Let’s start with a few elementary scripts to show that script writing isn’t difficult, and to illustrate some syntax and constructs.
Suppose that we have an editor such as jed which retains a copy of the previous version of a file by
to the filename. If at some time we realise that we really want the former version and to erase the new
one we could have a script called
restore that would perform the operation for an arbitrary file. It
might look something like this.
$1represents the first argument supplied on the command line. (There is more on this in Section 7.6.) If the command
restoreruns the script, then entering
splat.fand replace it with
The above script makes some assumptions. First it does not check whether or not the files exist. Let’s correct that.
Here we have introduced the if...then...endif construct. When the expression inside the parentheses is
true, the following statements are executed until there is an else or endif statement. In this case
&& -e $1 is true only when both files exist. The
-e is one of eight file operators for such things as file
access and type, and
&& is a logical AND.
The second assumption in
restore is that you’ll always remember to supply a file name.
Error: no file name suppliedif you forget the argument.
$#argvis the number of arguments supplied on the command line. Also notice the else if construct. The
==tests for equality.
Instead of an error message you could make the script prompt for a filename.
When the number of arguments is zero, this script issues the prompt
The file to restore
echo -n prevents a skip to a new line, and so allows you to enter your response
after the prompt string.
set file $< equates your response to a shell variable called
If on the other hand you gave the filename on the command line
set file = $1 assigns
that name to variable
file. When we come to use the variable in other commands it is
prefixed with a dollar, meaning “the value of the variable”. Finally, the script now tells you
which file or files are missing. Notice how the variable value can be included in the error
For your private scripts you need not be as rigorous as this, and in many cases the simplest script suffices. If you intend to use a script frequently and/or sharing with colleagues, it’s worth making a little extra effort such as adding commentary. Comments will help you remember what a lengthy script does after you’ve not run it for a while. It might seem tedious at the time, but you will thank yourself when you come to run it after a long interval. Comment lines begin with #. There are comment examples in some of the longer scripts later in the cookbook.