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<!--
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-->
<para>
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 &cv-link-ENV; construction variable
as the external environment
for executing commands.
</para>
<para>
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.
</para>
<para>
The default value of the &PATH; environment variable
on a POSIX system
is <literal>/usr/local/bin:/bin:/usr/bin</literal>.
The default value of the &PATH; environment variable
on a Win32 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 &cv-ENV; dictionary
in your construction environment.
</para>
<para>
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:
</para>
<programlisting>
path = ['/usr/local/bin', '/bin', '/usr/bin']
env = Environment(ENV = {'PATH' : path})
</programlisting>
<para>
Assign a dictionary to the &cv-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 &cv-ENV; and only
set the value of &PATH;,
the most straightforward way is probably:
</para>
<programlisting>
env['ENV']['PATH'] = ['/usr/local/bin', '/bin', '/usr/bin']
</programlisting>
<para>
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):
</para>
<programlisting>
env['ENV']['PATH'] = '/usr/local/bin:/bin:/usr/bin'
</programlisting>
<para>
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 likley system-specific, anyway).
</para>
<!--
<scons_example name="ex1">
<file name="SConstruct" printme="1">
env = Environment()
env.Command('foo', [], '__ROOT__/usr/bin/printenv.py')
</file>
<file name="__ROOT__/usr/bin/printenv.py" chmod="0755">
#!/usr/bin/env python
import os
import sys
if len(sys.argv) > 1:
keys = sys.argv[1:]
else:
keys = os.environ.keys()
keys.sort()
for key in keys:
print " " + key + "=" + os.environ[key]
</file>
</scons_example>
<para>
</para>
<scons_output example="ex1">
<scons_output_command>scons -Q</scons_output_command>
</scons_output>
-->
<section>
<title>Propagating &PATH; From the External Environment</title>
<para>
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 <literal>os.environ</literal>
dictionary,
which is Python's way of letting you
get at the external environment:
</para>
<programlisting>
import os
env = Environment(ENV = {'PATH' : os.environ['PATH']})
</programlisting>
<para>
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:
</para>
<programlisting>
import os
env = Environment(ENV = os.environ)
</programlisting>
<para>
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,
both the <literal>/bin</literal> and
<literal>/usr/local/bin</literal> 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.
</para>
</section>
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