chmod changes the permissions mode of a file or directory
in a CGI program.
chmod and Permission Strings files that belong to you
can be made private or world writeable. This is important because if your webpage is world
writeable, then hackers can change your page with outyour knowledge. When in your
Unix shell, you can type ls -l to get a printout of your files.
Such a print out might look like this:
-rw-r--r-- 1 etracsvc other 1157 Jan 27 23:35 gayla.htm
-rw-r--r-- 1 etracsvc other 13095 Dec 10 11:58 hotanim.gif
drwxr-xr-x 2 etracsvc other 4608 Mar 3 20:51 images/
-rw-r--r-- 1 etracsvc other 5257 Mar 3 08:35 index.htm
-rw-r--r-- 1 etracsvc other 4996 Feb 12 08:37 index.htmy
-rw-r--r-- 1 etracsvc other 1579 Jan 20 18:44 jasper1.htm
-rw-r--r-- 1 etracsvc other 13142 Jan 28 21:18 jeffsbio.htm
-rw-r--r-- 1 etracsvc other 5725 Jan 29 09:50 jukebox.htm
The first column of the print out are the permission strings. These tell the computer who may
or may not have access to files. There are 3 groups of letters sequenced, rwx (that is rwxrwxrwx).
Each of the 3 groups define permission parameters for different users. The first set pertains to
the owner, the second to the group and the third to the world. Without getting wrapped up in
technical babble, what all this means is, if you give others permission to write to your files,
your web page could get hacked. The letters pertain to specific characteristics:
d=directory (all directories must have this)
r=read (enables read for that group)
w=write (enables write for that group)
x=executable (makes file executable)
In the permission string above, index.html is defined as -rw-r--r--. This means that only the
owner can read and write to the file. The group (other) may read the file but not write, and the
world may read the file but not write. If you create a file but the permission string gets changed
to a world writeable file you must change the permission string by using the Unix command:
chmod The table below shows some of the common
chmod numbers: The command itself is,
chmod [number] filename.
400 r-------- files (won't let you accidently erase)
444 r--r--r-- files (lets everyone read)
600 rw------- files (no one else can read or see files)
644 rw-r--r-- files (getting the idea?)
664 rw-rw-r-- files
666 rw-rw-rw- files, (note: this could allow havoc of satanic proportions)
700 rwx------ programs and directories
750 rwxr-x--- programs and directories
755 rwxr-xr-x programs and directories
777 rwxrwxrwx programs and directories
Note: 777 is a wide open executable script, this is dangerous.
