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<?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 scons.xsd">
<title>Not Writing a Builder: the &Command; Builder</title>
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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
&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/' < $SOURCE > $TARGET")
</file>
<file name="foo.in">
foo.in
</file>
</scons_example>
<para>
When executed,
&SCons; runs the specified command,
substituting &cv-link-SOURCE; and &cv-link-TARGET;
as expected:
</para>
<scons_output example="builderscommands_ex1" suffix="1">
<scons_output_command>scons -Q</scons_output_command>
</scons_output>
<para>
This is often more convenient than
creating a &Builder; object
and adding it to the &cv-link-BUILDERS; variable
of a &consenv;
</para>
<para>
Note that the action you specify to the
&Command; &Builder; can be any legal &SCons; &Action;,
such as a Python function:
</para>
<scons_example name="builderscommands_ex2">
<file name="SConstruct" printme="1">
env = Environment()
def build(target, source, env):
# Whatever it takes to build
return None
env.Command('foo.out', 'foo.in', build)
</file>
<file name="foo.in">
foo.in
</file>
</scons_example>
<para>
Which executes as follows:
</para>
<scons_output example="builderscommands_ex2" suffix="1">
<scons_output_command>scons -Q</scons_output_command>
</scons_output>
<para>
Note that &cv-link-SOURCE; and &cv-link-TARGET; are expanded
in the source and target as well as of SCons 1.1,
so you can write:
</para>
<scons_example name="builderscommands_ex3">
<file name="SConstruct" printme="1">
env.Command('${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>
</chapter>
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