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Unix Programming - Unix Interface Design Patterns - The ed pattern
The ed pattern
All the previous patterns have very low interactivity; they use
only control information passed in at startup time, and separate
from the data. Many programs, of course, need to be driven by a
continuing dialog with the user after startup time.
In the Unix tradition, the simplest interactive design pattern
is exemplified by
ed(1),
the Unix line editor. Other classic examples of this pattern include
ftp(1)
and
sh(1),
the Unix shell. The
ed(1)
program takes a filename argument; it modifies that file. On its
input, it accepts command lines. Some of the commands result in output
to standard output, which is intended to be seen immediately by the
user as part of the dialog with the program.
An actual sample
ed(1)
session will be included in Chapter13.
Many browserlike and editorlike programs under Unix obey this
pattern, even when the named resource they edit is something other
than a text file. Consider
gdb(1),
the GNU symbolic debugger, as an example.
Programs obeying the ed interface
design pattern are not quite so scriptable as would be the simpler
interface types resembling filters. You can feed them commands on
standard input, but it is trickier to generate sequences of commands
(and interpret any output they might ship back) than it is to just set
environment variables and command-line options. If the action of the
commands is not so predictable that they can be run blind (e.g., with a
here-document as input and ignoring output), driving
ed-like programs requires a protocol, and a
corresponding state machine in the calling process. This raises the
problems we noted in Chapter7 during the discussion of slave process
control.
Nevertheless, this is the simplest and most scriptable
pattern that supports fully interactive programs. Accordingly, it
is still quite useful as a component of the “separated engine and
interface” pattern we'll describe below.
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