7.3. Controlling the Execution Environment for Issued Commands

When SCons builds a target file, it does not execute the commands with the same external environment that you used to execute SCons. Instead, it uses the dictionary stored in the $ENV construction variable as the external environment for executing commands.

The most important ramification of this behavior is that the PATH environment variable, which controls where the operating system will look for commands and utilities, is not the same as in the external environment from which you called SCons. This means that SCons will not, by default, necessarily find all of the tools that you can execute from the command line.

The default value of the PATH environment variable on a POSIX system is /usr/local/bin:/bin:/usr/bin. The default value of the PATH environment variable on a Windows system comes from the Windows registry value for the command interpreter. If you want to execute any commands--compilers, linkers, etc.--that are not in these default locations, you need to set the PATH value in the $ENV dictionary in your construction environment.

The simplest way to do this is to initialize explicitly the value when you create the construction environment; this is one way to do that:

path = ['/usr/local/bin', '/bin', '/usr/bin']
env = Environment(ENV = {'PATH' : path})

Assign a dictionary to the $ENV construction variable in this way completely resets the external environment so that the only variable that will be set when external commands are executed will be the PATH value. If you want to use the rest of the values in $ENV and only set the value of PATH, the most straightforward way is probably:

env['ENV']['PATH'] = ['/usr/local/bin', '/bin', '/usr/bin']

Note that SCons does allow you to define the directories in the PATH in a string, separated by the pathname-separator character for your system (':' on POSIX systems, ';' on Windows):

env['ENV']['PATH'] = '/usr/local/bin:/bin:/usr/bin'

But doing so makes your SConscript file less portable, (although in this case that may not be a huge concern since the directories you list are likely system-specific, anyway).

7.3.1. Propagating PATH From the External Environment

You may want to propagate the external PATH to the execution environment for commands. You do this by initializing the PATH variable with the PATH value from the os.environ dictionary, which is Python's way of letting you get at the external environment:

import os
env = Environment(ENV = {'PATH' : os.environ['PATH']})

Alternatively, you may find it easier to just propagate the entire external environment to the execution environment for commands. This is simpler to code than explicity selecting the PATH value:

import os
env = Environment(ENV = os.environ)

Either of these will guarantee that SCons will be able to execute any command that you can execute from the command line. The drawback is that the build can behave differently if it's run by people with different PATH values in their environment--for example, if both the /bin and /usr/local/bin directories have different cc commands, then which one will be used to compile programs will depend on which directory is listed first in the user's PATH variable.

7.3.2. Adding to PATH Values in the Execution Environment

One of the most common requirements for manipulating a variable in the execution environment is to add one or more custom directories to a search like the $PATH variable on Linux or POSIX systems, or the %PATH% variable on Windows, so that a locally-installed compiler or other utility can be found when SCons tries to execute it to update a target. SCons provides PrependENVPath and AppendENVPath functions to make adding things to execution variables convenient. You call these functions by specifying the variable to which you want the value added, and then value itself. So to add some /usr/local directories to the $PATH and $LIB variables, you might:

env = Environment(ENV = os.environ)
env.PrependENVPath('PATH', '/usr/local/bin')
env.AppendENVPath('LIB', '/usr/local/lib')

Note that the added values are strings, and if you want to add multiple directories to a variable like $PATH, you must include the path separate character (: on Linux or POSIX, ; on Windows) in the string.