UNIX for webmasters

Umenu is a menu system that allows you to issue simple UNIX commands through menu choices.  For the purposes of this class, you will work directly with UNIX.

Note:  There are many nodes on the blue cluster, so you may see a different node number each time you log in.

Case Study 1: 

Permissions

The basic permissions that may be granted are read, write, and execute.  Each of these permissions has a slightly different meaning when applied to either a file or a directory.
 
 

Permission	File	Directory
Read	view the contents of a file	list the contents of the directory (i.e. - do an Ls on the directory)
Write	write to a file, edit a file	create a new file in the directory
Execute	run the file if it is an executable file (e.g. - a cgi script)	navigate through that directory level to subdirectories

Usually you will have what are called bsions for yourself.  In other words, as the creator and owner of a file or directory, you have read, write, and execute permissions.

You may also decide to assign file and directory permissions for other people.  The three types of people you can assign permissions to are user (yourself), group (a predefined set of people), and other (everybody else).
 

People who may be granted permissions	Description
User	The person who created the file or directory. Sometimes called "owner".
Group	A group of users. By default, most people are already a member of the bch everyone with an account on blue.unix is a member. You can make a request to consult@virginia.edu to have a group created if you have need for multiple people to maintain the same files and directories.
Other	Everybody else

Fully displayed, permissions appear as follows:

          rwxrwxrwx

• The first section of rwx is for the user
• The second section of rwx if for the group
• The third section of rwx is for other (everyone else)

Each individual read, write, and execute permission can be turned on or off.  If a permission is b has been granted".  This is denoted by a r w or x entry.   If a permission is off, it means "this permission has not been granted". This is denoted by a - (i.e. - dash)
 

Example	Description
rwxr--r--	User has full permissions.  Group and other have only read permissions
rwxr-xr-x	User has full permissions.  Group and other have read and execute permissions
rwxrwxrwx	User, group, and other have full permissions.

Permission errors are probably the most common cause of Web pages not displaying correctly.  If you navigate to a Web page and receive the error message ban underlying permission error.

At a minimum, html and image files must be readable by other in order to appear appropriately, and the directories that contain the files must be executable by other.  Any cgi scripts or other executable files must be executable by other.

Many people choose to set all of the files in their public_html directory to be both readable and executable by other.  That way, the files will display properly and permission errors will be unlikely.  The downside of this approach is that people who have access to the server will be able to get a listing of all the files in your public_html directory (because the directory is readable).

Another issue to be aware of is that Windows (be it 95, 98, NT, or 2000) takes a different view of file permissions than UNIX.  Since Home Directory Service is a Windows utility that allows you to perform UNIX operations, it will often bult is that if you publish a page using Home Directory Service, the permissions may need to be modified in order for the page to display properly.

Viewing permissions is a simple modification on the Ls command, and looks like the following:

ls  -l

Exercise:  Viewing File Permissions

Permissions may be modified by the use of the chmod (change mode) command.  The syntax for chmod is as follows:

chmod mode filename

where mode is the set of permissions you want to grant and filename is the name of the file or directory for which you want to grant the permissions.  Some examples are as follows:
 

Example	What it means
chmod  go+r  stuff.html	Grant group and other read permissions for the file stuff.html
chmod  go+rx  images	Grant group and other read and execute permissions for the directory images
chmod  go-rwx private.html	Remove read, write, and execute permissions from the file private.html

Exercise:  Changing File Permissions

Departmental Web Sites

Most departmental Web sites are not stored on blue.unix, but on various servers such as bab Web sites on minerva are structured in one of two ways:

1. With a public_html directory
2. Under /www/doc

The login process, the URL for finding the site, and the file path for saving files on minerva differs greatly depending on which way the site is structured.  The following sections describe the two basic structures and the ramifications of each.  If you are in doubt about which method your department uses, you should check with your departmental computing support person.
 

With a public_html Directory (the old method)

Example: The Web site for the Classics Department at the University of Virginia may found at:
http://www.virginia.edu/~classics
 

• If you were the maintainer of this site, you would log into minerva with the username bhe password is for classics.
• After logging in, you would be at the home directory for the Classics department:

/home/classics
• You would then CD to the public_html directory:

/home/classics/public_html
in order to view, modify, or adjust file settings.

1. The URLs for this type of site have a tilde ~ in them, which can make them more difficult to remember.
2. The names for tilde accounts are limited to eight characters.
3. It is nearly impossible to tell who is making modifications to the pages.  If there were a group of four people working on the site, they would all log in as bssicsbp;

Under /www/doc (the new method)

Example: The Web site for the Center for National Security Law may found at:
http://www.virginia.edu/cnsl
 

• If you were the maintainer of this site, you would log into minerva with your own user name and the same password you would use on blue.unix.
• After logging in, you would navigate to /www/doc/cnsl in order to view, modify, or adjust file settings.

1. The URLs for this type of site do not have a tilde ~ in them.
2. Account names are not limited to eight characters.
3. Since files and directories are created by a people logged in as themselves, it is easier to track who created the individual files and directories.
4. If a site maintainer leaves the University, his or her account will be deleted (but not the files).  This effectively blocks former staff members from accessing files after they have left the University.

One issue that must be dealt with when using the /www/doc site arrangement is the creation and maintenance of groups (especially if you have many people maintaining the Web site). Since the site maintainers are logging in using their own computing ids (and not a generic account), groups must be created and members added to the groups in order to allow multiple people to access the files.
 

Appendix A: Web Troubleshooting Flowchart

Appendix B:  Solutions to Case Studies and Challenge Exercises

In order to solve this case, you will need to explore the directory structure by using the CD and Ls commands.
 
 

Action Steps:	Comments:
1. Navigate to the uwdclass directory by entering: cd  /home/mst3k/public_html/uwdclass
2. View the files in the uwdclass directory by entering: ls	Note that the file bnbsp; What other directories are available under uwdclass? (Remember that all directories are marked with a trailing slash)
3. Navigate to the images directory by entering: cd images
4. View the files in the images directory by entering: ls	Note that the file birectory.

 

Summary: The Web page isnb http://www.people.virginia.edu/~mst3k/uwdclass/case1.html because the case1.html file isnbectory;  it is stored in the images subdirectory of uwdclass, /home/mst3k/public_html/uwdclass/images/case1.html.

 

Case Study 2:  Open a Web browser and navigate to the following URL: http://www.people.virginia.edu/~mst3k/uwdclass where mst3k is your computing id.  The bexb to the fact that the uwdclass directory does not contain a file called home.htm(l) or index.htm(l).  This is generally considered to be an undesirable effect. Correct this problem by renaming the b in the /home/mst3k/public_html/uwdclass directory (where mst3k is your computing id). When you are done, refresh the page in your browser to view the effects.

 

Action Steps:	Comments:
1. Open a Web browser and go to the following URL: http://www.people.virginia.edu/~mst3k/ uwdclass	Note the b
2. In UNIX, navigate to the uwdclass directory by entering: cd  /home/mst3k/public_html/uwdclass
3. View the files in the uwdclass directory by entering: ls	Note the file bhere is no b/font>
4. Rename the nothome.html file to home.html by entering: mv  nothome.html  home.html
5. Verify that the change took place by entering: ls	Note that b b
6. Switch to your Web browser and reload the page to view the effects.

 

Summary:  Your public_html directory and all of its subdirectories should have a file called home.htm, home.html, index.htm, or index.html in order to avoid having an index listing appear when someone navigates to that directory.  In the case study above, you corrected the problem by renaming a file to home.html.

 

Case Study 3:  Open a Web browser and navigate to the following URL: http://www.people.virginia.edu/~mst3k/uwdclass/case3.html where mst3k is your computing id. You will receive an bss Deniedb Diagnose the cause of the problem and then take the appropriate actions to correct the situation.

 

Action Steps:	Comments:
1. Open a Web browser and navigate to the following URL: http://www.people.virginia.edu/~mst3k/ uwdclass/case3.html	Note that you get an bcess Forbiddenb
2. Switch back to UNIX and navigate to the uwdclass directory by entering: cd  /home/mst3k/public_html/uwdclass
3. View the permissions for this directory by entering: ls  -l	Note that bgroup or other.
4. Change the permissions for case3.html by entering: chmod  go+r  case3.html
5. Verify that the permissions have changed by entering: ls  -l	Note that band other.
6. Switch back to your Web browser and refresh the page.

 

Summary:  Html files must be readable by other in order to be viewable on the Web.  In this case study, you turned on the read permissions for case3.html, and then the page was viewable on the Web.

 
 

Challenge Exercises:  If you have extra time or would like further practice after class, diagnose and correct the problems you find at the following Web pages (for each, substitute your computing id for mst3k): http://www.people.virginia.edu/~mst3k/uwdclass/pics.html http://www.people.virginia.edu/~mst3k/uwdclass/challenge.html http://www.people.virginia.edu/~mst3k/uwdclass/freeforall.html (note:  this file will display correctly, but in its current state anybody else can modify it). For possible correct solutions, consult the back of this booklet (but there is more than one way to fix each problem).

Summary of Potential Solutions:

http://www.people.virginia.edu/~mst3k/uwdclass/pics.html
 

Problem:	Broken Image
Cause:	If you view the html source for pics.html, you will see that the image tag is looking for bges/bill.gifb file is not located under /home/mst3k/public_html/uwdclass/images, it is located directly under uwdclass.
Solution:	Move the bgifbe uwdclass directory and issuing the command mv  bill.gif  images.

http://www.people.virginia.edu/~mst3k/uwdclass/challenge.html
 

Problem:	Broken image
Cause (Part 1):	If you view the html source for challenge.html, you will see that the image tag is looking for b The rotunda_sky.jpg file is not located under /home/mst3k/public_html/uwdclass/images2, it is located directly under uwdclass.
Solution (Part 1):	Move the bs2 by going to the uwdclass directory and issuing the command mv  rotunda_sky.jpg  images2.  When you switch back to the Web browser and refresh, there will still be a broken image.
Cause (Part 2):	Switch back to UNIX and view the file permissions for rotunda_sky.jpg by going to the images2 directory (CD /home/mst3k/public_html/uwdclass/images2) and doing an ls  -l.  The permissions will appear to be correct.  At this point, you should check the permissions of the images2 directory itself.  Go to the uwdclass directory and do an ls  -l , and you will discover that the images2 directory is not executable.
Solution (Part 2):	Verify that you are at the uwdclass directory, and then issue the command chmod go+x images2.  If you refresh the Web page again, the image should appear successfully.

http://www.people.virginia.edu/~mst3k/uwdclass/freeforall.html
 

Problem:	Anyone with access to blue.unix can modify this page.
Cause:	Full permission are turned on for group and other.
Solution:	In UNIX, navigate to the uwdclass directory.  View the permissions for the directory by entering Ls  -l.  Remove the write permissions for group and other by entering chmod  go-w freeforall.html.

Appendix C:  Cleaning up After Class

Unfortunately, deleting all of the class exercise files in the method described above can be a long and laborious process, since you would need to delete each file, confirm the deletions, and then delete the directories.

If you would like to take a faster approach to deleting class files, perform the following actions:
 

Action Steps:	Comments:
1. Navigate to your public_html directory by entering cd /home/mst3k/public_html where mst3k is your computing id.
2. Enter the following command: rm  -Rf  uwdclass	Note! This operation will delete the uwdclass directory, all of its subdirectories, and all of its files.  You will not be able to undo this action. Caution! Do not perform this procedure with your other directories unless you are certain you want to perform the action.

If you would ever like to perform the exercises in this booklet again, you may reinstall the exercise files according to the instructions in the Getting Started section of this document.