How can I generate multi-line build commands?

Question

In SCons, my command generators create ridiculously long command lines. I'd like to be able to split these commands across multiple lines for readability in the build log.

eg I have a SConscipt like:

import os

# create dependency
def my_cmd_generator(source, target, env, for_signature):
    return r'''echo its a small world after all \
        its a small world after all'''

my_cmd_builder = Builder(generator=my_cmd_generator, suffix = '.foo')

env = Environment()
env.Append( BUILDERS = {'MyCmd' : my_cmd_builder } )

my_cmd = env.MyCmd('foo.foo',os.popen('which bash').read().strip())
AlwaysBuild(my_cmd)

When it executes, I get:

scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
echo its a small world after all \
its a small world after all
its a small world after all
sh: line 1: its: command not found
scons: *** [foo.foo] Error 127
scons: building terminated because of errors.

Doing this in the python shell with os.system and os.popen works -- I get a readable command string and the sub-shell process interprets all the lines as one command.

>>> import os
>>> cmd = r'''echo its a small world after all \
... its a small world after all'''
>>> print cmd
echo its a small world after all \
its a small world after all
>>> os.system( cmd)
its a small world after all its a small world after all
0

When I do this in SCons, it executes each line one at a time, which is not what I want.

I also want to avoid building up my commands into a shell-script and then executing the shell script, because that will create string escaping madness.

Is this possible?

UPDATE:
cournape,
Thanks for the clue about the $CCCOMSTR. Unfortunately, I'm not using any of the languages that SCons supports out of the box, so I'm creating my own command generator. Using a generator, how can I get SCons to do:

echo its a small world after all its a small world after all' 

but print

echo its a small world after all \
    its a small world after all

?

Answer 1

Thanks to cournape's tip about Actions versus Generators ( and eclipse pydev debugger), I've finally figured out what I need to do. You want to pass in your function to the 'Builder' class as an 'action' not a 'generator'. This will allow you to actually execute the os.system or os.popen call directly. Here's the updated code:

import os

def my_action(source, target, env):
    cmd = r'''echo its a small world after all \
        its a small world after all'''
    print cmd
    return os.system(cmd)

my_cmd_builder = Builder(
    action=my_action,  # <-- CRUCIAL PIECE OF SOLUTION
    suffix = '.foo')

env = Environment()
env.Append( BUILDERS = {'MyCmd' : my_cmd_builder } )

my_cmd = env.MyCmd('foo.foo',os.popen('which bash').read().strip())

This SConstruct file will produce the following output:

scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
my_action(["foo.foo"], ["/bin/bash"])
echo its a small world after all \
        its a small world after all
its a small world after all its a small world after all
scons: done building targets.

The other crucial piece is to remember that switching from a 'generator' to an 'action' means the target you're building no longer has an implicit dependency on the actual string that you are passing to the sub-process shell. You can re-create this dependency by adding the string into your environment.

eg, the solution that I personally want looks like:

import os

cmd = r'''echo its a small world after all \
        its a small world after all'''

def my_action(source, target, env):
    print cmd
    return os.system(cmd)

my_cmd_builder = Builder(
    action=my_action,
    suffix = '.foo')

env = Environment()
env['_MY_CMD'] = cmd  # <-- CREATE IMPLICIT DEPENDENCY ON CMD STRING
env.Append( BUILDERS = {'MyCmd' : my_cmd_builder } )

my_cmd = env.MyCmd('foo.foo',os.popen('which bash').read().strip())

Answer 2

You are mixing two totally different things: the command to be executed, and its representation in the command line. By default, scons prints the command line, but if you split the command line, you are changing the commands executed.

Now, scons has a mechanism to change the printed commands. They are registered per Action instances, and many default ones are available:

env = Environment()
env['CCCOMSTR']  = "CC                 $SOURCE"
env['CXXCOMSTR'] = "CXX                $SOURCE"
env['LINKCOM']   = "LINK               $SOURCE"

Will print, assuming only C and CXX sources:

CC    foo.c
CC    bla.c
CXX   yo.cc
LINK  yo.o bla.o foo.o