.MH "Advanced Techniques"
.bq
.pp
This section deals with several of the more
advanced features of the Subsystem.
.eq
.SH "Command Files"
As an illustration, let us take an operation that finds use quite
frequently: making printed listings of all the Ratfor source code
in a directory.
.nh
Command language programs, or "shell[bl]programs," greatly
simplify the automation of this process.
.hy
Shell programs are files
containing commands to be executed when human intervention is
not required.
.pp
Suppose that we put the following commands in a file named "mklist"
(note the use of i/o redirection here):
.be 5
lf  -c  >temp1
temp1>  find  .r  >temp2
temp2>  change  %  "sp "  >temp3
temp3>  sh
del  temp1  temp2  temp3
.ee
Then, whenever we want a listing of all the Ratfor
source code in the current directory, we just type:
.be
mklist
.ee
The only price we must pay for this convenience is to
ensure that the names of all files containing Ratfor programs
end in ".r".
(If the 'find', 'change', and 'sp' commands mystify you, 'help' can offer
explanations.)
.#
.#
.SH "Pipes"
.#
.#
Pipes are another handy feature of the Subsystem.  A "pipe"
between two programs
simply connects the standard output of the first to the
standard input of the second; and two or more programs connected in this
manner form a "pipeline." With pipes,
programs are easily combined as cooperating tools to perform any
number of complex tasks that would otherwise require special-purpose
programs.
.pp
The command interpreter provides a simple and intuitive way to specify
these combinations:
.be
prog1 | prog2
.ee
Essentially, two or more complete commands are typed on the same
line, separated by vertical bars ("|").
(One or more spaces [ul must] appear on both sides of this symbol.)
The command interpreter
then does all the work in connecting them together so that whatever the
program on the left of the bar writes on its standard output, the one on the
right reads from its standard input.
.pp
Take our shell program to create listings as an example; that series of
commands involved the creation of three temporary files.
Not only is this distracting, in that it takes our attention away
from the real work at hand, but it also leads to wasted storage
space, since one all too frequently forgets to delete temporary files
after they have served their function. Using pipes, we could just as
easily have done the same thing like this:
.be
lf  -c  |  find  .r  |  change  %  "sp "  |  sh
.ee
and the command interpreter would have taken care of all the details
that before we had to attend to ourselves. In addition to being
much cleaner looking, the pipeline's function is also more obvious.
.#
.#
.SH "Additional I/O Redirectors"
.#
.#
The last advanced features of the Subsystem that we will examine are
the two remaining i/o redirection operators,
represented by two variations of the double funnel (">>").
.pp
.ne 14
In the first variation,
.be
>>xyz          (read "append to xyz")
.ee
causes standard output to be appended to the file named "xyz".
Whereas
.be
cat file1 >file2
.ee
would copy the contents of file1 into file2, destroying whatever was
previously in file2,
.be
cat file1 >>file2
.ee
would copy the contents of file1 to the end of file2, without destroying
anything that was there to start with.
.pp
In the second variation, the double funnel is used without a file name
.be
>>             (read "from command input")
.ee
to connect standard input to the current shell program.
For example, suppose we wanted to make a shell program that extracted the
first ten lines of a file, and deleted all the rest.
The shell program might
look something like this:
.be 4
>>  ed file
11,$d
w
q
.ee
">>" is frequently used in this way for the editor to read commands
from the shell program, without having to have a separate script file.
.ne 7
.pp
This is only a very small sample of the power made available by the
features of the Subsystem. As is the case with any craft, given the
proper tools and an hospitable environment in which to work, the only
limit to the variety of things that can be done is the imagination and
ingenuity of the craftsman himself.