path: root/doc/user/command-line.xml

<!--

  __COPYRIGHT__

  Permission is hereby granted, free of charge, to any person obtaining
  a copy of this software and associated documentation files (the
  "Software"), to deal in the Software without restriction, including
  without limitation the rights to use, copy, modify, merge, publish,
  distribute, sublicense, and/or sell copies of the Software, and to
  permit persons to whom the Software is furnished to do so, subject to
  the following conditions:

  The above copyright notice and this permission notice shall be included
  in all copies or substantial portions of the Software.

  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY
  KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE
  WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
  NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
  LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
  OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
  WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

-->

  <para>

  &SCons; provides a number of ways
  for the writer of the &SConscript; files
  to give the users who will run &SCons;
  a great deal of control over the build execution.
  The arguments that the user can specify on
  the command line are broken down into three types:

  </para>

  <variablelist>

    <varlistentry>
    <term>Options</term>

    <listitem>
    <para>

    Command-line options always begin with
    one or two <literal>-</literal> (hyphen) characters.
    &SCons; provides ways for you to examine
    and set options values from within your &SConscript; files,
    as well as the ability to define your own
    custom options.
    See <xref linkend="sect-command-line-options"></xref>, below.

    </para>
    </listitem>
    </varlistentry>

    <varlistentry>
    <term>Variables</term>

    <listitem>
    <para>

    Any command-line argument containing an <literal>=</literal>
    (equal sign) is considered a variable setting with the form
    <varname>variable</varname>=<varname>value</varname>
    &SCons; provides direct access to
    all of the command-line variable settings,
    the ability to apply command-line variable settings
    to construction environments,
    and functions for configuring 
    specific types of variables
    (Boolean values, path names, etc.)
    with automatic validation of the user's specified values.
    See <xref linkend="sect-command-line-variables"></xref>, below.

    </para>
    </listitem>
    </varlistentry>

    <varlistentry>
    <term>Targets</term>

    <listitem>
    <para>

    Any command-line argument that is not an option
    or a variable setting
    (does not begin with a hyphen
    and does not contain an equal sign)
    is considered a target that the user
    (presumably) wants &SCons; to build.
    A list of Node objects representing
    the target or targets to build.
    &SCons; provides access to the list of specified targets,
    as well as ways to set the default list of targets
    from within the &SConscript; files.
    See <xref linkend="sect-command-line-targets"></xref>, below.

    </para>
    </listitem>
    </varlistentry>

  </variablelist>

  <section id="sect-command-line-options">
  <title>Command-Line Options</title>

    <para>

    &SCons; has many <emphasis>command-line options</emphasis>
    that control its behavior.
    A &SCons; <emphasis>command-line option</emphasis>
    always begins with one or two <literal>-</literal> (hyphen)
    characters.

    </para>

    <section>
    <title>Not Having to Specify Command-Line Options Each Time:  the &SCONSFLAGS; Environment Variable</title>

      <para>

      Users may find themselves supplying
      the same command-line options every time
      they run &SCons;.
      For example, you might find it saves time
      to specify a value of <literal>-j 2</literal>
      to have &SCons; run up to two build commands in parallel.
      To avoid having to type <literal>-j 2</literal> by hand
      every time,
      you can set the external environment variable
      &SCONSFLAGS; to a string containing
      command-line options that you want &SCons; to use.

      </para>

      <para>

      If, for example,
      you're using a POSIX shell that's
      compatible with the Bourne shell,
      and you always want &SCons; to use the
      <literal>-Q</literal> option,
      you can set the &SCONSFLAGS;
      environment as follows:

      </para>

      

      <screen>
        % <userinput>scons</userinput>
        scons: Reading SConscript files ...
        scons: done reading SConscript files.
        scons: Building targets ...
            ... [build output] ...
        scons: done building targets.
        % <userinput>export SCONSFLAGS="-Q"</userinput>
        % <userinput>scons</userinput>
            ... [build output] ...
      </screen>

      <para>

      Users of &csh;-style shells on POSIX systems
      can set the &SCONSFLAGS; environment as follows:

      </para>

      <screen>
        $ <userinput>setenv SCONSFLAGS "-Q"</userinput>
      </screen>

      <para>

      Windows users may typically want to set the
      &SCONSFLAGS; in the appropriate tab of the
      <literal>System Properties</literal> window.

      </para>

    </section>

    <section>
    <title>Getting Values Set by Command-Line Options:  the &GetOption; Function</title>

      <para>

      &SCons; provides the &GetOption; function
      to get the values set by the various command-line options.
      One common use of this is to check whether or not
      the <literal>-h</literal> or <literal>--help</literal> option
      has been specified.
      Normally, &SCons; does not print its help text
      until after it has read all of the &SConscript; files,
      because it's possible that help text has been added
      by some subsidiary &SConscript; file deep in the
      source tree hierarchy.
      Of course, reading all of the &SConscript; files
      takes extra time.

      </para>

      <para>

      If you know that your configuration does not define
      any additional help text in subsidiary &SConscript; files,
      you can speed up the command-line help available to users
      by using the &GetOption; function to load the
      subsidiary &SConscript; files only if the
      the user has <emphasis>not</emphasis> specified
      the <literal>-h</literal> or <literal>--help</literal> option,
      like so:

      </para>

      <programlisting></programlisting>

      <para>

      In general, the string that you pass to the
      &GetOption; function to fetch the value of a command-line
      option setting is the same as the "most common" long option name
      (beginning with two hyphen characters),
      although there are some exceptions.
      The list of &SCons; command-line options
      and the &GetOption; strings for fetching them,
      are available in the
      <xref linkend="sect-command-line-option-strings"></xref> section,
      below.

      </para>

    </section>

    <section>
    <title>Setting Values of Command-Line Options:  the &SetOption; Function</title>

      <para>

      You can also set the values of &SCons;
      command-line options from within the &SConscript; files
      by using the &SetOption; function.
      The strings that you use to set the values of &SCons;
      command-line options are available in the
      <xref linkend="sect-command-line-option-strings"></xref> section,
      below.

      </para>

      <para>

      One use of the &SetOption; function is to
      specify a value for the <literal>-j</literal>
      or <literal>--jobs</literal> option,
      so that users get the improved performance
      of a parallel build without having to specify the option by hand.
      A complicating factor is that a good value
      for the <literal>-j</literal> option is
      somewhat system-dependent.
      One rough guideline is that the more processors
      your system has,
      the higher you want to set the
      <literal>-j</literal> value,
      in order to take advantage of the number of CPUs.

      </para>

      <para>

      For example, suppose the administrators
      of your development systems
      have standardized on setting a
      <varname>NUM_CPU</varname> environment variable
      to the number of processors on each system.
      A little bit of Python code
      to access the environment variable
      and the &SetOption; function
      provide the right level of flexibility:

      </para>

      <programlisting>
        import os
        num_cpu = int(os.environ.get('NUM_CPU', 2))
        SetOption('num_jobs', num_cpu)
        print "running with -j", GetOption('num_jobs')
      </programlisting>

      <para>

      The above snippet of code
      sets the value of the <literal>--jobs</literal> option
      to the value specified in the
      <varname>$NUM_CPU</varname> environment variable.
      (This is one of the exception cases
      where the string is spelled differently from
      the from command-line option.
      The string for fetching or setting the <literal>--jobs</literal>
      value is <literal>num_jobs</literal>
      for historical reasons.)
      The code in this example prints the <literal>num_jobs</literal>
      value for illustrative purposes.
      It uses a default value of <literal>2</literal>
      to provide some minimal parallelism even on
      single-processor systems:

      </para>

      <screen>
        % <userinput>scons -Q</userinput>
        running with -j 2
        scons: `.' is up to date.
      </screen>

      <para>

      But if the <varname>$NUM_CPU</varname>
      environment variable is set,
      then we use that for the default number of jobs:

      </para>

      <screen>
        % <userinput>export NUM_CPU="4"</userinput>
        % <userinput>scons -Q</userinput>
        running with -j 4
        scons: `.' is up to date.
      </screen>

      <para>

      But any explicit
      <literal>-j</literal> or <literal>--jobs</literal>
      value the user specifies an the command line is used first,
      regardless of whether or not
      the <varname>$NUM_CPU</varname> environment
      variable is set:

      </para>

      <screen>
        % <userinput>scons -Q -j 7</userinput>
        running with -j 7
        scons: `.' is up to date.
        % <userinput>export NUM_CPU="4"</userinput>
        % <userinput>scons -Q -j 3</userinput>
        running with -j 3
        scons: `.' is up to date.
      </screen>

    </section>

    <section id="sect-command-line-option-strings">
    <title>Strings for Getting or Setting Values of &SCons; Command-Line Options</title>

      <para>

      The strings that you can pass to the &GetOption;
      and &SetOption; functions usually correspond to the
      first long-form option name
      (beginning with two hyphen characters:  <literal>--</literal>),
      after replacing any remaining hyphen characters
      with underscores.

      </para>

      <para>

      The full list of strings and the variables they
      correspond to is as follows:

      </para>

      <informaltable>
      <tgroup cols="2" align="left">

      <thead>

      <row>
      <entry>String for &GetOption; and &SetOption;</entry>
      <entry>Command-Line Option(s)</entry>
      </row>

      </thead>

      <tbody>

      <row>
      <entry><literal>cache_debug</literal></entry>
      <entry><option>--cache-debug</option></entry>
      </row>

      <row>
      <entry><literal>cache_disable</literal></entry>
      <entry><option>--cache-disable</option></entry>
      </row>

      <row>
      <entry><literal>cache_force</literal></entry>
      <entry><option>--cache-force</option></entry>
      </row>

      <row>
      <entry><literal>cache_show</literal></entry>
      <entry><option>--cache-show</option></entry>
      </row>

      <row>
      <entry><literal>clean</literal></entry>
      <entry><option>-c</option>,
           <option>--clean</option>,
           <option>--remove</option></entry>
      </row>

      <row>
      <entry><literal>config</literal></entry>
      <entry><option>--config</option></entry>
      </row>

      <row>
      <entry><literal>directory</literal></entry>
      <entry><option>-C</option>,
             <option>--directory</option></entry>
      </row>

      <row>
      <entry><literal>diskcheck</literal></entry>
      <entry><option>--diskcheck</option></entry>
      </row>

      <row>
      <entry><literal>duplicate</literal></entry>
      <entry><option>--duplicate</option></entry>
      </row>

      <row>
      <entry><literal>file</literal></entry>
      <entry><option>-f</option>,
             <option>--file</option>,
             <option>--makefile </option>,
             <option>--sconstruct</option></entry>
      </row>

      <row>
      <entry><literal>help</literal></entry>
      <entry><option>-h</option>,
             <option>--help</option></entry>
      </row>

      <row>
      <entry><literal>ignore_errors</literal></entry>
      <entry><option>--ignore-errors</option></entry>
      </row>

      <row>
      <entry><literal>implicit_cache</literal></entry>
      <entry><option>--implicit-cache</option></entry>
      </row>

      <row>
      <entry><literal>implicit_deps_changed</literal></entry>
      <entry><option>--implicit-deps-changed</option></entry>
      </row>

      <row>
      <entry><literal>implicit_deps_unchanged</literal></entry>
      <entry><option>--implicit-deps-unchanged</option></entry>
      </row>

      <row>
      <entry><literal>interactive</literal></entry>
      <entry><option>--interact</option>,
             <option>--interactive</option></entry>
      </row>

      <row>
      <entry><literal>keep_going</literal></entry>
      <entry><option>-k</option>,
             <option>--keep-going</option></entry>
      </row>

      <row>
      <entry><literal>max_drift</literal></entry>
      <entry><option>--max-drift</option></entry>
      </row>

      <row>
      <entry><literal>no_exec</literal></entry>
      <entry><option>-n</option>,
             <option>--no-exec</option>,
             <option>--just-print</option>,
             <option>--dry-run</option>,
             <option>--recon</option></entry>
      </row>

      <row>
      <entry><literal>no_site_dir</literal></entry>
      <entry><option>--no-site-dir</option></entry>
      </row>

      <row>
      <entry><literal>num_jobs</literal></entry>
      <entry><option>-j</option>,
             <option>--jobs</option></entry>
      </row>

      <row>
      <entry><literal>profile_file</literal></entry>
      <entry><option>--profile</option></entry>
      </row>

      <row>
      <entry><literal>question</literal></entry>
      <entry><option>-q</option>,
             <option>--question</option></entry>
      </row>

      <row>
      <entry><literal>random</literal></entry>
      <entry><option>--random</option></entry>
      </row>

      <row>
      <entry><literal>repository</literal></entry>
      <entry><option>-Y</option>,
             <option>--repository</option>,
             <option>--srcdir</option></entry>
      </row>

      <row>
      <entry><literal>silent</literal></entry>
      <entry><option>-s</option>,
             <option>--silent</option>,
             <option>--quiet</option></entry>
      </row>

      <row>
      <entry><literal>site_dir</literal></entry>
      <entry><option>--site-dir</option></entry>
      </row>

      <row>
      <entry><literal>stack_size</literal></entry>
      <entry><option>--stack-size</option></entry>
      </row>

      <row>
      <entry><literal>taskmastertrace_file</literal></entry>
      <entry><option>--taskmastertrace</option></entry>
      </row>

      <row>
      <entry><literal>warn</literal></entry>
      <entry><option>--warn</option> <option>--warning</option></entry>
      </row>

      </tbody>

      </tgroup>
      </informaltable>

    </section>

    <section>
    <title>Adding Custom Command-Line Options:  the &AddOption; Function</title>

      <para>

      &SCons; also allows you to define your own
      command-line options with the &AddOption; function.
      The &AddOption; function takes the same arguments
      as the <function>optparse.add_option</function> function
      from the standard Python library.
      <footnote>
      <para>
      The &AddOption; function is,
      in fact, implemented using a subclass
      of the <classname>optparse.OptionParser</classname>.
      </para>
      </footnote>
      Once you have added a custom command-line option
      with the &AddOption; function,
      the value of the option (if any) is immediately available
      using the standard &GetOption; function.
      (The value can also be set using &SetOption;,
      although that's not very useful in practice
      because a default value can be specified in
      directly in the &AddOption; call.)

      </para>

      <para>

      One useful example of using this functionality
      is to provide a <option>--prefix</option> for users:

      </para>

      <programlisting>
        AddOption('--prefix',
                  dest='prefix',
                  type='string',
                  nargs=1,
                  action='store',
                  metavar='DIR',
                  help='installation prefix')

        env = Environment(PREFIX = GetOption('prefix'))

        installed_foo = env.Install('$PREFIX/usr/bin', 'foo.in')
        Default(installed_foo)
      </programlisting>

      <para>

      The above code uses the &GetOption; function
      to set the <varname>$PREFIX</varname>
      construction variable to any
      value that the user specifies with a command-line
      option of <literal>--prefix</literal>.
      Because <varname>$PREFIX</varname>
      will expand to a null string if it's not initialized,
      running &SCons; without the
      option of <literal>--prefix</literal>
      will install the file in the
      <filename>/usr/bin/</filename> directory:

      </para>

      <screen>
        % <userinput>scons -Q -n</userinput>
        Install file: "foo.in" as "/usr/bin/foo.in"
      </screen>

      <para>

      But specifying <literal>--prefix=/tmp/install</literal>
      on the command line causes the file to be installed in the
      <filename>/tmp/install/usr/bin/</filename> directory:

      </para>

      <screen>
        % <userinput>scons -Q -n --prefix=/tmp/install</userinput>
        Install file: "foo.in" as "/tmp/install/usr/bin/foo.in"
      </screen>

    </section>

  </section>

  <section id="sect-command-line-variables">
  <title>Command-Line <varname>variable</varname>=<varname>value</varname> Build Variables</title>

    <para>

    You may want to control various aspects
    of your build by allowing the user
    to specify <varname>variable</varname>=<varname>value</varname>
    values on the command line.
    For example, suppose you
    want users to be able to
    build a debug version of a program
    by running &SCons; as follows:

    </para>

    <screen>
      % <userinput>scons -Q debug=1</userinput>
    </screen>

    <para>

    &SCons; provides an &ARGUMENTS; dictionary
    that stores all of the
    <varname>variable</varname>=<varname>value</varname>
    assignments from the command line.
    This allows you to modify
    aspects of your build in response
    to specifications on the command line.
    (Note that unless you want to require
    that users <emphasis>always</emphasis>
    specify a variable,
    you probably want to use
    the Python
    <literal>ARGUMENTS.get()</literal> function,
    which allows you to specify a default value
    to be used if there is no specification
    on the command line.)

    </para>

    <para>

    The following code sets the &cv-link-CCFLAGS; construction
    variable in response to the <varname>debug</varname>
    flag being set in the &ARGUMENTS; dictionary:

    </para>

    <programlisting>
       env = Environment()
       debug = ARGUMENTS.get('debug', 0)
       if int(debug):
           env.Append(CCFLAGS = '-g')
       env.Program('prog.c')
    </programlisting>

    <para>

    This results in the <varname>-g</varname>
    compiler option being used when
    <literal>debug=1</literal>
    is used on the command line:

    </para>

    <screen>
       % <userinput>scons -Q debug=0</userinput>
       cc -o prog.o -c prog.c
       cc -o prog prog.o
       % <userinput>scons -Q debug=0</userinput>
       scons: `.' is up to date.
       % <userinput>scons -Q debug=1</userinput>
       cc -o prog.o -c -g prog.c
       cc -o prog prog.o
       % <userinput>scons -Q debug=1</userinput>
       scons: `.' is up to date.
    </screen>

    <para>

    Notice that &SCons; keeps track of
    the last values used to build the object files,
    and as a result correctly rebuilds
    the object and executable files
    only when the value of the <literal>debug</literal>
    argument has changed.

    </para>

    <para>

    The &ARGUMENTS; dictionary has two minor drawbacks.
    First, because it is a dictionary,
    it can only store one value for each specified keyword,
    and thus only "remembers" the last setting
    for each keyword on the command line.
    This makes the &ARGUMENTS; dictionary
    inappropriate if users should be able to
    specify multiple values
    on the command line for a given keyword.
    Second, it does not preserve
    the order in which the variable settings
    were specified,
    which is a problem if
    you want the configuration to
    behave differently in response
    to the order in which the build
    variable settings were specified on the command line.

    </para>

    <para>

    To accomodate these requirements,
    &SCons; provides an &ARGLIST; variable
    that gives you direct access to
    <varname>variable</varname>=<varname>value</varname>
    settings on the command line,
    in the exact order they were specified,
    and without removing any duplicate settings.
    Each element in the &ARGLIST; variable
    is itself a two-element list
    containing the keyword and the value
    of the setting,
    and you must loop through,
    or otherwise select from,
    the elements of &ARGLIST; to
    process the specific settings you want
    in whatever way is appropriate for your configuration.
    For example,
    the following code to let the user
    add to the &CPPDEFINES; construction variable
    by specifying multiple
    <varname>define=</varname>
    settings on the command line:

    </para>

    <programlisting>
       cppdefines = []
       for key, value in ARGLIST:
           if key == 'define':
               cppdefines.append(value)
       env = Environment(CPPDEFINES = cppdefines)
       env.Object('prog.c')
    </programlisting>

    <para>

    Yields the following output:

    </para>

    <screen>
       % <userinput>scons -Q define=FOO</userinput>
       cc -o prog.o -c -DFOO prog.c
       % <userinput>scons -Q define=FOO define=BAR</userinput>
       cc -o prog.o -c -DFOO -DBAR prog.c
    </screen>

    <para>

    Note that the &ARGLIST; and &ARGUMENTS;
    variables do not interfere with each other,
    but merely provide slightly different views
    into how the user specified
    <varname>variable</varname>=<varname>value</varname>
    settings on the command line.
    You can use both variables in the same
    &SCons; configuration.
    In general, the &ARGUMENTS; dictionary
    is more convenient to use,
    (since you can just fetch variable
    settings through a dictionary access),
    and the &ARGLIST; list
    is more flexible
    (since you can examine the
    specific order in which
    the user's command-line variabe settings).

    </para>

    <section>
    <title>Controlling Command-Line Build Variables</title>

      <para>

      Being able to use a command-line build variable like
      <literal>debug=1</literal> is handy,
      but it can be a chore to write specific Python code
      to recognize each such variable,
      check for errors and provide appropriate messages,
      and apply the values to a construction variable.
      To help with this,
      &SCons; supports a class to
      define such build variables easily,
      and a mechanism to apply the
      build variables to a construction environment.
      This allows you to control how the build variables affect
      construction environments.

      </para>

      <para>

      For example, suppose that you want users to set
      a &RELEASE; construction variable on the
      command line whenever the time comes to build
      a program for release,
      and that the value of this variable
      should be added to the command line
      with the appropriate <literal>-D</literal> option
      (or other command line option)
      to pass the value to the C compiler.
      Here's how you might do that by setting
      the appropriate value in a dictionary for the
      &cv-link-CPPDEFINES; construction variable:

      </para>

      <programlisting>
           vars = Variables()
           vars.Add('RELEASE', 'Set to 1 to build for release', 0)
           env = Environment(variables = vars,
                             CPPDEFINES={'RELEASE_BUILD' : '${RELEASE}'})
           env.Program(['foo.c', 'bar.c'])
      </programlisting>

      <para>

      This &SConstruct; file first creates a &Variables; object
      (the <literal>vars = Variables()</literal> call),
      and then uses the object's &Add;
      method to indicate that the &RELEASE;
      variable can be set on the command line,
      and that its default value will be <literal>0</literal>
      (the third argument to the &Add; method).
      The second argument is a line of help text;
      we'll learn how to use it in the next section.

      </para>

      <para>

      We then pass the created &Variables;
      object as a &variables; keyword argument
      to the &Environment; call
      used to create the construction environment.
      This then allows a user to set the
      &RELEASE; build variable on the command line
      and have the variable show up in
      the command line used to build each object from
      a C source file:

      </para>

      <screen>
        % <userinput>scons -Q RELEASE=1</userinput>
        cc -o bar.o -c -DRELEASE_BUILD=1 bar.c
        cc -o foo.o -c -DRELEASE_BUILD=1 foo.c
        cc -o foo foo.o bar.o
      </screen>

      <para>

      NOTE:  Before &SCons; release 0.98.1, these build variables
      were known as "command-line build options."
      The class was actually named the &Options; class,
      and in the sections below,
      the various functions were named
      &BoolOption;, &EnumOption;, &ListOption;,
      &PathOption;, &PackageOption; and &AddOptions;.
      These older names still work,
      and you may encounter them in older
      &SConscript; fles,
      but they have been officially deprecated
      as of &SCons; version 2.0.

      </para>

    </section>

    <section>
    <title>Providing Help for Command-Line Build Variables</title>

      <para>

      To make command-line build variables most useful,
      you ideally want to provide
      some help text that will describe
      the available variables
      when the user runs <literal>scons -h</literal>.
      You could write this text by hand,
      but &SCons; provides an easier way.
      &Variables; objects support a
      &GenerateHelpText; method
      that will, as its name suggests,
      generate text that describes
      the various variables that
      have been added to it.
      You then pass the output from this method to
      the &Help; function:

      </para>

      <programlisting>
           vars = Variables('custom.py')
           vars.Add('RELEASE', 'Set to 1 to build for release', 0)
           env = Environment(variables = vars)
           Help(vars.GenerateHelpText(env))
      </programlisting>

      <para>

      &SCons; will now display some useful text
      when the <literal>-h</literal> option is used:

      </para>

      <screen>
        % <userinput>scons -Q -h</userinput>
        
        RELEASE: Set to 1 to build for release
            default: 0
            actual: 0
        
        Use scons -H for help about command-line options.
      </screen>

      <para>

      Notice that the help output shows the default value,
      and the current actual value of the build variable.

      </para>

    </section>

    <section>
    <title>Reading Build Variables From a File</title>

      <para>

      Giving the user a way to specify the
      value of a build variable on the command line
      is useful,
      but can still be tedious
      if users must specify the variable
      every time they run &SCons;.
      We can let users provide customized build variable settings
      in a local file by providing a
      file name when we create the
      &Variables; object:

      </para>

      <programlisting>
           vars = Variables('custom.py')
           vars.Add('RELEASE', 'Set to 1 to build for release', 0)
           env = Environment(variables = vars,
                             CPPDEFINES={'RELEASE_BUILD' : '${RELEASE}'})
           env.Program(['foo.c', 'bar.c'])
           Help(vars.GenerateHelpText(env))
      </programlisting>

      <para>

      This then allows the user to control the &RELEASE;
      variable by setting it in the &custom_py; file:

      </para>

      <programlisting>
        RELEASE = 1
        </programlisting>

      <para>

      Note that this file is actually executed
      like a Python script.
      Now when we run &SCons;:

      </para>

      <screen>
        % <userinput>scons -Q</userinput>
        cc -o bar.o -c -DRELEASE_BUILD=1 bar.c
        cc -o foo.o -c -DRELEASE_BUILD=1 foo.c
        cc -o foo foo.o bar.o
      </screen>

      <para>

      And if we change the contents of &custom_py; to:

      </para>

      <programlisting>
        RELEASE = 0
      </programlisting>

      <para>

      The object files are rebuilt appropriately
      with the new variable:

      </para>

      <screen>
        % <userinput>scons -Q</userinput>
        cc -o bar.o -c -DRELEASE_BUILD=0 bar.c
        cc -o foo.o -c -DRELEASE_BUILD=0 foo.c
        cc -o foo foo.o bar.o
      </screen>

    </section>

    <section>
    <title>Pre-Defined Build Variable Functions</title>

      <para>

      &SCons; provides a number of functions
      that provide ready-made behaviors
      for various types of command-line build variables.

      </para>

      <section>
      <title>True/False Values:  the &BoolVariable; Build Variable Function</title>

        <para>

        It's often handy to be able to specify a
        variable that controls a simple Boolean variable
        with a &true; or &false; value.
        It would be even more handy to accomodate
        users who have different preferences for how to represent
        &true; or &false; values.
        The &BoolVariable; function
        makes it easy to accomodate these
        common representations of
        &true; or &false;.

        </para>

        <para>

        The &BoolVariable; function takes three arguments:
        the name of the build variable,
        the default value of the build variable,
        and the help string for the variable.
        It then returns appropriate information for
        passing to the &Add; method of a &Variables; object, like so:

        </para>

        <programlisting>
             vars = Variables('custom.py')
             vars.Add(BoolVariable('RELEASE', 'Set to build for release', 0))
             env = Environment(variables = vars,
                               CPPDEFINES={'RELEASE_BUILD' : '${RELEASE}'})
             env.Program('foo.c')
        </programlisting>

        <para>

        With this build variable,
        the &RELEASE; variable can now be enabled by
        setting it to the value <literal>yes</literal>
        or <literal>t</literal>:

        </para>

        <screen>
          % <userinput>scons -Q RELEASE=yes foo.o</userinput>
          cc -o foo.o -c -DRELEASE_BUILD=True foo.c
        </screen>

        <screen>
          % <userinput>scons -Q RELEASE=t foo.o</userinput>
          cc -o foo.o -c -DRELEASE_BUILD=True foo.c
        </screen>

        <para>

        Other values that equate to &true; include
        <literal>y</literal>,
        <literal>1</literal>,
        <literal>on</literal>
        and
        <literal>all</literal>.

        </para>

        <para>

        Conversely, &RELEASE; may now be given a &false;
        value by setting it to
        <literal>no</literal>
        or
        <literal>f</literal>:

        </para>

        <screen>
          % <userinput>scons -Q RELEASE=no foo.o</userinput>
          cc -o foo.o -c -DRELEASE_BUILD=False foo.c
        </screen>

        <screen>
          % <userinput>scons -Q RELEASE=f foo.o</userinput>
          cc -o foo.o -c -DRELEASE_BUILD=False foo.c
        </screen>

        <para>

        Other values that equate to &false; include
        <literal>n</literal>,
        <literal>0</literal>,
        <literal>off</literal>
        and
        <literal>none</literal>.

        </para>

        <para>

        Lastly, if a user tries to specify
        any other value,
        &SCons; supplies an appropriate error message:

        </para>

        <screen>
          % <userinput>scons -Q RELEASE=bad_value foo.o</userinput>
          
          scons: *** Error converting option: RELEASE
          Invalid value for boolean option: bad_value
          File "/home/my/project/SConstruct", line 4, in &lt;module&gt;
        </screen>

      </section>

      <section>
      <title>Single Value From a List:  the &EnumVariable; Build Variable Function</title>

        <para>

        Suppose that we want a user to be able to
        set a &COLOR; variable
        that selects a background color to be
        displayed by an application,
        but that we want to restrict the
        choices to a specific set of allowed colors.
        This can be set up quite easily
        using the &EnumVariable;,
        which takes a list of &allowed_values;
        in addition to the variable name,
        default value,
        and help text arguments:

        </para>

        <programlisting>
             vars = Variables('custom.py')
             vars.Add(EnumVariable('COLOR', 'Set background color', 'red',
                                 allowed_values=('red', 'green', 'blue')))
             env = Environment(variables = vars,
                               CPPDEFINES={'COLOR' : '"${COLOR}"'})
             env.Program('foo.c')
        </programlisting>

        <para>

        The user can now explicity set the &COLOR; build variable
        to any of the specified allowed values:

        </para>

        <screen>
          % <userinput>scons -Q COLOR=red foo.o</userinput>
          cc -o foo.o -c -DCOLOR="red" foo.c
          % <userinput>scons -Q COLOR=blue foo.o</userinput>
          cc -o foo.o -c -DCOLOR="blue" foo.c
          % <userinput>scons -Q COLOR=green foo.o</userinput>
          cc -o foo.o -c -DCOLOR="green" foo.c
        </screen>

        <para>

        But, almost more importantly,
        an attempt to set &COLOR;
        to a value that's not in the list
        generates an error message:

        </para>

        <screen>
          % <userinput>scons -Q COLOR=magenta foo.o</userinput>
          
          scons: *** Invalid value for option COLOR: magenta
          File "/home/my/project/SConstruct", line 5, in &lt;module&gt;
        </screen>

        <para>

        The &EnumVariable; function also supports a way
        to map alternate names to allowed values.
        Suppose, for example,
        that we want to allow the user
        to use the word <literal>navy</literal> as a synonym for
        <literal>blue</literal>.
        We do this by adding a &map; dictionary
        that will map its key values
        to the desired legal value:

        </para>

        <programlisting>
             vars = Variables('custom.py')
             vars.Add(EnumVariable('COLOR', 'Set background color', 'red',
                                 allowed_values=('red', 'green', 'blue'),
                                 map={'navy':'blue'}))
             env = Environment(variables = vars,
                               CPPDEFINES={'COLOR' : '"${COLOR}"'})
             env.Program('foo.c')
        </programlisting>

        <para>

        As desired, the user can then use
        <literal>navy</literal> on the command line,
        and &SCons; will translate it into <literal>blue</literal>
        when it comes time to use the &COLOR;
        variable to build a target:

        </para>

        <screen>
          % <userinput>scons -Q COLOR=navy foo.o</userinput>
          cc -o foo.o -c -DCOLOR="blue" foo.c
        </screen>

        <para>

        By default, when using the &EnumVariable; function,
        arguments that differ
        from the legal values
        only in case
        are treated as illegal values:

        </para>

        <screen>
          % <userinput>scons -Q COLOR=Red foo.o</userinput>
          
          scons: *** Invalid value for option COLOR: Red
          File "/home/my/project/SConstruct", line 5, in &lt;module&gt;
          % <userinput>scons -Q COLOR=BLUE foo.o</userinput>
          
          scons: *** Invalid value for option COLOR: BLUE
          File "/home/my/project/SConstruct", line 5, in &lt;module&gt;
          % <userinput>scons -Q COLOR=nAvY foo.o</userinput>
          
          scons: *** Invalid value for option COLOR: nAvY
          File "/home/my/project/SConstruct", line 5, in &lt;module&gt;
        </screen>

        <para>

        The &EnumVariable; function can take an additional
        &ignorecase; keyword argument that,
        when set to <literal>1</literal>,
        tells &SCons; to allow case differences
        when the values are specified:

        </para>

        <programlisting>
             vars = Variables('custom.py')
             vars.Add(EnumVariable('COLOR', 'Set background color', 'red',
                                 allowed_values=('red', 'green', 'blue'),
                                 map={'navy':'blue'},
                                 ignorecase=1))
             env = Environment(variables = vars,
                               CPPDEFINES={'COLOR' : '"${COLOR}"'})
             env.Program('foo.c')
        </programlisting>

        <para>

        Which yields the output:

        </para>

        <screen>
          % <userinput>scons -Q COLOR=Red foo.o</userinput>
          cc -o foo.o -c -DCOLOR="Red" foo.c
          % <userinput>scons -Q COLOR=BLUE foo.o</userinput>
          cc -o foo.o -c -DCOLOR="BLUE" foo.c
          % <userinput>scons -Q COLOR=nAvY foo.o</userinput>
          cc -o foo.o -c -DCOLOR="blue" foo.c
          % <userinput>scons -Q COLOR=green foo.o</userinput>
          cc -o foo.o -c -DCOLOR="green" foo.c
        </screen>

        <para>

        Notice that an &ignorecase; value of <literal>1</literal>
        preserves the case-spelling that the user supplied.
        If you want &SCons; to translate the names
        into lower-case,
        regardless of the case used by the user,
        specify an &ignorecase; value of <literal>2</literal>:

        </para>

        <programlisting>
             vars = Variables('custom.py')
             vars.Add(EnumVariable('COLOR', 'Set background color', 'red',
                                 allowed_values=('red', 'green', 'blue'),
                                 map={'navy':'blue'},
                                 ignorecase=2))
             env = Environment(variables = vars,
                               CPPDEFINES={'COLOR' : '"${COLOR}"'})
             env.Program('foo.c')
        </programlisting>

        <para>

        Now &SCons; will use values of
        <literal>red</literal>,
        <literal>green</literal> or
        <literal>blue</literal>
        regardless of how the user spells
        those values on the command line:

        </para>

        <screen>
          % <userinput>scons -Q COLOR=Red foo.o</userinput>
          cc -o foo.o -c -DCOLOR="red" foo.c
          % <userinput>scons -Q COLOR=nAvY foo.o</userinput>
          cc -o foo.o -c -DCOLOR="blue" foo.c
          % <userinput>scons -Q COLOR=GREEN foo.o</userinput>
          cc -o foo.o -c -DCOLOR="green" foo.c
        </screen>

      </section>

      <section>
      <title>Multiple Values From a List:  the &ListVariable; Build Variable Function</title>

        <para>

        Another way in which you might want to allow users
        to control a build variable is to
        specify a list of one or more legal values.
        &SCons; supports this through the &ListVariable; function.
        If, for example, we want a user to be able to set a
        &COLORS; variable to one or more of the legal list of values:

        </para>

        <programlisting>
             vars = Variables('custom.py')
             vars.Add(ListVariable('COLORS', 'List of colors', 0,
                                 ['red', 'green', 'blue']))
             env = Environment(variables = vars,
                               CPPDEFINES={'COLORS' : '"${COLORS}"'})
             env.Program('foo.c')
        </programlisting>

        <para>

        A user can now specify a comma-separated list
        of legal values,
        which will get translated into a space-separated
        list for passing to the any build commands:

        </para>

        <screen>
          % <userinput>scons -Q COLORS=red,blue foo.o</userinput>
          cc -o foo.o -c -DCOLORS="red blue" foo.c
          % <userinput>scons -Q COLORS=blue,green,red foo.o</userinput>
          cc -o foo.o -c -DCOLORS="blue green red" foo.c
        </screen>

        <para>

        In addition, the &ListVariable; function
        allows the user to specify explicit keywords of
        &all; or &none;
        to select all of the legal values,
        or none of them, respectively:

        </para>

        <screen>
          % <userinput>scons -Q COLORS=all foo.o</userinput>
          cc -o foo.o -c -DCOLORS="red green blue" foo.c
          % <userinput>scons -Q COLORS=none foo.o</userinput>
          cc -o foo.o -c -DCOLORS="" foo.c
        </screen>

        <para>

        And, of course, an illegal value
        still generates an error message:

        </para>

        <screen>
          % <userinput>scons -Q COLORS=magenta foo.o</userinput>
          
          scons: *** Error converting option: COLORS
          Invalid value(s) for option: magenta
          File "/home/my/project/SConstruct", line 5, in &lt;module&gt;
        </screen>

      </section>

      <section>
      <title>Path Names:  the &PathVariable; Build Variable Function</title>

        <para>

        &SCons; supports a &PathVariable; function
        to make it easy to create a build variable
        to control an expected path name.
        If, for example, you need to
        define a variable in the preprocessor
        that controls the location of a
        configuration file:

        </para>

        <programlisting>
             vars = Variables('custom.py')
             vars.Add(PathVariable('CONFIG',
                                 'Path to configuration file',
                                 '/etc/my_config'))
             env = Environment(variables = vars,
                               CPPDEFINES={'CONFIG_FILE' : '"$CONFIG"'})
             env.Program('foo.c')
        </programlisting>

        <para>

        This then allows the user to
        override the &CONFIG; build variable
        on the command line as necessary:

        </para>

        <screen>
          % <userinput>scons -Q foo.o</userinput>
          cc -o foo.o -c -DCONFIG_FILE="/etc/my_config" foo.c
          % <userinput>scons -Q CONFIG=/usr/local/etc/other_config foo.o</userinput>
          scons: `foo.o' is up to date.
        </screen>

        <para>

        By default, &PathVariable; checks to make sure
        that the specified path exists and generates an error if it
        doesn't:

        </para>

        <screen>
          % <userinput>scons -Q CONFIG=/does/not/exist foo.o</userinput>
          
          scons: *** Path for option CONFIG does not exist: /does/not/exist
          File "/home/my/project/SConstruct", line 6, in &lt;module&gt;
        </screen>

        <para>

        &PathVariable; provides a number of methods
        that you can use to change this behavior.
        If you want to ensure that any specified paths are,
        in fact, files and not directories,
        use the &PathVariable_PathIsFile; method:

        </para>

        <programlisting>
             vars = Variables('custom.py')
             vars.Add(PathVariable('CONFIG',
                                 'Path to configuration file',
                                 '/etc/my_config',
                                 PathVariable.PathIsFile))
             env = Environment(variables = vars,
                               CPPDEFINES={'CONFIG_FILE' : '"$CONFIG"'})
             env.Program('foo.c')
        </programlisting>

        <para>

        Conversely, to ensure that any specified paths are
        directories and not files,
        use the &PathVariable_PathIsDir; method:

        </para>

        <programlisting>
             vars = Variables('custom.py')
             vars.Add(PathVariable('DBDIR',
                                 'Path to database directory',
                                 '/var/my_dbdir',
                                 PathVariable.PathIsDir))
             env = Environment(variables = vars,
                               CPPDEFINES={'DBDIR' : '"$DBDIR"'})
             env.Program('foo.c')
        </programlisting>

        <para>

        If you want to make sure that any specified paths
        are directories,
        and you would like the directory created
        if it doesn't already exist,
        use the &PathVariable_PathIsDirCreate; method:

        </para>

        <programlisting>
             vars = Variables('custom.py')
             vars.Add(PathVariable('DBDIR',
                                 'Path to database directory',
                                 '/var/my_dbdir',
                                 PathVariable.PathIsDirCreate))
             env = Environment(variables = vars,
                               CPPDEFINES={'DBDIR' : '"$DBDIR"'})
             env.Program('foo.c')
        </programlisting>

        <para>

        Lastly, if you don't care whether the path exists,
        is a file, or a directory,
        use the &PathVariable_PathAccept; method
        to accept any path that the user supplies:

        </para>

        <programlisting>
             vars = Variables('custom.py')
             vars.Add(PathVariable('OUTPUT',
                                 'Path to output file or directory',
                                 None,
                                 PathVariable.PathAccept))
             env = Environment(variables = vars,
                               CPPDEFINES={'OUTPUT' : '"$OUTPUT"'})
             env.Program('foo.c')
        </programlisting>

      </section>

      <section>
      <title>Enabled/Disabled Path Names: the &PackageVariable; Build Variable Function</title>

        <para>

        Sometimes you want to give users
        even more control over a path name variable,
        allowing them to explicitly enable or
        disable the path name
        by using <literal>yes</literal> or <literal>no</literal> keywords,
        in addition to allow them
        to supply an explicit path name.
        &SCons; supports the &PackageVariable;
        function to support this:

        </para>

        <programlisting>
             vars = Variables('custom.py')
             vars.Add(PackageVariable('PACKAGE',
                                    'Location package',
                                    '/opt/location'))
             env = Environment(variables = vars,
                               CPPDEFINES={'PACKAGE' : '"$PACKAGE"'})
             env.Program('foo.c')
        </programlisting>

        <para>

        When the &SConscript; file uses the &PackageVariable; funciton,
        user can now still use the default
        or supply an overriding path name,
        but can now explicitly set the
        specified variable to a value
        that indicates the package should be enabled
        (in which case the default should be used)
        or disabled:

        </para>

        <screen>
          % <userinput>scons -Q foo.o</userinput>
          cc -o foo.o -c -DPACKAGE="/opt/location" foo.c
          % <userinput>scons -Q PACKAGE=/usr/local/location foo.o</userinput>
          cc -o foo.o -c -DPACKAGE="/usr/local/location" foo.c
          % <userinput>scons -Q PACKAGE=yes foo.o</userinput>
          cc -o foo.o -c -DPACKAGE="True" foo.c
          % <userinput>scons -Q PACKAGE=no foo.o</userinput>
          cc -o foo.o -c -DPACKAGE="False" foo.c
        </screen>

      </section>

    </section>

    <section>
    <title>Adding Multiple Command-Line Build Variables at Once</title>

      <para>

      Lastly, &SCons; provides a way to add
      multiple build variables to a &Variables; object at once.
      Instead of having to call the &Add; method
      multiple times,
      you can call the &AddVariables;
      method with a list of build variables
      to be added to the object.
      Each build variable is specified
      as either a tuple of arguments,
      just like you'd pass to the &Add; method itself,
      or as a call to one of the pre-defined
      functions for pre-packaged command-line build variables.
      in any order:

      </para>

      <programlisting>
          vars = Variables()
          vars.AddVariables(
              ('RELEASE', 'Set to 1 to build for release', 0),
              ('CONFIG', 'Configuration file', '/etc/my_config'),
              BoolVariable('warnings', 'compilation with -Wall and similiar', 1),
              EnumVariable('debug', 'debug output and symbols', 'no',
                         allowed_values=('yes', 'no', 'full'),
                         map={}, ignorecase=0),  # case sensitive
              ListVariable('shared',
                         'libraries to build as shared libraries',
                         'all',
                         names = list_of_libs),
              PackageVariable('x11',
                            'use X11 installed here (yes = search some places)',
                            'yes'),
              PathVariable('qtdir', 'where the root of Qt is installed', qtdir),
          )
      </programlisting>

      <para>
      </para>

    </section>

    <section>
    <title>Handling Unknown Command-Line Build Variables:  the &UnknownVariables; Function</title>

      <para>

      Users may, of course,
      occasionally misspell variable names in their command-line settings.
      &SCons; does not generate an error or warning
      for any unknown variables the users specifies on the command line.
      (This is in no small part because you may be
      processing the arguments directly using the &ARGUMENTS; dictionary,
      and therefore &SCons; can't know in the general case
      whether a given "misspelled" variable is
      really unknown and a potential problem,
      or something that your &SConscript; file
      will handle directly with some Python code.)

      </para>

      <para>

      If, however, you're using a &Variables; object to
      define a specific set of command-line build variables
      that you expect users to be able to set,
      you may want to provide an error
      message or warning of your own
      if the user supplies a variable setting
      that is <emphasis>not</emphasis> among
      the defined list of variable names known to the &Variables; object.
      You can do this by calling the &UnknownVariables;
      method of the &Variables; object:

      </para>

      <programlisting>
           vars = Variables(None)
           vars.Add('RELEASE', 'Set to 1 to build for release', 0)
           env = Environment(variables = vars,
                             CPPDEFINES={'RELEASE_BUILD' : '${RELEASE}'})
           unknown = vars.UnknownVariables()
           if unknown:
               print "Unknown variables:", unknown.keys()
               Exit(1)
           env.Program('foo.c')
      </programlisting>

      <para>

      The &UnknownVariables; method returns a dictionary
      containing the keywords and values
      of any variables the user specified on the command line
      that are <emphasis>not</emphasis>
      among the variables known to the &Variables; object
      (from having been specified using
      the &Variables; object's&Add; method).
      In the examble above,
      we check for whether the dictionary
      returned by the &UnknownVariables; is non-empty,
      and if so print the Python list
      containing the names of the unknwown variables
      and then call the &Exit; function
      to terminate &SCons;:

      </para>

      <screen>
        % <userinput>scons -Q NOT_KNOWN=foo</userinput>
        Unknown variables: ['NOT_KNOWN']
      </screen>

      <para>

      Of course, you can process the items in the
      dictionary returned by the &UnknownVariables; function
      in any way appropriate to your build configuration,
      including just printing a warning message
      but not exiting,
      logging an error somewhere,
      etc.

      </para>

      <para>

      Note that you must delay the call of &UnknownVariables;
      until after you have applied the &Variables; object
      to a construction environment
      with the <literal>variables=</literal>
      keyword argument of an &Environment; call.

      </para>

    </section>

  </section>

  <section id="sect-command-line-targets">
  <title>Command-Line Targets</title>

    <section>
    <title>Fetching Command-Line Targets: the &COMMAND_LINE_TARGETS; Variable</title>

      <para>

      &SCons; supports a &COMMAND_LINE_TARGETS; variable
      that lets you fetch the list of targets that the
      user specified on the command line.
      You can use the targets to manipulate the
      build in any way you wish.
      As a simple example,
      suppose that you want to print a reminder
      to the user whenever a specific program is built.
      You can do this by checking for the
      target in the &COMMAND_LINE_TARGETS; list:

      </para>

      <programlisting>
        if 'bar' in COMMAND_LINE_TARGETS:
            print "Don't forget to copy `bar' to the archive!"
        Default(Program('foo.c'))
        Program('bar.c')
      </programlisting>

      <para>

      Then, running &SCons; with the default target
      works as it always does,
      but explicity specifying the &bar; target
      on the command line generates the warning message:

      </para>

      <screen>
        % <userinput>scons -Q</userinput>
        cc -o foo.o -c foo.c
        cc -o foo foo.o
        % <userinput>scons -Q bar</userinput>
        Don't forget to copy `bar' to the archive!
        cc -o bar.o -c bar.c
        cc -o bar bar.o
      </screen>

      <para>

      Another practical use for the &COMMAND_LINE_TARGETS; variable
      might be to speed up a build
      by only reading certain subsidiary &SConscript;
      files if a specific target is requested.

      </para>

    </section>

    <section>
    <title>Controlling the Default Targets:  the &Default; Function</title>

      <para>

      One of the most basic things you can control
      is which targets &SCons; will build by default--that is,
      when there are no targets specified on the command line.
      As mentioned previously,
      &SCons; will normally build every target
      in or below the current directory
      by default--that is, when you don't
      explicitly specify one or more targets
      on the command line.
      Sometimes, however, you may want
      to specify explicitly that only
      certain programs, or programs in certain directories,
      should be built by default.
      You do this with the &Default; function:

      </para>

      <programlisting>
         env = Environment()
         hello = env.Program('hello.c')
         env.Program('goodbye.c')
         Default(hello)
      </programlisting>

      <para>

      This &SConstruct; file knows how to build two programs,
      &hello; and &goodbye;,
      but only builds the
      &hello; program by default:

      </para>

      <screen>
         % <userinput>scons -Q</userinput>
         cc -o hello.o -c hello.c
         cc -o hello hello.o
         % <userinput>scons -Q</userinput>
         scons: `hello' is up to date.
         % <userinput>scons -Q goodbye</userinput>
         cc -o goodbye.o -c goodbye.c
         cc -o goodbye goodbye.o
      </screen>

      <para>

      Note that, even when you use the &Default;
      function in your &SConstruct; file,
      you can still explicitly specify the current directory
      (<literal>.</literal>) on the command line
      to tell &SCons; to build
      everything in (or below) the current directory:

      </para>

      <screen>
         % <userinput>scons -Q .</userinput>
         cc -o goodbye.o -c goodbye.c
         cc -o goodbye goodbye.o
         cc -o hello.o -c hello.c
         cc -o hello hello.o
      </screen>

      <para>

      You can also call the &Default;
      function more than once,
      in which case each call
      adds to the list of targets to be built by default:

      </para>

      <programlisting>
         env = Environment()
         prog1 = env.Program('prog1.c')
         Default(prog1)
         prog2 = env.Program('prog2.c')
         prog3 = env.Program('prog3.c')
         Default(prog3)
      </programlisting>

      <para>

      Or you can specify more than one target
      in a single call to the &Default; function:

      </para>

      <programlisting>
         env = Environment()
         prog1 = env.Program('prog1.c')
         prog2 = env.Program('prog2.c')
         prog3 = env.Program('prog3.c')
         Default(prog1, prog3)
      </programlisting>

      <para>

      Either of these last two examples
      will build only the
      <application>prog1</application>
      and
      <application>prog3</application>
      programs by default:

      </para>

      <screen>
         % <userinput>scons -Q</userinput>
         cc -o prog1.o -c prog1.c
         cc -o prog1 prog1.o
         cc -o prog3.o -c prog3.c
         cc -o prog3 prog3.o
         % <userinput>scons -Q .</userinput>
         cc -o prog2.o -c prog2.c
         cc -o prog2 prog2.o
      </screen>

      <para>

      You can list a directory as
      an argument to &Default;:

      </para>

      <programlisting>
         env = Environment()
         env.Program(['prog1/main.c', 'prog1/foo.c'])
         env.Program(['prog2/main.c', 'prog2/bar.c'])
         Default('prog1')
      </programlisting>

      <para>

      In which case only the target(s) in that
      directory will be built by default:

      </para>

      <screen>
         % <userinput>scons -Q</userinput>
         cc -o prog1/foo.o -c prog1/foo.c
         cc -o prog1/main.o -c prog1/main.c
         cc -o prog1/main prog1/main.o prog1/foo.o
         % <userinput>scons -Q</userinput>
         scons: `prog1' is up to date.
         % <userinput>scons -Q .</userinput>
         cc -o prog2/bar.o -c prog2/bar.c
         cc -o prog2/main.o -c prog2/main.c
         cc -o prog2/main prog2/main.o prog2/bar.o
      </screen>

      <para>

      Lastly, if for some reason you don't want
      any targets built by default,
      you can use the Python <literal>None</literal>
      variable:

      </para>

      <programlisting>
         env = Environment()
         prog1 = env.Program('prog1.c')
         prog2 = env.Program('prog2.c')
         Default(None)
      </programlisting>

      <para>

      Which would produce build output like:

      </para>

      <screen>
         % <userinput>scons -Q</userinput>
         scons: *** No targets specified and no Default() targets found.  Stop.
         % <userinput>scons -Q .</userinput>
         cc -o prog1.o -c prog1.c
         cc -o prog1 prog1.o
         cc -o prog2.o -c prog2.c
         cc -o prog2 prog2.o
      </screen>

      <section>
      <title>Fetching the List of Default Targets: the &DEFAULT_TARGETS; Variable</title>

        <para>

        &SCons; supports a &DEFAULT_TARGETS; variable
        that lets you get at the current list of default targets.
        The &DEFAULT_TARGETS; variable has
        two important differences from the &COMMAND_LINE_TARGETS; variable.
        First, the &DEFAULT_TARGETS; variable is a list of
        internal &SCons; nodes,
        so you need to convert the list elements to strings
        if you want to print them or look for a specific target name.
        Fortunately, you can do this easily
        by using the Python <function>map</function> function
        to run the list through <function>str</function>:

        </para>

        <programlisting>
           prog1 = Program('prog1.c')
           Default(prog1)
           print "DEFAULT_TARGETS is", map(str, DEFAULT_TARGETS)
        </programlisting>

        <para>

        (Keep in mind that all of the manipulation of the
        &DEFAULT_TARGETS; list takes place during the
        first phase when &SCons; is reading up the &SConscript; files,
        which is obvious if
        we leave off the <literal>-Q</literal> flag when we run &SCons;:)

        </para>

        <screen>
           % <userinput>scons</userinput>
           scons: Reading SConscript files ...
           DEFAULT_TARGETS is ['prog1']
           scons: done reading SConscript files.
           scons: Building targets ...
           cc -o prog1.o -c prog1.c
           cc -o prog1 prog1.o
           scons: done building targets.
        </screen>

        <para>

        Second,
        the contents of the &DEFAULT_TARGETS; list change
        in response to calls to the &Default;: function,
        as you can see from the following &SConstruct; file:

        </para>

        <programlisting>
           prog1 = Program('prog1.c')
           Default(prog1)
           print "DEFAULT_TARGETS is now", map(str, DEFAULT_TARGETS)
           prog2 = Program('prog2.c')
           Default(prog2)
           print "DEFAULT_TARGETS is now", map(str, DEFAULT_TARGETS)
        </programlisting>

        <para>

        Which yields the output:

        </para>

        <screen>
           % <userinput>scons</userinput>
           scons: Reading SConscript files ...
           DEFAULT_TARGETS is now ['prog1']
           DEFAULT_TARGETS is now ['prog1', 'prog2']
           scons: done reading SConscript files.
           scons: Building targets ...
           cc -o prog1.o -c prog1.c
           cc -o prog1 prog1.o
           cc -o prog2.o -c prog2.c
           cc -o prog2 prog2.o
           scons: done building targets.
        </screen>

        <para>

        In practice, this simply means that you
        need to pay attention to the order in
        which you call the &Default; function
        and refer to the &DEFAULT_TARGETS; list,
        to make sure that you don't examine the
        list before you've added the default targets
        you expect to find in it.

        </para>

      </section>

    </section>

    <section>
    <title>Fetching the List of Build Targets, Regardless of Origin: the &BUILD_TARGETS; Variable</title>

      <para>

      We've already been introduced to the
      &COMMAND_LINE_TARGETS; variable,
      which contains a list of targets specified on the command line,
      and the &DEFAULT_TARGETS; variable,
      which contains a list of targets specified
      via calls to the &Default; method or function.
      Sometimes, however,
      you want a list of whatever targets
      &SCons; will try to build,
      regardless of whether the targets came from the
      command line or a &Default; call.
      You could code this up by hand, as follows:

      </para>

      <programlisting>
        if COMMAND_LINE_TARGETS:
            targets = COMMAND_LINE_TARGETS
        else:
            targets = DEFAULT_TARGETS
      </programlisting>

      <para>

      &SCons;, however, provides a convenient
      &BUILD_TARGETS; variable
      that eliminates the need for this by-hand manipulation.
      Essentially, the &BUILD_TARGETS; variable
      contains a list of the command-line targets,
      if any were specified,
      and if no command-line targets were specified,
      it contains a list of the targets specified
      via the &Default; method or function.

      </para>

      <para>

      Because &BUILD_TARGETS; may contain a list of &SCons; nodes,
      you must convert the list elements to strings
      if you want to print them or look for a specific target name,
      just like the &DEFAULT_TARGETS; list:

      </para>

      <programlisting>
        prog1 = Program('prog1.c')
        Program('prog2.c')
        Default(prog1)
        print "BUILD_TARGETS is", map(str, BUILD_TARGETS)
      </programlisting>

      <para>

      Notice how the value of &BUILD_TARGETS;
      changes depending on whether a target is
      specified on the command line:

      </para>

      <screen>
        % <userinput>scons -Q</userinput>
        BUILD_TARGETS is ['prog1']
        cc -o prog1.o -c prog1.c
        cc -o prog1 prog1.o
        % <userinput>scons -Q prog2</userinput>
        BUILD_TARGETS is ['prog2']
        cc -o prog2.o -c prog2.c
        cc -o prog2 prog2.o
        % <userinput>scons -Q -c .</userinput>
        BUILD_TARGETS is ['.']
        Removed prog1.o
        Removed prog1
        Removed prog2.o
        Removed prog2
      </screen>

    </section>

  </section>