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
appending
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.
$1
represents the first argument supplied on the
command line. (There is more on this in Section 7.6.) If the command restore
runs the script, then
entering
would remove splat.f
and replace it with
splat.f
.
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
&& -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 supplied
if you forget the
argument. $#argv
is 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
>
. The 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 file
.
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
message.
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.