Introduction to Unix

Directory

When you want to prepare Web pages that are going to be accessible by anyone in the world, you need to put these pages in a directory called public_html. Hence you need to create this directory in your home directory. You are also required to create a directory called WebDev in which you will store your tutorial work for this module. As this work needs to be accessible by anyone, this directory will have to be in your public_html directory.

Access Rights

When you type ls or ls -l in your terminal window, you will see lots of details about the contents of your directory:

11 grizzly.cscs.wmin.ac.uk% ls -l
 total 41978
 drwxr-xr-x   2 coloma   800       512 Feb 11 16:25 lect01
 -rw-r--r--   1 coloma   800     25126 Jan 02 09:23 lect01.html 
 drwxr-xr-x   2 coloma   800       512 Feb 20 08:54 lect02
 -rw-r--r--   1 coloma   800     29164 Feb 13 10:12 lect02.html
 drwxr-xr-x   2 coloma   800       512 Feb 22 17:43 lect03
 -rw-r--r--   1 coloma   800     29816 Feb 20 11:52 lect03.html
 drwx--x--x   2 coloma   800       512 Feb 28 12:47 lect04
 -rw-r--r--   1 coloma   800     29935 Mar 14 09:54 lect04.html
12 grizzly.cscs.wmin.ac.uk%

In the left-hand column is a 10 symbol ‘string’ consisting of symbols d, r,w, x, -. If d is present, it will be at the left-hand end of the string, and indicates a directory. Otherwise ‘-’ will be the starting symbol of the string.

The 9 remaining symbols indicate the permissions, or access rights, and are taken as three groups of 3:

• The left group of 3 gives the file permissions for the user that owns the file (or directory) (coloma in the above example);
• the middle group gives the permissions for the group of people to whom the file (or directory) belongs (800 in the above example);
• the rightmost group gives the permissions for all others.

The symbols r, w, etc., have slightly different meanings depending on whether they refer to a simple file or to a directory.

Access rights on files

• r (or -), indicates read permission (or otherwise), that is, the presence or absence of permission to read and copy the file
• w (or -), indicates write permission (or otherwise), that is, the permission (or otherwise) to modify a file
• x (or -), indicates execution permission (or otherwise), that is, the permission to execute a file, where appropriate

Access rights on directories

• r allows users to list files in the directory;
• w allows users to make changes to the content of the directory, including delete files from the directory or move files into it;
• x means the right to access files in the directory. This implies that users may read files in the directory provided they have read permission on the individual files.

Changing access right

Only the owner of a file can use chmod to change the permissions of a file. The options of chmod are as follows:

For example, to remove read write and execute permissions on the file welcome.html for the group and others, type:

chmod go-rwx welcome.html

To give read and write permissions on the file welcome.html to all:

chmod a+rw welcome.html

Alternatively, it is also possible to change access rights using the corresponding octal value of the permission: read permission is given the value 4, write permission the value 2 and execute permission 1:

rwx
421

These values are added together for any one user category:

So access permissions can be expressed as three digits. For example:

                            user    group   others

      chmod 644 file1       rw-     r--     r--
      chmod 711 dir1       drwx     --x     --x
      chmod 600 file2       rw-     ---     ---

When creating web pages and setting up web sites, your pages need to be accessible by anyone in the world. You need to set permissions to directories and files as follows:

cd 
chmod 711 .
cd 
chmod 711 public_html
cd public_html
mkdir WebDev
chmod 711 WebDev

cd WebDev
chmod 644 tut1.html