command_howto.txt

操作系统文件管理

							1999-03-02
							Emil Brink

			Defining a command

1. INTRODUCTION
This little text describes how to define a new command in gentoo. Such
a command is commonly referred to as a "user command". There are two
kinds of commands in gentoo; those that are built-in ("native"), and
those that aren't. The latter are what this text is about.
	In a perfect world, this file wouldn't exist, since this would
all be covered in the Complete Gentoo Documentation. Unfortunately,
that have yet to be written. In the meantime, perhaps this text can
shed some light on the topic.
	Defining your own commands to do the things you want is an
important part of utilizing gentoo, so this could be important stuff.
Read on, and don't hesitate to mail me (at <[e-mail]>) if you
have questions, corrections, or whatever.


2. WHERE IS IT DONE?
To create a user command, open up gentoo's configuration dialog window,
and click on the tab labeled "Commands". This displays a list of all
currently defined user commands, plus interface controls that allow you
to modify, add and delete commands.
	All operations described below refer to the current command. To
make a command current, click on its name in the command listing. As the
command becomes current, the interface reflects this by showing the
settings for it.
	It will probably be a lot easier to follow this text if you
open up the relevant window, so you can check out the interface along
with reading about it...


3. WHAT'S A COMMAND?
A user command has basically two parts: a name and a definition. The
name is just some word you choose; typically it should be descriptive
so you can understand what the command does from its name. Command names
should begin with a lower-case letter, to distinguish them from the
built-ins.
	The definition of a user command consists of a sequence of rows.
Each row has a type; its either "built-in" or "external". In the former
case, all the row does is envoke one of gentoo's built-in commands. In
the latter case, the row causes an external program (such as a shell
command or another application) to execute.


4. DEFINITION EDITING
In the frame labeled "Definition", the current command's definition is
shown. The main part of the frame is taken up by a list, showing the
command's rows. To the left of the list are buttons for adding,
duplicating, moving and deleting rows. These buttons all act on the
current row; to make a row current just click it in the list.


4.1 ROW EDITING
Once a row has been selected, you can modify it in various ways. These
modifications are carried out through the interface below the row list.
	You can change a row's type, its contents, and the flags from
this interface.


4.2 ROW TYPE
To change a row's type, select the row and then locate the big popup-
menu-button (or GtkOptionMenu as its known in the GTK+ widget set) in
the top left corner of the row editing interface. It will read as either
"Built-in" or "External", showing you the row's current type. Click on
the button, and a popup-menu appears with these two types in it. Point
at the desired type, and click again. The row's type changes to the one
selected.


4.3 ROW DEFINITION STRING
The most important part of a row is its definition string. This is a
line of text that you can enter and edit in order to control what the
row should do when executed.
	The meaning of the definition string is type-dependant.


4.3.1 BUILT-IN DEFINITION
For a row of type "Built-In", the definition string is really simple:
it's just the name of a gentoo built-in command to run. For example,
you could enter "Quit" to envoke the quitting command.
	The tiny button labeled "..." to the right of the string entry
field pops up a window allowing you to pick a command name from a list
of built-ins. The name you select will replace whatever is in the entry
box.


4.3.2 EXTERNAL DEFINITION
The definition of an external command is a lot more flexible and power-
ful than that for built-ins. As a side effect, it is also considerably
more complex.
	Basically, what you want to do in a external row definition is
tell gentoo which external command to run, and with which arguments.
	You can use special selection codes (spelled using braces) in
the definition string to make gentoo supply information that you want
to include in the execution.
	As a simple example, you could use a definition of "echo {f}"
(without the quotes) to print out the name of the first selected item
in the pane that is active when the command is run. In this example,
'echo' is the name of the standard shell command to print text; '{f}'
is a info selection code that is replaced by the name of the first
selected file.
	There are plenty of these data selection codes available. For
a list (with brief explanations), click on the "..."-button located to
the right of the entry field. The code you select will be appended at
the end of the current definition.

4.3.3 EXTERNAL FLAGS
External commands have a number of flags, that control their behaviour.
	There are three categories of flags: General, Before and After.
	The General flags control general aspects of the command exe-
cution. The currently available flags are:
Run in Background?
	Starts up the command in the background, letting it run
	without tying up gentoo's interface (as it otherwise would).
Kill Previous Instance?
	Instructs gentoo to kill the process that the last execution
	of this command resulted in. Requires background execution.
Survive Quit?
	Normally, gentoo will attempt to kill all child processes
	spawned from it when you quit. If this flag is set, the corre-
	sponding process will not be killed. Requires background.
Capture Output?
	When this is set, gentoo will open up a window into which the
	command's standard output and error streams will be directed.

The Before and After flag categories contain flags that are applied
either before or after the command has executed. It works like this:
first, the Before flags are applied. Then the child process that is
going to run the command is spawned. When the child terminates, the
After flags are applied.
	The flags available in the two categories are the same, with
the exception of the "Change Directory" group, which is missing for
obvious reasons in the After category.


5. NOTES
Here are a few notes and general "words of wisdom" regarding user
commands, their definition & execution:

* The order in which rows are executed is unknown for rows that have
  the "Run in Background?" flag set.
* Rows that don't run in the background must complete before the next
  row starts to execute. This means that you the sequence of rows
  really is a sequence; commands further down will run after those
  closer to the top, and thus can depend on their results.
* External rows are executed by the execvp(3) system call, which
  means that you don't need to include paths in the command name.
  It also means that all the stuff normally done by the shell is
  missing (e.g. $- & ~-expansion, pipes, redirection with < & >).
  Dollar and tildeexpansion are provided through {}-codes. If you
  need pipes and/or redirection, run your program through the shell
  (e.g. use a string of 'sh "my_program <input >output | prog2"',
  where the single quotes should not be included, but the double
  ones should).
* If a {}-code is likely to result in a string containing spaces,
  you may want to surround it with quotes in the command definition.
* It is never necessary to define a user command containing just
  a single row of type "Built-In", since wherever a user command
  name is accepted, you can always enter the name of a built-in
  directly. This is how most of the example config's buttons work.