16.6. Guaranteeing local copies of files

If the repository tree contains the complete results of a build, and we try to build from the repository without any files in our local tree, something moderately surprising happens:

% mkdir $HOME/build2
% cd $HOME/build2
% scons -Q -Y /usr/all/repository hello
scons: `hello' is up-to-date.

Why does SCons say that the hello program is up-to-date when there is no hello program in the local build directory? Because the repository (not the local directory) contains the up-to-date hello program, and SCons correctly determines that nothing needs to be done to rebuild that up-to-date copy of the file.

There are, however, many times when you want to ensure that a local copy of a file always exists. A packaging or testing script, for example, may assume that certain generated files exist locally. To tell SCons to make a copy of any up-to-date repository file in the local build directory, use the Local function:

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

If we then run the same command, SCons will make a local copy of the program from the repository copy, and tell you that it is doing so:

% scons -Y /usr/all/repository hello
Local copy of hello from /usr/all/repository/hello
scons: `hello' is up-to-date.

(Notice that, because the act of making the local copy is not considered a "build" of the hello file, SCons still reports that it is up-to-date.)