Software Engineering for Students A Programming Approach

260 Chapter 18 ■ Scripting
GNU/Linux. For reasons that we shall shortly see, Unix can act as an excellent basis
for scripting.
Unix provides:
■ a textual command language, based on command verbs followed by parameters
■ the facility to write programs in the command language
■ a filing system, with tree-structured directories (folders)
■ a set of useful utility programs, called filters, e.g. a file copy tool
■ a facility, called pipes, for joining filters together.
These facilities are now commonly found in many operating systems, but Unix was
the first system to provide them.
Unix consists of a small kernel, augmented by a rich set of small utility programs, the
filters. An example of a filter is a program to display a list of the files within a particular
directory. Perhaps because only two people designed it, Unix is built around a few
simple concepts. One of the fundamental ideas of Unix is the notion that software
(including Unix itself ) should be built from small general-purpose components that are
developed individually. These components can be used individually but also can be
combined as necessary in order to satisfy a new requirement. A filter is a program that
inputs a serial stream of information, processes it and outputs a second serial stream.
Other examples of Unix-provided filters:
■ count the number of lines or characters in a file
■ a file copy program
■ print a file with specified formatting
■ spool a file to the printer
■ print all lines in a file that contain a specified textual pattern
■ a lexical analyzer.
Filters are combined by taking the output from one and feeding it as input to
another. The stream of data that flows from one filter to another is known as a pipe.
This combination of filters and pipes is carried out by Unix under the control of the
command language.
An example of using a filter is the command:
ls
When you type a Unix command such as this, you invoke the filter with the same
name. The filter named ls displays on the screen the names of all the files in the current
directory, one per line. Another tool, named wc, inputs a file and gives a line, word and
character count for that file. The output of wc can be controlled by providing parameters,
and the parameter -l specifies that lines should be counted, so that the command:
wc -l file