For our first shell script, we'll just write a script which says
"Hello World". We will then try to get more out of a Hello World
program than any other tutorial you've ever read :-)
Use a simple text editor (not a word processor) to create a plain text file named
first.sh with this content (or download it from the link below):
#!/bin/sh # This is a comment! echo Hello World # This is a comment, too!
The first line tells Unix that the file is to be executed by
This is the standard location of a Bourne-compatible shell on just about every
Unix/Linux system. If you're using GNU/Linux,
/bin/sh is normally a symbolic link to the
bash shell, or possibly
dash. We'll look at the
#! part in a moment.
The second line begins with a special symbol:
#. This marks
the line as a comment, and it is ignored completely by the shell.
The only exception is when the very first line of the file starts with
#! ... as ours does. This is a special directive
which Unix treats specially. It means that even if you are using csh,
ksh, or anything else as your interactive shell, that what follows
should be interpreted by the Bourne shell.
Similarly, a Perl script may start with the line
#!/usr/bin/perl to tell your interactive shell that the program which follows should be executed
by perl. For Bourne shell programming, we shall stick to
The third line runs a command:
echo, with two parameters, or
arguments - the first is
Hello; the second is
echo will automatically put a single space between its parameters.
# symbol on line 3 still marks a comment; the
# and anything
following it (up to the end of the line) is ignored by the shell.
chmod 755 first.sh to make the text file executable,
and then run
Your screen should then look like this:
$ chmod 755 first.sh $ ./first.sh Hello World $
You will probably have expected that! You could even just run:
$ echo Hello World Hello World $
But now, let's make a few changes.
The best way to learn about anything is to change it, poke it, prod it and see what happens.
First, note that
echo puts ONE space between its parameters.
Put a few spaces between
"World". What do you
expect the output to be? What about putting a TAB character between them?
As always with shell programming, try it and see.
$ echo Hello World Hello World $
The output is exactly the same! We are calling the
with two arguments; it doesn't care any more than
about the gaps in between them.
Now modify the code again:
#!/bin/sh # This is a comment! echo "Hello World" # This is a comment, too!
This time it works. You probably expected that, too, if you have experience
of other programming languages. But the key to understanding what is going
on with more complex command and shell script, is to understand and be able
to explain: WHY?
echo has now been called with just ONE argument - the string
"Hello World". It prints this out exactly.
The point to understand here is that the shell parses the arguments BEFORE passing them on to the program being called. In this case, it strips the quotes but passes the string as one argument.
As a final example, type in the following script. Try to predict the outcome before you run it:
#!/bin/sh # This is a comment! echo "Hello World" # This is a comment, too! echo "Hello World" echo "Hello * World" echo Hello * World echo Hello World echo "Hello" World echo Hello " " World echo "Hello "*" World" echo `hello` world echo 'hello' world
Is everything as you expected? If not, don't worry! These are just some
of the things we will be covering in this tutorial ... and yes, we will
be using more powerful commands than