File Permissions

In Linux, file permissions are expressed in a 3 digit octal (base 8) number.
These numbers stand for the following permissions:

0 - No access.
1 - Search or execute access.
2 - Write access.
3 - Write plus search or execute access.
4 - Read access.
5 - Read plus search or execute access.
6 - Read and write access.
7 - Read, write and search or execute access.

When listing the file permissions ('ls -l'), you will notice that there are 10 file attribute characters:

    total 11
    drwxr-xr-x        3 ROOT    SYS1    0 Mar 12 19:32 tmp
    drwxrwxrwx        4 ROOT    SYS1    0 Mar 12 19:32 usr
    drwxr-xr-x        2 ROOT    SYS1    0 Mar 12 19:32 bin
    -rwxr--r--        1 ROOT    SYS1  572 Mar 12 19:32 foo
    -rwxr--r--        1 ROOT    SYS1  640 Mar 12 19:33 abc

The first one indicates the type of file, this is either:

- - Normal file.
c - Character special file.
d - Directory.
e - External link.
l - Symbolic link.
p - FIFO.
s - Socket file type.

The rest describe the 3 digit access mentioned above for difference user groups and can be a combination of:

r - Read access.
w - Write access.
x - Search or execute access.

The first set of 3 digits is the access for the file owner, the second set of 3 digits is the access for the file group and the last set of 3 digits is the access for anyone else.

So for example, file access of 600 (shown as '-rw-------'), means that the file owner has read and write access, but no-one else has any access.