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How to Create and Run a Script
Why are we starting with shell script programming? The reason is that they
are easy. Honest, they *are* easy. So easy, there are several ways to make
them.
First, let's walk though the Pico way to create a simple script.
1) Open an editor program. We'll use the easiest one: Pico. At the prompt in
your shell account, simply type in "pico hackphile." ("Hackfile" will be the
name of the script you will create. If you don't like that name, open Pico
with the name you like, for example "pico myfilename.")
 This brings up a screen that looks a lot like the Pine email program's
"compose mail" screen.
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Evil genius tip: If your shell account is half-way decent, you will have
Pine and it will allow you to choose whatever editor you want for composing
email. Default is Pico. But you may configure it to use other editors such
as the far more powerful vi or emacs. Just go to the main menu on Pine, then
to Setup, then to Configure, then scroll down almost to the end o f all the
options. There will be a line "editor = pico." Put in your favorite editor!
If you regularly use Pine to compose email, you will keep in practice by
using its editor, making it much easier to write programs.
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Here's what your Pico screen should look like:
UW PICO(tm) 2.9 File: hackphile
[ New file ]
^G Get Help ^O WriteOut ^R Read File ^Y Prev Pg ^K Cut Text ^C Cur Pos
^X Exit ^J Justify ^W Where is ^V Next Pg ^U UnCut Text^T To Spell
At the bottom is some fast help, a list of commonly used Pico commands.
That "^" thingy means to hold down the control key while hitting the letter
of the alphabet that follows. Besides these commands, some others that it
helps to know for Pico are:
^e moves the cursor to the end of a line
^a moves the cursor to the beginning of a line
^d deletes a character
^f moves the cursor forward (or use the -> arrow key if it wo rks)
^b moves the cursor backward (or use the
^p moves the cursor up (or use the up arrow key if it works)
^n moves the cursor down (or use the down arrow key if it works)
^t checks spelling
2) Write in some Unix commands. Here are some fun ones:
echo I am a programmer and one heck of a hacker!
echo Today I am going to
echo $1 $2 $3 $4 $5 $6 $7 $8 $9
3) Now exit Pico. Hold down the control key while pressing "x." Pico will
ask you if you want to save the file. Hit the "y" key to save. It will ask
you whether you want to save it with the name "hackphile." Unless your
change your mind, just hit the "enter" key and you are done.
4) Next make it executable. On most systems, you can do this by typing
"chmod 700 hackphile." On some computers the command "chmod +x hackphile"
will work. On other computers you might have to write a line in your shell
script "#!/bin/bash" (or "#!/bin/tcsh" or "#!/bin/csh" etc. depending on the
path to whatever shell you are using) to make it work. Sorry to be so
complicated on this instruction, but there are a lot of different kinds of
Unix and Unix shells out there. Groan.
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Newbie note: That "chmod" command sets permissions. Making a file executable
is only one of the many things that magical command does. It also controls
who can execute it, who can read it, and who can write it.
Damian Bates of Rt66 Internet points out that you could set the permissions
so only you could execute that shell script by typing "chmod u+rx filename"
(u=you). If you are in a Unix "group," you could allow your group to execute
it by typing "chmod g+rx filename" (g=group) or you could give everyone else
execute permissions by typing "chmod o+rx filename" (o=other). Any of these
can be done in combination such as "chmod ug+rx filename (user and group can
read and execute but not write) or "chmod g-rwx filename"
If you hate typing all that stuff, you can use numbers as in "chmod 700,"
which gives you, and only you read, write and execute permission. To add
permission to read and execute, but not write, to everyone else, use "chmod
755." To learn more on how to use the number chmod commands, use the command
"man chmod."
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5) Now type in: "hackphile forge email from Santa Claus." Press "enter" and
you will see on your screen: "I am a programmer and one heck of a hacker!
Today I am going to forge email from Santa Claus."
Pretty cool, huh? What that last echo command does is find the first word
you typed after the "hackphile" command, which is held in the memory
location $1, the second word in $2, and so on. Unlike more sophisticated
programming languages, you don't need to set up those dollar sign variables
in advance -- the stuff you type on the command line after the name of the
script automatically goes into those memory locations!
Now suppose you want a script to actually forge email from Santa Claus.
Unfortunately, this is where you learn the limitations of shell scripts. You
can put in the command "telnet foobar.com 25" and be ready to forge email.
But if the next command in your shell script is "mail from:
santa@north.pole.com," it just won't happen. The problem is that you are no
longer in your Unix shell. You now are running a mail program on foobar.com,
which does not bring up the rest in your sequence of shell commands.
But help is on the way. The programming languages of Perl and C will do the
job for you much more easily than a shell script. More on these in later
Guides, I promise!
How about more fun ways to make shell scripts?
Shell Scripts on the Fly
In a rush? Do you always do things perfectly? If so, try the "cat" command
to create shell scripts.
Here's an example of a useful one. Type in:
cat > list
ls -alK|more
w|more
Then hold down the control key while hitting the letter "d." This will
automatically end the "cat" command while saving the commands "ls -alK|more"
and "w|more" in the file "list." Then make it executable with the command:
"chmod 700 list." (If chmod 700 doesn't work on your system, try the
alternative ways to make it executable in 4) above.)
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