echo of var to file in SConscript

Question

In an SConscript file I have a env variable BUILDID_STR that contains a C string that I wish to output to a file.

bstr = env['BUILDID_STR']
print(bstr)

which when printed, print(bstr) correctly shows this:

//this file is automatically generated
static char* build_str="0|0.1.0|2014-05-29_16:16:51";

However, I can't get the var expanded/exported correctly, just the literal string is output instead of the above text:

cat src/log/src/version.c
env[BUILDID_STR]

Here's the pertinent part of my SConscript file

env.Command(target='#/src/log/src/version.c',
        source=libSrcfiles,
        action="echo env['BUILDID_STR'] > $TARGET")


env.SharedLibrary('log', [libSrcfiles, '#/src/log/src/version.c'])

I've also tried the code in a function and also passing to a shell script, all with the same result. The reason I have .../version.c in the SharedLibrary is that my goal is to have the .c file generated only when on of the libSrcfiles is built, thereby version.c is compiled-in.

Answer 1

The " textfile " Tool offers two Builders Textfile() and Substfile() for cases like this. You probably want to use the first:

env = Environment(tools=['textfile'])
env['BUILDID_STR'] = 'A test'
env.Textfile('test.txt', ['$BUILDID_STR'])

Answer 2

As you have seen, the action is not expanding the env variable. In the action, you can refer to env variables with the $VAR syntax (just like referring to the $SOURCE and $TARGET variables provided by SCons):

env.Command(target='#/src/log/src/version.c',
    source=libSrcfiles,
    action="echo $BUILDID_STR > $TARGET")

This solution may not handle a BUILDID_STR containing multiple lines.

You may want to investigate a system that builds your source file from a template (instead of constructing the file contents entirely within a string). The Substfile Builder referenced in this previous question might be a good starting point -- you can provide a list of (key, value) pairs to be substituted in an input file. The Substfile Builder is built into recent versions of SCons.