if the directory that contains that file allows them to access it (via execute permission). As for the directory, users in group student have read/execute permissions on that directory, so they can use ls to see the contents of the directory, and they can access files that they have permission to look at via the file permissions.
The last 3 characters are the permissions for "other" (everyone not yourself or in the group named "student"). In this example, they can read file1 if the directory permissions allow it. Since they have no permissions on directory1, they can't do anything with the files in that directory, or with the directory itself.
You can use the chmod command to give and remove permissions. For example:
chmod g+r file1 Adds read permission for file1 for members of the group. chmod g-rx directory1 Removes read and execute permission from directory1 for members of the group. chmod o-r file1 Removes read permission from file1 for "others".
to get a complete explanation of how you can set and use your permissions.