When SCons builds a target file,
it does not execute the commands with
the external environment
that you used to execute SCons.
Instead, it builds an execution environment from the values
stored in the $ENV construction variable
and uses that 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,
will almost certainly not be the same as in the external environment
from which you called SCons.
This means that SCons might not
necessarily find all of the tools
that you can successfully execute from the command line.
The default value of the PATH environment variable
on a POSIX system
is /usr/local/bin:/opt/bin:/bin:/usr/bin:/snap/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})
Assigning a dictionary to the $ENV
construction variable in this way
completely resets the execution 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, you can assign a value only
to that variable:
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 with paths
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,
since it will be correct only for the system type that
matches the separator. You can use the Python
os.pathsep for for greater portability -
don't worry too much if this Python syntax doesn't make sense
since there are other ways available:
import os
env['ENV']['PATH'] = os.pathsep.join(['/usr/local/bin', '/bin', '/usr/bin'])
You may want to propagate the external environment 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.copy())
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.
One of the most common requirements
for manipulating a variable in the execution environment
is to add one or more custom directories to a path search variable
like PATH on Linux or POSIX systems,
or %PATH% 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 env.PrependENVPath
and env.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.copy())
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 separator character
in the string
(: on Linux or POSIX,
; on Windows, or use
os.pathsep for portability).