Shell scripts and .profiles

• Aliases are very useful, but only if you want to do the same thing many times. (Actually aliases are slightly more flexible than this, but only slightly.) Suppose that you often copy files from the directory /usr/fs/sbj/public to your current working directory; that means that you often type commands like:

   
       cp /usr/fs/sbj/public/xxx xxx

  where xxx is something different each time. You can place a template for this command in a file and then ask ksh to execute the template using a particular file name for xxx each time. This kind of file is called a shell script.

  Use vi (or your favourite editor) to create a file called, say, cpp, and in it put the line

  
         cp /usr/fs/sbj/public/$1 $1

  ($1 stands for ``the first parameter''). You must inform ksh that cpp is something that it can execute by changing its mode using the changemode command

  
         chmod u+x cpp

  (This gives (+) the owner (u, for `user') execute (x) permission for the file cpp.)

  Now if you type the command

  
         cpp boxes.p

  then ksh will execute

  
         cp /usr/fs/sbj/public/boxes.p boxes.p

• Shell scripts can contain sequences of commands rather than just one; in fact ksh has its own programming language for building scripts - see the manual entry. Any command that you can type at the prompt is valid inside a script, and, similarly, any script command is valid at the prompt; ksh makes no significant distinction between keyboard and script: both are places to read commands from.

• Since the script is in a file, it is a permanent feature - until you remove the file.

• You have seen a little about creating shell scripts to simplify some activities. Unix relies heavily on shell scripts for various purposes.

  In particular, you have a shell script in your home directory called .profile (note: you need to use ls -a to see files that have names starting with . in a directory listing). .profile is executed automatically each time that you open a new xterm window, to initialize the ksh that runs in that window. It is usual to include shell commands in .profile to set up configuration information and perform useful actions (such as checking for mail or new bulletins). If there is anything that you need to have done in each window, then you can put appropriate commands in your .profile.

• Try adding the following ksh commands to .profile as an experiment (you can change it later if you don't like it):

  
         echo Greetings. Your every wish is my command ...
         PS1='Well, what now? '
         alias lspas='ls *.p'

  Now open a new window (option in root window mouse menu). You are greeted, the command prompt has changed to a rather impatient sounding question, and the alias lspas has been set up.

  echo is a built in ksh command.

  PS1 is called a shell variable - see next.

• The shell variable PS1 is one of a large collection that are used by ksh and other programs to control your working environment. Some have numerical values, and some have string values (like PS1).

  ksh uses the current value of PS1 as the command prompt; you can change it any time you want by typing a command like the one in the .profile (i.e. PS1=...) - but note that the change applies only in the current window.

  If you type the command

  
       set

  then ksh will give you a listing of all the shell variables that are currently set, together with their current values.

This is the end of the useful tips about Unix - the tips have covered a variety of the commonly used facilities, and hopefully they have given some pointers to the other facilities on offer. If you want to know more about using ksh, then read the manual pages produced by the command man ksh - but this is definitely not to be recommended for the faint hearted, because it is very long and quite indigestible!