with your domain name:
http://www..com/cgi-bin/admin/vs/imanager.pl
This page will prompt you for your root password. Simply enter your VirtuaLand VirtualServer root password and click on the Start iManager button. When iManager is started, you will see a screen that shows you your current working directory on your server followed by a list of entries in that directory. You can move around the directories by modifying the Path Specification box and clicking on the Change Directory button. If you want to change to a directory below your current working directory, you can also simply click on the directory name that is in the list below the Change Directory button.
If your list of entries within your current working directory contains a graphic file or a home page file, you can click on the name of that file to bring up a small browser window that will show you the graphic image or the home page file. You will also notice that the list of entries shows you not only the name, but also the file size, the date the file was last modified, the mode or permissions of the file, and a series of options to edit, delete, copy, move, link, or chmod (change permissions) for that file. These options will be discussed in more detail below.
The iManager extension allows you to edit the text files, such as html files, from within your web browser. This is useful if you need to make a quick change and do not want to telnet to your server to do it.
When you start iManager and look at the list of entries you will notice the column titled Edit. The entries will either have ED, CD, or a blank in this column. If the entry has ED, then it is an editable text file. Click on the ED to begin editing. If the entry has the CD option in the Edit column, then you know the entry is a directory. By clicking on the CD you will change to that directory as your current working directory and a new entry list will appear. If the entry has no option in the Edit column, then you know that the file is neither a directory nor an editable file. Graphic images will appear like this.
The iManager extension also allows you to delete files and directories from your server without actually telneting to your server to do it.
When you start iManager and look at the list of entries you will notice the column titled Delete. All the file and directory entries below will have the RM listed in this column. The RM stands for the rm (remove) unix command. Clicking on the RM will take you to a screen to confirm that you want to delete the selected file or directory.
The iManager extension can copy files on your server to a new file and a new location.
When you start iManager you will see the column titled Copy. Click on the CP option for any of your files or directories and you will be taken to the Copy File or Copy Directory page. On this page you will be prompted to enter the name of the new copy you are creating for the file or directory you selected. You are also able to change the path for your copy if you would like to place it in a directory other than your current working directory.
Moving a file in iManager is also quite simple. When iManager is running you will see the column titled Move. Click on the MV option for any of your entries and you will be taken to the Move (Rename) File or Move (Rename) Directory page.
This page is very similar to the Copy File or Copy Directory page. However, this page uses the mv Unix command which actually moves the file or directory to a new location or renames the file or directory.
The iManager also allows you to link files with symbolic links. Symbolic links enable you to have one file actually point to another file. For example, if you have a few files with different names that are all actually the same file, you can link them all to one file. This way if you need to change this file you can change it once and it will affect all the linked files.
For a more extensive understanding of symbolic links see "Creating Symbolic Links".
To link a file with iManager, click on the LN under the Link column. This will take you to the Create Link to File or Create Link to Directory page. At this point you will be prompted to enter the name and path of the file you want to link to the file you selected.
If you have a file called index.html and want to link a new file called welcome.html to it, select the LN next to your index.html file. On the Create Link to File page enter the Filename as welcome.html. Now when someone accesses welcome.html from the web it will actually give them the index.html file. If you edit your index.html file, the welcome.html file will show the changes.
iManager allows users to change the permissions of files from their browser. To change the permission of a file click on the CM next to the filename. If you are unsure about what file permissions you need for a file or directory then leave them alone.
You can use iManager to upload a file for you from your local computer to your VirtuaLand VirtualServer without the need of an FTP client. From the iManager main screen go to the Upload File to Current Working Directory section. In this section enter the Local Filename of the file located on your computer that you want to upload to your VirtuaLand VirtualServer. You can use the Browse button if you need to look around your computer to find the proper file.
Then enter the Remote Filename or name that you will want to call the file once it is loaded onto your server. You can also change the Remote Path if want to upload the file to a directory other than your current working directory.
When you have selected the correct file and correct remote filename and path, click on the Upload File button.
Within iManager you are able to add a new directory to your VirtuaLand VirtualServer under your current working directory. Simply go to the Make New Directory section of iManager and enter the new directory name. Click on the Make New Directory button and it will be created.
The iRoot plug-in will enable you to do the most common tasks associated with maintaining your server. These tasks include:
- Adding Email and FTP users
- Changing Email and FTP users' passwords
- Removing Email and FTP users
- Adding, Deleting, and Updating your Email aliases
- Changing your server Root password
- Creating a Digital Certificate for Allowing SSL (NetscapeTM compatible) Encryption on your server
- Installing a Digital Certificate on your server
To install iRoot, telnet to your VirtuaLand VirtualServer and perform the following steps:
- Make your home directory the current working directory, type "cd" at your telnet prompt and then hit the return key.
- Untar the iRoot archive file, type the following:
tar -xvf /usr/local/contrib/iroot.tar
You can now use the iRoot utility on your server by accessing the URL:
http://your_domain_name/cgi-bin/admin/gateway.cgi or
http://your_domain_name/cgi-bin/admin/vs/gateway.cgi
You can either open the URL with your browser directly, or link the URL to either text or an image on one (or more) of your web pages. The iManager utility will look for the existence of the iRoot plugin and if found will include an authentication form for the iRoot utility.
The iRoot plug-in is run from your web browser. Enter the following URL into your web browser substituting with your domain name:
http://www..com/cgi-bin/admin/vs/iroot.cgi
This page will prompt you for your root password. Simply enter your VirtuaLand VirtualServer root password and click on the Start iRoot button.
Once you have the iRoot plug-in running, follow these steps:
- Go to the vadduser Wizard - Username section.
- Enter a new username and click on the Next button.
- You will be taken to the Password screen. Enter the new users password and confirm then click on the Next button.
- At the Full Name screen enter the users full name and then click on the Next button.
- At the FTP Privileges screen, decide whether or not you want the user to have FTP privileges. If you choose No, click on the Next button to proceed on to the final screen to confirm the user settings you have chosen. If you want them to have FTP privileges, choose Yes and click on the Next button to proceed on to the Home Directory selection screen.
- At the Home Directory selection screen, decide which home directory you would like the user to have then click on the Next button.
- At the FTP Quota screen, choose the amount of your disk space you would like to allocate to this user then click on the Next button. This will take you to the final screen to confirm the user settings you have chosen.
While using the vadduser Wizard or any of the iRoot Wizards, you can use the Prev and Next buttons to navigate forward and back to make any changes you need to before you complete the final step.
Once you have the iRoot plug-in running, follow these steps:
- Go to the vpasswd Wizard - Username section.
- Select a username and click on the Next button.
- This will take you to the Password screen.
- Enter the users new password and confirm your entry. Then click on the Next button to take you to the Confirmation screen.
- If the settings are correct, click on the Change Password button to change the password. If they are not correct, click on the Cancel button to quit or the Prev button to change your entries.
Once you have the iRoot plug-in running, follow these steps:
- Go to the vrmuser Wizard - Username section and select a user to delete.
- Click on the Next button to go to the confirmation page.
- If the user is the one you want to delete, click on the Remove User button.
Once you have the iRoot plug-in running, follow these steps:
- Go to the Add Alias Wizard - Alias Name section to add an Email alias.
- Choose an alias name and click on the Next button to go to the Alias Type screen.
- Select a simple alias (see "Creating Email Aliases"), an include list (see "Creating Email Lists"), or an autoreply (see "Creating Email Autoresponders"). The rest of the setup is fairly easy to follow. If you have questions as to which alias you need or exactly how you need to implement the alias, include list, or autoreply, please reference the above sections of the manual.
- You will finally be prompted to click on the New Aliases button to run the vnewaliases command so your changes will take effect.
Once you have the iRoot plug-in running, follow these steps:
- Go to the Delete Alias Wizard - Alias Name section.
- Select an alias name to delete from the list and click on the Next button.
- This will take you to the confirmation screen to verify the alias to delete.
- Click on the Remove Alias button to remove your selection, or click on the Prev or Cancel button to start over.
- You will finally be prompted to click on the New Aliases button to run the vnewaliases command so your changes will take effect.
Once you have the iRoot plug-in running, follow these steps:
- Go to the vnewaliases Wizard section.
- Click on the New Aliases button to update your aliases file.
Once you have the iRoot plug-in running, follow these steps:
- Enter your current root password.
- Enter your choice for a new password and confirm it.
- Click on the Change Password button to update your password.
Once you have iRoot running, go to the Digital Certificate section to create and install a digital certificate for your server.
A digital certificate allows you to have secure home pages via SSL (Secure Sockets Layer) Encryption. This is especially important if you are asking customers to enter their credit card data into your on-line forms.
For more information on digital certificates and encrypted home pages send us an Email request.
The VirtuaLand VirtualServer system is a powerful system that allows you to do many things that you simply cannot with a virtual "host." This chapter is designed to introduce you to some commands and information that will help you make the most out of your VirtuaLand VirtualServer.
The quota command is used to help you identify how much disk space you have on your VirtuaLand VirtualServer, and of that space, how much you have already used. At a telnet command prompt, simply type quota.
yourserver: {1} % quota
The command will output your blocks, limit, files, and file limit.
- To see what your quota is, look at the limit column. A block is 1024 bytes. If for example, your limit says 51200, you would divide that number by 1024 to get your disk space in Megabytes. In this example, your total space would be 50 Meg.
- To see how much space you have used, look at the blocks column. Divide this number by 1024 to see how much space you have used. If for example, your blocks say 37851, you would be using roughly 37 Meg of your space.
- The files and file limit columns that display tell you the total number of files you can have on your system.
The vdiskuse command is useful in showing you where most of your disk space utilization is going. The command will return a listing of your directory structure along with the amount of disk space used in each directory.
To use the vdiskuse command, simply telnet to your server and type the following:
yourserver {1} % vdiskuse
If the output is too long to fit onto one screen, you can add the more command. The more command will show just one screen of information. When you hit the space bar the next screen will scroll into view.
yourserver {2} % vdiskuse | more
You can also have this report Emailed to you if you don't want to read it off your screen. Type this at the telnet command line and replace yourname@domian.com with your Email address:
yourserver {3}% vdiskuse | mail yourname@domain.com
Sometimes you will have files that are counted against your quota that you cannot see with the vdiskuse command. For example you will run quota and it will show you as using 25 Meg but vdiskuse will show you as using only 10 Meg.
This happens when you try to delete files that are opened for some reason. The most common cause of this is trying to delete your log files when they are actually being written to by the server. That is why it is so important to use the vnukelog command.
However, if this happens type the following at a telnet command prompt replacing with your login name:
yourserver {4}% ps -aux | grep
It will return a list of processes that might have one that looks similar to this:
4769 0.0 0.3 1436 196 ?? IW 16Oct96 0:00.06 httpd
Look for processes that are over a day old such as this one and kill them by process id. The process id is the first number listed after your username. In the above example the process id is 4769. This will free up the space the locked files were taking:
yourserver {5}% kill
Your access log files will likely be your largest user of disk space. It is wise to reset or nuke your log files when they get large. (For more information on your log files see "Monitoring Your Virtual Web Server Log Files" on page 18) You can do this by simply typing vnukelog at a telnet command prompt.
yourserver: {1} % vnukelog
Successfully deleted messages log.
Successfully deleted access_log.
Successfully deleted error_log.
Successfully deleted agent_log.
Successfully deleted referer_log.
When users have trouble reaching their VirtuaLand VirtualServer, or their connection time is slow, the traceroute command can be used to find out where the trouble is. This command can be issued from the users local machine running Microsoft's Windows 95, or from the VirtuaLand VirtualServer itself if you are able to telnet to the server.
The traceroute command follows the data route from the machine where you begin the route to the machine you route to. The route returns many different bits of information, but the most important thing to look for are asterisks ( * ). These indicate packet loss along the route. Packet loss along the route translates to slow, or completely inaccessible service.
If the packet loss occurs from your ISP computer systems, you can call them and ask why things are so slow. If it occurs somewhere in between your ISP and your VirtuaLand VirtualServer, you will need to wait it out or try another ISP that might have an alternate route. If the packet loss occurs within the VirtuaLand VirtualServers computers, send Email to us to see when it will be cleared up.
To run the command from your VirtuaLand VirtualServer, simply telnet in and type /usr/sbin/traceroute www.domain.com (substituting your ISP's domain name for www.domain.com). For example, if you were having trouble connecting from your service provider whose domain name was acmenet.com, you would type the following on your VirtuaLand VirtualServer:
yourserver: {1} % /usr/sbin/traceroute www.acmenet.com
If you were having trouble even logging in to your VirtuaLand VirtualServer, you can run a traceroute from your Windows 95 machine. At a DOS prompt you type tracert www.domain.com. You would substitute www.domain.com with the domain name of your VirtuaLand VirtualServer. For example, if your VirtuaLand VirtualServer domain name was acmenet.com, you would type the following:
C:\Windows>tracert www.acmenet.com
As a VirtuaLand VirtualServer reseller/client, you have access to a large array of programs that can really enhance the functionality of your VirtuaLand VirtualServer. These programs are all located in the "contributed" area, or /usr/local/contrib. This /usr/local/contrib is not a directory within your directory structure, but a directory of the host machine root from which you are able to copy programs.
The programs and utilities that are located in the /usr/local/contrib are not directly supported by our technical support since they were largely developed by third parties. Many of the contributed programs come with a large amount of documentation that should be reviewed and understood. Furthermore, many of the programs have license agreements that you should make yourself familiar with and agree to.
For the most current listing of programs available for you in the Contrib directory Email us. The following is a list of programs and their functionality at the time of this printing:
- Formmail.pl
This script is an easy way to handle form content and have it mailed to some specified address. The form can also be modified to send an auotmated response back to the remote client.
- PGP (Pretty Good Privacy)
PGP(tm) uses public-key encryption to protect E-mail and data files. PGP is well featured and fast, with sophisticated key management, digital signatures, data compression, and good ergonomic design.
- PGPformmail.pl
This script is an easy way to handle form content and have it mailed to some specified address encrypted with your PGP(tm) public key. The form can also be modified to send an automated response back to the remote client.
- Whois Gateway
A simple CGI to perform lookups on domain names.
- WWW Board 2.0
WWW Board is a web bulletin board message system. The script provides some administrative utilities to manage the bulletin board.
- WWW Count 2.2
A CGI program to keep record of the raw hits of a web page. It generates a GIF image of the number of hits and returns to the browser as an in-lined image.
- WWW Stat 1.0
An access statistics package with a web interface.
You will also find links to our free CGI library and Java Applet examples at this site.
You might find it necessary at one time or another to create symbolic links between files, or even directories. On the VirtuaLand VirtualServer system, we have created a symbolic link for the usr/local/etc/httpd directory. The link is simply www. From your home directory, you can type cd www and it will place you in the same directory as if you had typed cd usr/local/etc/httpd.
Links can be for your convenience in navigating around your site, or can be used to create a series of home pages that actually point to a different page. One of the most used links on the VirtuaLand VirtualServer system is the link to the index.html.
The default home page that comes up for each directory of your web server is the file named index.html. If you do not have an index.html file, then a directory listing will come up in the users web browser. However, some people do not want to call their default page index.html for various reasons. There are two ways around this. First, you can change the DirectoryIndex parameter in your srm.conf file (see Section 2.2.2 on page 10), or you can create a simple symbolic link.
To create the link, telnet to your server and go to the directory where your files are located. Assuming you have a file called welcome.htm and you want it to be the default home page for that directory, type this at the command prompt:
yourserver: {1} % ln -s welcome.htm index.html
This will establish a link that points index.html to welcome.htm.
Unlike a traditional introduction to Unix, the emphasis of this one is on philosophy and brevity. When you understand how the creators of Unix intended you to use it, you'll approach Unix on it's "best side". This introduction intends to help a new Unix user get started on the right foot quickly. For more information, readers are referred to the Unix manuals and other listed references. As little detail as possible has been duplicated from the manual.
In some ways, Unix is "old technology" - it was invented in the late 1960's for a small computer with a 64K-byte address space, it is largely character oriented (not graphic). Why is it still here? Why is it spreading to more and more systems from PC's to Cray Supercomputers? One answer is that Unix is written in a mostly machine independent way (in the high level language "C") and is therefore more easily moved to new machines. Once Unix has moved, a large base of applications also moves easily and your investment in learning Unix continues to pay off. Another answer is that many problems are still character oriented (or at least can be approached that way) and for these problems, like a sharp tool in the hands of a skilled user, Unix really helps you get your work done. Also, you can use Unix from any kind of terminal and over dial-up phone lines or computer network connections.
In the space below, I hope to convey, with a minimum of specific information, the essence of "The Unix Philosophy" so that you can use and enjoy Unix at its best. To try to summarize in just two sentences (for those who really believe in such brevity): Unix comes with a rich set of connectable tools which, even if they don't directly address the problem at hand, can be conveniently composed (using the programmability of the command interpreter) into a solution. Unix also imposes relatively few arbitrary limits and assumptions on the user or the problem domain and has thereby proven to be a suitable platform on which to build many useful and highly portable research and commercial applications.
Before I can realistically hope to say more about Unix in general, or give meaningful examples, I must briefly explain some Unix commands and concepts. These descriptions are intentionally minimal. You will soon see how to find more detail in the manuals.
Unix is a multi-user operating system. This means that several users can share the computer simultaneously. To protect each user's data from damage by other users, Unix requires each user "login" to the system to identify him/herself (with a login name) and authenticate him/herself (with a password). During the login process, a user's defaults and "terminal type" are usually established. The mechanism Unix uses to allow concurrent users also allows each user to have more than one program (also called "process" or "commands") running concurrently. You will see shortly how convenient this is.
Once you have logged in, you will be running a program called your "login shell". The shell is a program which executes the commands you type in and prompts you when it is ready for input. One of the nice features of the Unix shell is that it is a powerful programming language unto itself, however one need not program it to use Unix. There are several different "shell" programs in common use: csh (c-shell), sh (bourne-shell), ksh (korn-shell), vsh (visual-shell) to name a few. Most people use "csh".
Unix commands consist of a program name followed by options (or arguments) to that program (if any). One or more spaces follow the program name and separate arguments. Each program examines its argument list and modifies its behavior accordingly. By convention, arguments which begin with a dash are called "switches" or "flags" and they are used to request various non-default program behavior or to introduce other arguments. It is occasionally important to remember that it is the shell which does filename expansion (such as turning "*.old" into "a.old list.old program.old"). Programs normally don't ever see unexpanded argument lists. Many Unix programs can also take implicit arguments. These are available (to every program you run) via the "environment". Your "terminal type", stored in an environment variable called TERM, is an example of this. The manual for each program you use should list the environment vari-ables it examines and the manual for your shell explains environment variables in detail.
Before getting into any specific commands and examples, note that most Unix systems have both on-line and printed manuals. Many commands will be mentioned below in passing without explanation. It is assumed that the interested reader will look them up in the manual.
The on-line manuals generally contain only the numbered sections of the printed manuals. The tutorials and in-depth articles are usually only in printed form. This introduction intends to reproduce as little of the information contained in the Unix manuals as possible. For more information on any Unix command, type "man command" ("man man", for example gets you "the manpage" for the on-line manual command: man). (Note: if you are prompted with the word "more", you are interacting with the "more" program. Three quick things to know: you may type a space to get the next screenful, the letter "q" to quit, or "?" for a help screen.)
Among other things, the man-page for the "man" command points out that "man -k word" will list the summary line of all on-line man-pages in which the keyword: word is present. For example, "man -k sort", will produce something like this:
comm (1) - select or reject lines common to two sorted files
look (1) - find lines in a sorted list
qsort (3) - quicker sort
qsort (3F) - quick sort
scandir, alphasort (3)- scan a directory
sort (1) - sort or merge files
sortbib (1) - sort bibliographic database
tsort (1) - topological sort
This tells you that section 1 (user commands) of the manual has man-pages for comm, look, sort, sortbib, tsort. Use the man command on any of these to learn more. The other numbered sections of the Unix manual are for system calls, subroutines, file formats, etc. You can find out about each section of the manual by saying, for example, "man 2 intro". Enough about manuals.
By convention, whenever possible, Unix programs don't explicitly specify from-where to read input or to-where to write output. Instead, programs usually read from "standard input" (stdin for short) and write to "standard output" (stdout). By default, standard input is the keyboard you logged in on and standard output is the associated display, however, the shell allows you to re-direct the standard output of one program either to a "file" or to the standard input of another. Standard input can be similarly redirected. Perhaps Unix's greatest success comes from the ability to combine programs easily (by joining their standard inputs and outputs together forming a pipeline) to solve poten-
tially complex problems.
"Standard error" (stderr) is not usually re-directed, hence programs which write warnings, prompts, errors, etc. to stderr will write them to the display even when normal input and output is usefully re-directed. (Note that since I/O devices are implemented as files on Unix, I/O re-direction also works to and from physical devices.) The syntax for I/O re-direction is fully described in the manual for the shell you are using (probably csh).
The following are some simple examples of I/O re-direction. For clarity, the shell's ready-for-input-prompt has been shown as "Ready%" and explanations have been inserted in italics. Everything the user would type is shown in slightly bold type after the Ready% prompt.
Running the "date" command prints today's date and time on standard output
Ready% date
Wed Mar 22 13:06:30 PST 1989
Ready%
Put the standard output from the date command in a file called "myfile"
Ready% date > myfile
Ready%
Use the word-count program to count the number of lines, words, characters in "myfile"
Ready% wc < myfile
1 6 29
Ready%
Pipe the output of the date command directly into the word count command. Note that commands in a pipeline such as this can run simultaneously.
Ready% date | wc
1 6 29
Ready%
Use output from one program as command line arguments to another
Ready% echo My computer, `hostname`, thinks today is `date`
My computer, samburu, thinks today is Wed Mar 22 13:06:30 PST 1989
Ready%
Look in the on-line dictionary for words beginning with "pe" and count how many are found
Ready% look pe | wc
294 294 2548
Ready%
Pipe those 294 lines through cat -n to insert line numbers and then through sed to select only lines 5-8
Ready% look pe | cat -n | sed -n 5,8p
5 peaceful
6 peacemake
7 peacetime
8 peach
Ready%
Now, from those 294 words, select only those containing "va" somewhere and re-direct them into the argument list of the echo command
Ready% echo I found these: `look pe | grep va`.
I found these: Pennsylvania Percival pervade pervasion pervasive.
Ready%
Grep (search) through all files with names ending in ".c" for lines beginning with "#define". (Grep -l lists the file names containing the lines which match instead of the lines themselves). These file names are redirected to form the command line of the vi editor - hence, edit all ".c" files which contain "define" statements.
Ready% vi `grep ^#define *.c`
The depiction of an interactive session with the "vi" editor is omitted.
Ready%
When a program reads from a file or from a pipe it can tell when there is no more to read. This condition is called reading the "end-of-file" or EOF. When standard input is a terminal, the EOF must be explicitly typed because the program must otherwise assume you are still typing. Normally EOF is typed as a CONTROL-D (indicated in print as ^D). Think of the control key as another SHIFT key - it must be pressed and held when the D is typed. If the EOF is not the first thing on a line, two must be typed.
If you are running a program and you wish to interrupt it completely, you can often do so by typing ^C. You can try this with the "wc" program (run wc then interrupt it):
Ready% wc
sample input
^C
Ready%
run wc then type EOF
Ready% wc
sample input
^D
1 2 13
Ready%
Note that both ^D and ^C ended the program however, ^D allowed the program to finish normally but ^C killed it (and produced no output). If, for some reason, you want to type a special character such as ^C and actually have it sent to your program and not generate an interrupt, you can "quote it" by typing a backslash (or sometimes a ^V) before it. The backslash also "quotes" shell "meta-characters" such as asterisk, question mark, double-quote, backslash, etc.
"Job control" is the name given to an extremely convenient feature of many modern versions of Unix. Job control allows one to suspend a program and resume it later. If you are in the middle of running some program when the phone rings, you can type ^Z to suspend the program (and get back to your shell prompt) without interrupting or exiting that program. After you handle the phone call, you can type "fg" to resume the original program right where you left off. Unix permits one to have a fairly large number of suspended jobs and to resume them in any order. Csh's "jobs" command displays which jobs are stopped. (In some ways, job control is "a poor man's window system"; however, even on Unix systems with windows, many people find job control indispensable.) For more information on job control, see the "csh" man-page.
Unix files exist in directories. Every user has a "home directory", which is the "current directory" after logging in. A user can make "sub directories" with the "mkdir", command and make them the current directory with the "cd" command. You can print your current directory with the "pwd" command and you can refer to the parent directory as ".." (two dots). You can get back to your home directory by typing "cd" with no arguments.
Files and directories have permissions called "modes" which determine whether you, "your group", or everyone can: read, write, or execute the file. Permissions are changed with the "chmod" command. The main reason for bringing this up now is to point out that a collection of commands which can be typed to the shell can also be put in a file, given a name, made executable and subsequently invoked as a new command by that name. This type of file is called a "shell script" and is one of the main ways Unix is customized to the work habits and chores of its users.
When a user types a command, s/he usually doesn't type the full (and unambiguous) path name of the program: (/bin/date for example) but instead types only the last component of the path name, date, thus requesting the system to search for it. To achieve predictability and efficiency, the system searches only those directories listed in your PATH environment variable and it searches them in that order. By placing your own version of a program in a directory you search before the system directories, you can override a system command with your own version of it.
Your version can be anything from an entirely different program to a simple shell script which supplies some arguments you always use and then calls the standard version. The command "echo $PATH" will print the value of the PATH environment variable to stdout. The procedure for setting environment variables such as PATH differs from shell to shell. See the man-page for the shell you use.
Well, so much for the nitty-gritty. I will now try to explain "The Unix Philosophy" in a bit more detail. Basically, the idea is that rather than have a custom program for each little thing you want to do, Unix has a collection of useful tools each of which does a specific job (and does it well). To get a job done, one combines the pieces either on the command line or in a shell script. For example, on Unix, a user would not expect an application to provide an input text editor. Instead, one would expect to be able to use one's favorite (and standard) "text editor" (probably "vi", perhaps "emacs") for all instances of editing text. Electronic mail, C programs, shell scripts, documents-to-typeset can all be edited with the same text editor. By convention, applications invoke the text editor you have specified in your EDITOR environment variable.
Even though Unix editors are generally very powerful and capable programs, they too recognize that they are just tools and they allow you to pipe all or part of your "editor buffer" through any pipeline of Unix commands in order to do something special for which there isn't a built-in editor command. (The editor buffer is that private copy of your file to which the editor makes changes before you save them.)
Unlike most other operating systems, Unix has only one "file type". Any program which can read or write standard I/O can read/write any "file" (even if it is a device such as a terminal, printer or disk). Granted, not every program can make sense out of the data in every file, however, that is strictly between the program and the data - nothing imposed by Unix. The single file-type contributes greatly to the modular/re-usable pipes-and-filters approach to problem solving.
So, what is to be learned from all this? Just that it is good to construct solutions to your problems in as general and modular a fashion as possible. You will undoubtedly find that a somewhat general program (or shell script) you wrote as part of the solution to one problem will be just what you need as part of the solution to some future problem and it will be simple to hook up.
Let's assume the following problem, inspired by a real-world situation. You are a professor of English and someone walks into your office with an old manuscript claiming it is an undiscovered work by Shakespeare. You postulate (correctly) that you can use statistics about frequency of word usage to help determine its authenticity. The problem, therefore, is to come up with a histogram (count) of the number of times each word is used.
You could, of course, write a program from scratch in C or FORTRAN to do it, however a partial solution comes to mind using "awk", a programmable text processing tool which has 2 particularly useful features: 1) lines are read and processed automatically; 2) arrays can have text-string subscripts. So, if you hadn't already written a "histogram" shell script, you write one now. (Keep it around, you will find a use for it again.) The file "histogram" has the following contents (de-mystified somewhat below):
awk `
NF > 0 { counts[$0] = counts[$0] + 1; }
END { for (word in counts) print counts[word], word; }
`
For each line with NF > 0 (NF is awk-talk for number-of-fields-on-this-line, hence for each non-empty line), add 1 to that particular counter hereby associated with the-text-on-this-line ($0 is awk-talk for the-text-on-this-line). Then, at the END of input, for each unique input line; print that line preceded by the count of how many times it was seen.
So, now the task is simply getting the input into a format where all punctuation marks are removed and each word appears on a line by itself. Again, you could write a program to do it; you could manually reformat the text with an editor; or you could notice that Unix has a translate command "tr" which will do just what you want when used in two steps as shown:
tr -dc "a-zA-Z' \012" | tr " " "\012"
The first "tr" command has options -dc (delete the complement of the indicated characters) so it will delete from standard input all characters except those which are listed (letters, apostrophe, space, and octal 012 (newline)). The resulting output has no punctuation. The second "tr" translates all spaces into newlines, thus causing at most one word to be on each line.
Piping the output of these two commands into "histogram" will give us word counts. Piping the output of histogram into "sort -n" will sort the histogram in numerical order. Putting the whole thing in a file and making it executable makes it available as conveniently as if it had been built into Unix.
Here then is some sample input and the output our script produces:
One black bug bled blue black blood
while another blue bug bled black.
And the output of tr ... | tr ... | histogram | sort:
1 One
1 another
1 blood
1 while
2 bled
2 blue
2 bug
3 black
Note that other simple solutions to the problem exist. Our awk-based histogram program can be replaced by "sort | uniq -c" (but that is less intuitive than the awk solution and not necessarily any better). Also, "sed" could have been used in place of either or both of the "tr" commands. (Sed is much more powerful than tr however the sed command line would have been less intuitive.)
Probably the two biggest advantages of concurrent execution of commands in a pipeline are: 1) No disk space is required for intermediate data which flows through pipelines. 2) output can start coming out the end of the pipeline before the entire input is processed through the first program in the pipeline.
For example, imagine you want to compute a histogram for a very large file which is compressed and your disk is too full to hold the uncompressed version. You can uncompress it to standard output and pipe that directly into your histogram pipeline.
Now imagine you have a pipeline which takes 30 minutes to compute and produces data which takes 30 minutes to print. If you first computed and then printed, it would take 60 minutes. If you re-direct the output of the pipeline to the printer, the whole process only takes 30 minutes. (Note: you can output directly to a device such as a printer but in a multi-user environment the normal printing mechanism is to spool the output in a file (with "lpr") and print it after the computation finishes.)
On Unix you can run any number of programs "in the background", which means that the shell doesn't wait for them to finish before giving you a new prompt. Read more about this in the manual for your shell.
You can also have programs started for you automatically at certain times of the day, week, month, etc. (read about "at" and "cron") or when certain events happen, such as when electronic mail arrives.
Since it is not the intent to duplicate information from the Unix manual in this introduction I won't give many details about the following programs, however, I would like to point them out so you can look them up in the manual if you are interested. Most manual pages have a "SEE ALSO" section at the end. Consider yourself invited to read up on those programs as well. (If you really want to know everything, look up every program in every directory in your $PATH!)
- learn An interactive tutorial on a few subjects. (Not available on all systems). Probably most useful for learning the "vi" editor. Type "learn vi" to try it.
- vi, emacs, ex, ed, pico "vi" is the most common Unix screen-oriented text editor. Emacs can be another good choice. ("ed", the original Unix text editor is essentially subsumed by vi and is much harder to use.) "ex" is really just vi in a non-screen-oriented (ed-like) mode. "pico" is a very easy to use but not very powerful screen-oriented editor. There are substantial printed manuals on vi, ex and emacs. Whichever editor you choose, you will eventually want to read everything there is to know about it. Unix editors are very powerful and knowing how to use that power really helps a lot.
- rm, mv, cp, rmdir Remove; move (rename); copy a file; remove a directory.
- ls List directories. More options than just about any other program. Filenames which begin with dot are not listed unless the -a option is used.
- stty, tset Set such aspects of terminal I/O as: number of lines on display device, input character- or line-at-a-time; whether keyboard typing is visible.
- cat Concatenate files to standard output.
- more, less, page, pr Display data a screen or page at a time. Search and skip forward to a page of interest.
- cmp, comm, diff, diffmk Show differences between two files.
- grep Find lines which match specified pattern (incredibly useful).
- rlogin, rsh, rcp Login to remote Unix system, run a command on remote Unix system, copy a file to remote Unix system. Similar to below.
- telnet, ftp Connect to remote system of arbitrary type, copy a file.
- talk, mail, mh, mm Connect your terminal to another user for interactive communication. Send or read electronic mail.
- crypt, pgp Encrypt or decrypt data.
- compress Compress data or files, typical compressions are 2-3 to 1.
- tar, cpio Archive and restore files and directories into/from a single file on disk or removable media.
- sed Probably the single most useful command for rearranging or extracting pieces of data quickly. (A bit cryptic for many users, though.)
- awk More powerful than sed but somewhat slower; almost a general purpose programming language but definitely tailored to filtering text from stdin to stdout.
- head, tail First, last part of a file or stdin.
- find Locate files which meet specified criteria.
- look, spell Look up words in an on-line dictionary. Find spelling errors.
- sum Compute a CRC (checksum) for comparison with supposedly identical data on a remote system.
- od Display an octal (or hex) dump of input data. This lets you see every byte of your data as a bunch of numbers.
- du, df Display disk usage and free disk space.
- script Keep a transcript of your session in a file.
- who, whoami, su Who is on the system. What is my username? Become another user temporarily.
- ps, kill Process-status lists attributes and resources associated with each process. Kill sends to a process a "signal" which (depending upon the signal sent) will cause the process to terminate in various ways. See also the man-page for "signal".
- 4.4BSD Unix Manuals, U.C. Berkeley, published by O'Reilly & Associates, Inc. 103 Morris Street, Suite A, Sebastopol, CA 95472
- The Unix Programming Environment, Kernighan, B.W. and R. Pike, Prentice Hall, Engelwood Cliffs, N.J.
- Welcome to Unix, Rick Ells, Academic Computing Services, University of Washington, Seattle, Washington.
- Introducing the Unix System, Henry McGilton, McGraw-Hill Software Series.
- The C Programming Language, Kernighan, B.W. and Ritchie, D.M., Prentice Hall, Englewood Cliffs, N.J.
- Introducing Unix System V, Morgan, R., McGilton, H., McGraw-Hill Software Series.
- Unix for People, Birns, P., Brown, P., Muster, J.C.C, Prentice Hall, Englewood Cliffs, N.J.
- Unix for Dummies, John R. Levine and Margaret Levine Young, IDG Books Worldwide, Inc., Foster City, CA.
- Advanced Unix Programming, Marc J. Rochkind, Prentice-Hall, Englewood Cliffs, N.J.
Footnotes
- (1)
- See ftp://nic.merit.edu
- (2)
- See http://tig.com/IBC/statistics.html and http://www.umich.edu/~sgupta/conres.html
- (3)
- Significately less than $20,000 if you use a Virtual Internet Server from ISI.
- (4)
- Dr. Dobb's Developer Update, Volume 2 Number 5 May 1995
- (5)
- This is not a complete list of web registration sites but will give you a good start.
- (6)
- Prices vary with ISP and area.
- (7)
- As the old UNIX saying goes: "UNIX gives you enough rope to hang yourself plus a couple of extra feet."
- (8)
- See http://hoohoo.ncsa.uiuc.edu/docs/Overview.html for more information about NCSA's httpd server.
- (9)
- The imagemap CGI program has a small bug that requires the last line in imagemap.conf to have a line return at the end of the line. Therefore, when you are editing imagemap.conf be sure to hit the "enter" key when you are done with the last line.
- (10)
- Getstats is written by Kevin Hughes (kevinh@eit.com). See http://www.eit.com/software/getstats/getstats.html for more information.
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TABLE 1. Distribution of Internet Users
