10.3. Command-Line Targets

10.3.1. Fetching Command-Line Targets: the COMMAND_LINE_TARGETS Variable

SCons supports a COMMAND_LINE_TARGETS variable that lets you fetch the list of targets that the user specified on the command line. You can use the targets to manipulate the build in any way you wish. As a simple example, suppose that you want to print a reminder to the user whenever a specific program is built. You can do this by checking for the target in the COMMAND_LINE_TARGETS list:

if 'bar' in COMMAND_LINE_TARGETS:
    print("Don't forget to copy `bar' to the archive!")
Default(Program('foo.c'))
Program('bar.c')
        

Then, running SCons with the default target works as it always does, but explicity specifying the bar target on the command line generates the warning message:

% scons -Q
cc -o foo.o -c foo.c
cc -o foo foo.o
% scons -Q bar
Don't forget to copy `bar' to the archive!
cc -o bar.o -c bar.c
cc -o bar bar.o

Another practical use for the COMMAND_LINE_TARGETS variable might be to speed up a build by only reading certain subsidiary SConscript files if a specific target is requested.

10.3.2. Controlling the Default Targets: the Default Function

You can control which targets SCons will build by default - that is, when there are no targets specified on the command line. As mentioned previously, SCons will normally build every target in or below the current directory unless you explicitly specify one or more targets on the command line. Sometimes, however, you may want to specify that only certain programs, or programs in certain directories, should be built by default. You do this with the Default function:

env = Environment()
hello = env.Program('hello.c')
env.Program('goodbye.c')
Default(hello)
         

This SConstruct file knows how to build two programs, hello and goodbye, but only builds the hello program by default:

% scons -Q
cc -o hello.o -c hello.c
cc -o hello hello.o
% scons -Q
scons: `hello' is up to date.
% scons -Q goodbye
cc -o goodbye.o -c goodbye.c
cc -o goodbye goodbye.o

Note that, even when you use the Default function in your SConstruct file, you can still explicitly specify the current directory (.) on the command line to tell SCons to build everything in (or below) the current directory:

% scons -Q .
cc -o goodbye.o -c goodbye.c
cc -o goodbye goodbye.o
cc -o hello.o -c hello.c
cc -o hello hello.o

You can also call the Default function more than once, in which case each call adds to the list of targets to be built by default:

env = Environment()
prog1 = env.Program('prog1.c')
Default(prog1)
prog2 = env.Program('prog2.c')
prog3 = env.Program('prog3.c')
Default(prog3)
         

Or you can specify more than one target in a single call to the Default function:

env = Environment()
prog1 = env.Program('prog1.c')
prog2 = env.Program('prog2.c')
prog3 = env.Program('prog3.c')
Default(prog1, prog3)
      

Either of these last two examples will build only the prog1 and prog3 programs by default:

% scons -Q
cc -o prog1.o -c prog1.c
cc -o prog1 prog1.o
cc -o prog3.o -c prog3.c
cc -o prog3 prog3.o
% scons -Q .
cc -o prog2.o -c prog2.c
cc -o prog2 prog2.o

You can list a directory as an argument to Default:

env = Environment()
env.Program(['prog1/main.c', 'prog1/foo.c'])
env.Program(['prog2/main.c', 'prog2/bar.c'])
Default('prog1')
         

In which case only the target(s) in that directory will be built by default:

% scons -Q
cc -o prog1/foo.o -c prog1/foo.c
cc -o prog1/main.o -c prog1/main.c
cc -o prog1/main prog1/main.o prog1/foo.o
% scons -Q
scons: `prog1' is up to date.
% scons -Q .
cc -o prog2/bar.o -c prog2/bar.c
cc -o prog2/main.o -c prog2/main.c
cc -o prog2/main prog2/main.o prog2/bar.o

Lastly, if for some reason you don't want any targets built by default, you can use the Python None variable:

env = Environment()
prog1 = env.Program('prog1.c')
prog2 = env.Program('prog2.c')
Default(None)
         

Which would produce build output like:

% scons -Q
scons: *** No targets specified and no Default() targets found.  Stop.
Found nothing to build
% scons -Q .
cc -o prog1.o -c prog1.c
cc -o prog1 prog1.o
cc -o prog2.o -c prog2.c
cc -o prog2 prog2.o

10.3.2.1. Fetching the List of Default Targets: the DEFAULT_TARGETS Variable

SCons supports a DEFAULT_TARGETS variable that lets you get at the current list of default targets specified by calls to the Default function or method. The DEFAULT_TARGETS variable has two important differences from the COMMAND_LINE_TARGETS variable. First, the DEFAULT_TARGETS variable is a list of internal SCons nodes, so you need to convert the list elements to strings if you want to print them or look for a specific target name. You can do this easily by calling the str on the elements in a list comprehension:

prog1 = Program('prog1.c')
Default(prog1)
print("DEFAULT_TARGETS is %s" % [str(t) for t in DEFAULT_TARGETS])
           

(Keep in mind that all of the manipulation of the DEFAULT_TARGETS list takes place during the first phase when SCons is reading up the SConscript files, which is obvious if we leave off the -Q flag when we run SCons:)

% scons
scons: Reading SConscript files ...
DEFAULT_TARGETS is ['prog1']
scons: done reading SConscript files.
scons: Building targets ...
cc -o prog1.o -c prog1.c
cc -o prog1 prog1.o
scons: done building targets.

Second, the contents of the DEFAULT_TARGETS list changes in response to calls to the Default function, as you can see from the following SConstruct file:

prog1 = Program('prog1.c')
Default(prog1)
print("DEFAULT_TARGETS is now %s" % [str(t) for t in DEFAULT_TARGETS])
prog2 = Program('prog2.c')
Default(prog2)
print("DEFAULT_TARGETS is now %s" % [str(t) for t in DEFAULT_TARGETS])
           

Which yields the output:

% scons
scons: Reading SConscript files ...
DEFAULT_TARGETS is now ['prog1']
DEFAULT_TARGETS is now ['prog1', 'prog2']
scons: done reading SConscript files.
scons: Building targets ...
cc -o prog1.o -c prog1.c
cc -o prog1 prog1.o
cc -o prog2.o -c prog2.c
cc -o prog2 prog2.o
scons: done building targets.

In practice, this simply means that you need to pay attention to the order in which you call the Default function and refer to the DEFAULT_TARGETS list, to make sure that you don't examine the list before you've added the default targets you expect to find in it.

10.3.3. Fetching the List of Build Targets, Regardless of Origin: the BUILD_TARGETS Variable

We've already been introduced to the COMMAND_LINE_TARGETS variable, which contains a list of targets specified on the command line, and the DEFAULT_TARGETS variable, which contains a list of targets specified via calls to the Default method or function. Sometimes, however, you want a list of whatever targets SCons will try to build, regardless of whether the targets came from the command line or a Default call. You could code this up by hand, as follows:

if COMMAND_LINE_TARGETS:
    targets = COMMAND_LINE_TARGETS
else:
    targets = DEFAULT_TARGETS
      

SCons, however, provides a convenient BUILD_TARGETS variable that eliminates the need for this by-hand manipulation. Essentially, the BUILD_TARGETS variable contains a list of the command-line targets, if any were specified, and if no command-line targets were specified, it contains a list of the targets specified via the Default method or function.

Because BUILD_TARGETS may contain a list of SCons nodes, you must convert the list elements to strings if you want to print them or look for a specific target name, just like the DEFAULT_TARGETS list:

prog1 = Program('prog1.c')
Program('prog2.c')
Default(prog1)
print ("BUILD_TARGETS is %s" % [str(t) for t in BUILD_TARGETS])
        

Notice how the value of BUILD_TARGETS changes depending on whether a target is specified on the command line - BUILD_TARGETS takes from DEFAULT_TARGETS only if there are no COMMAND_LINE_TARGETS:

% scons -Q
BUILD_TARGETS is ['prog1']
cc -o prog1.o -c prog1.c
cc -o prog1 prog1.o
% scons -Q prog2
BUILD_TARGETS is ['prog2']
cc -o prog2.o -c prog2.c
cc -o prog2 prog2.o
% scons -Q -c .
BUILD_TARGETS is ['.']
Removed prog1.o
Removed prog1
Removed prog2.o
Removed prog2