More Unix

Every person or user accessing a Unix server is running a completely separate user session.

A shell program provides the user interface you see - i.e. the prompt. The shell basically enables you to run programs on demand. Common shell programs are the 'Bourne', 'C', or 'BASH' shells.

There are a number of settings that can be individually set for each user - and these settings are used by the shell to manage your interaction together. These settings are held in environment variables.

For example, there will be environment variables defining what type of terminal you're using, how big it is, what your username is and where your home directory is.

All environment variables start with the dollar sign '$'. $HOME is your home directory. if you type
echo 'Home dir=' $HOME you'll get a response like:

 Home dir=/user/yourname

Try the following commands:

• env
  
  Displays a list of all your environment variables.
  
  
• cat .bashhistory
  
  Show the history of what you've typed. Note, you can access this list through 'hot keys' to save typing. (Usually the up and down arrows on the keyboard)
  
  
• cat .bashrc
  
  Shows your login script - this script is run every time you open a terminal window.
  

All the Unix commands are files too. They live in a directory like '/bin' or '/usr/bin' on the machine. The reason you can access all the Unix commands from any directory is simply because a search path is set up...

A search path is an environment variable that tells the Unix shell where to look for programs. When you type a command, the shell looks in all the directories listed in the $PATH environment variable. You can type echo $PATH to see how the path is set up for your session.

You can (of course) alter the contents of environment variable. (That why they're called 'variables'.) You could, for example, extend the search path to include an extra directory.

INFO: You can create your own unix commands either by creating aliases (i.e. alias ll='ls -al';) which you'd place in a startup file like .bashrc ... ...or by writing small executable files and placing them in the search path: Use the cat command to open a new file. Type the unix commands which have the desired effect. Close the file. Make it executable using the chmod command Then you can run it like any other command.

Unix comes with a number of extremely powerful tools (Note, there are man pages for them all):

• All Unixes come with a 'C' compiler. C is a powerful general-purpose programming language. Most of Unix itself is written in C.
  
  
• Perl is a programming language that's widely used on the Internet. Web servers
  
  
• 'awk', and 'sed' are powerful file manipulation programs. If you needed to format a thousand pages of text, or automatically strip the email address from every email you're sent and store it, you could use one of these programs.
  
  
• 'grep' is a program that performs pattern matching. You'd use grep to find a particular bit of text in a file.
  
  
• Much of the above 3 programs power comes from their ability to handle regular expressions. This is a language that can describe generalised patterns of characters. For an explanation of what they are, and how they work, see the resources section for links.
  
  
• Unix has 2 built in editors, 'ed' and 'vi'. ed is a 'line editor', and its operation is so obscure by today's standards, hardly anyone uses it.
  
  

  vi, on the other hand, while still very strange if you're used to windows, is very powerful, quite easy to use - once you get the hang of it, and has a huge user base. It's also the only reasonable editor you'll get on a character-based system.

You never really need to, but it's often desirable to edit files on your webserver directly. Files like system settings, .qmail files, Perl scripts etc. are sometimes better edited on site...

Consider debugging a Perl script... The debugging cycle is: edit, save then run. If you have to upload as well, the cycle becomes: edit, save, upload, change file permissions (chmod), run. These extra two steps slow the process down a lot!

INFO: All your web pages should be edited on your PC, then uploaded to your webserver. This way you always have the most recent versions in one place - on your PC.

There are 3 ways to edit files that are on your webserver:

1. Edit on PC and Upload.
2. Edit on server using vi.
3. Edit on server using a remote editor - there's only 1 I know of: Direct FTP from CoffeeCup.com.
   
   

vi is an editor that inspires love and hate amongst its users. I recommend you read this article: The VI Story and try it on your own webserver.

If after this time, you don't want to use it, then try one of the alternatives.