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SCons / doc / user / builders-commands.in

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  <!--

  =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
  &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="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="ex1">
    <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="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="ex2">
    <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 as of SCons 1.1,
  so you can write:

  </para>

  <scons_example name="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>
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