<?xml version='1.0'?>
<!DOCTYPE sconsdoc [
    <!ENTITY % scons SYSTEM "../scons.mod">
    %scons;
    
    <!ENTITY % builders-mod SYSTEM "../generated/builders.mod">
    %builders-mod;
    <!ENTITY % functions-mod SYSTEM "../generated/functions.mod">
    %functions-mod;
    <!ENTITY % tools-mod SYSTEM "../generated/tools.mod">
    %tools-mod;
    <!ENTITY % variables-mod SYSTEM "../generated/variables.mod">
    %variables-mod;

]>

<chapter id="chap-builders-commands"
         xmlns="http://www.scons.org/dbxsd/v1.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://www.scons.org/dbxsd/v1.0 http://www.scons.org/dbxsd/v1.0/scons.xsd">
<title>Not Writing a Builder:  the &Command; Builder</title>

<!--

  __COPYRIGHT__

  Permission is hereby granted, free of charge, to any person obtaining
  a copy of this software and associated documentation files (the
  "Software"), to deal in the Software without restriction, including
  without limitation the rights to use, copy, modify, merge, publish,
  distribute, sublicense, and/or sell copies of the Software, and to
  permit persons to whom the Software is furnished to do so, subject to
  the following conditions:

  The above copyright notice and this permission notice shall be included
  in all copies or substantial portions of the Software.

  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY
  KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE
  WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
  NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
  LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
  OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
  WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

-->

  <!--

  =head2 The C<Command> method


  The C<Command> method is called as follows:

    Command $env <target>, <inputs>, <build action>;

  The target is made dependent upon the list of input files specified, and the
  inputs must be built successfully or Cons will not attempt to build the
  target.

  To specify a command with multiple targets, you can specify a reference to a
  list of targets. In Perl, a list reference can be created by enclosing a
  list in square brackets. Hence the following command:

    Command $env ['foo.h', 'foo.c'], 'foo.template', q(
  	gen %1
    );

  could be used in a case where the command C<gen> creates two files, both
  F<foo.h> and F<foo.c>.

  -->

  <para>

  Creating a &Builder; and attaching it to a &consenv;
  allows for a lot of flexibility when you
  want to re-use actions
  to build multiple files of the same type.
  This can, however, be cumbersome
  if you only need to execute one specific command
  to build a single file (or group of files).
  For these situations, &SCons; supports a
  &f-link-Command; builder that arranges
  for a specific action to be executed
  to build a specific file or files.
  This looks a lot like the other builders
  (like &b-link-Program;, &b-link-Object;, etc.),
  but takes as an additional argument
  the command to be executed to build the file:

  </para>

  <scons_example name="builderscommands_ex1">
     <file name="SConstruct" printme="1">
env = Environment()
env.Command('foo.out', 'foo.in', "sed 's/x/y/' &lt; $SOURCE > $TARGET")
     </file>
     <file name="foo.in">
foo.in
     </file>
  </scons_example>

  <para>

  When executed,
  &SCons; runs the specified command,
  substituting &cv-link-SOURCE; and &cv-link-TARGET;
  as expected:

  </para>

  <scons_output example="builderscommands_ex1" suffix="1">
    <scons_output_command>scons -Q</scons_output_command>
  </scons_output>

  <para>

  This is often more convenient than
  creating a &Builder; object
  and adding it to the &cv-link-BUILDERS; variable
  of a &consenv;.

  </para>

  <para>

  Note that the action you specify to the
  &Command; &Builder; can be any legal &SCons; &Action;,
  such as a Python function:

  </para>

  <scons_example name="builderscommands_ex2">
     <file name="SConstruct" printme="1">
env = Environment()

def build(target, source, env):
    # Whatever it takes to build
    return None

env.Command('foo.out', 'foo.in', build)
     </file>
     <file name="foo.in">
foo.in
     </file>
  </scons_example>

  <para>

  Which executes as follows:

  </para>

  <scons_output example="builderscommands_ex2" suffix="1">
    <scons_output_command>scons -Q</scons_output_command>
  </scons_output>

  <para>

  Note that &cv-link-SOURCE; and &cv-link-TARGET; are expanded 
  in the source and target as well, so you can write:

  </para>

  <scons_example name="builderscommands_ex3">
     <file name="SConstruct" printme="1">
env.Command('${SOURCE.basename}.out', 'foo.in', build)
     </file>
  </scons_example>

  <para>

  which does the same thing as the previous example, but allows you
  to avoid repeating yourself.

  </para>

  <para>

  It may be helpful to use the <parameter>action</parameter>
  keyword to specify the action, is this makes things more clear
  to the reader:

  </para>

  <scons_example name="builderscommands_ex4">
     <file name="SConstruct" printme="1">
env.Command('${SOURCE.basename}.out', 'foo.in', action=build)
     </file>
  </scons_example>

  <para>

  The method described in
  <xref linkend="sect-controlling-build-output"/> for controlling
  build output works well when used with pre-defined builders which
  have pre-defined <literal>*COMSTR</literal> variables for that purpose,
  but that is not the case when calling &f-Command;,
  where &SCons; has no specific knowledge of the action ahead of time.
  If the action argument to &f-Command; is not already an &Action; object,
  it will construct one for you with suitable defaults,
  which include a message based on the type of action.
  However, you can also construct the &Action; object yourself
  to pass to &f-Command;, which gives you much more control.
  Here's an evolution of the example from above showing this approach:

  </para>

  <scons_example name="builderscommands_ex5">
     <file name="SConstruct" printme="1">
env = Environment()

def build(target, source, env):
    # Whatever it takes to build
    return None

act = Action(build, cmdstr="Building ${TARGET}")
env.Command('foo.out', 'foo.in', action=act)
     </file>
     <file name="foo.in">
foo.in
     </file>
  </scons_example>

  <para>

  Which executes as follows:

  </para>

  <scons_output example="builderscommands_ex5" suffix="1">
    <scons_output_command>scons -Q</scons_output_command>
  </scons_output>

</chapter>
