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<chapter id="chap-command-line"
         xmlns="http://www.scons.org/dbxsd/v1.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://www.scons.org/dbxsd/v1.0 http://www.scons.org/dbxsd/v1.0/scons.xsd">
<title>Controlling a Build From the Command Line</title>

  <para>

  Software builds are rarely completely static,
  so &SCons; gives you a number of ways to help control
  build execution via instructions on the command line.
  The arguments that can be specified on
  the command line are broken down into three types:

  </para>

  <variablelist>

    <varlistentry>
    <term>Options</term>

    <listitem>
    <para>

    Command-line arguments that begin with a
    <literal>-</literal> (hyphen) characters
    are called <firstterm>options</firstterm>.
    &SCons; provides ways for you to examine
    and act on options and their values,
    as well as the ability to define custom options
    for your project.
    See <xref linkend="sect-command-line-options"></xref>, below.

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

    <varlistentry>
    <term>Variables</term>

    <listitem>
    <para>

    Command-line arguments containing an <literal>=</literal>
    (equal sign) character are called <firstterm>build variables</firstterm>
    (or just <firstterm>variables</firstterm>).
    &SCons; provides direct access to
    all of the build variable settings from the command line,
    as well as a higher-level interface that lets you
    define known build variables,
    including defining types, default values, help text,
    and automatic validation,
    as well as applying those to a &consenv;.
    See <xref linkend="sect-command-line-variables"></xref>, below.

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

    <varlistentry>
    <term>Targets</term>

    <listitem>
    <para>

    Command-line arguments that are neither options
    nor build variables
    (that is, do not begin with a hyphen
    and do not contain an equal sign)
    are considered <firstterm>targets</firstterm>
    that you are telling &SCons; 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 command-line options that control its behavior.
    A command-line option always begins with one
    or two hyphen (<literal>-</literal>) characters.
    The &SCons; manual page contains the description of
    the current options
    (see <ulink url="https://scons.org/doc/production/HTML/scons-man.html"/>).

    </para>

    <section id="sect-SCONSFLAGS">
    <title>How To Avoid Typing Command-Line Options Each Time:  the &SCONSFLAGS; Environment Variable</title>

      <para>

      You may find yourself using
      certain command-line options every time
      you run &SCons;.
      For example, you might find it saves time
      to specify <userinput>-j 2</userinput>
      to have &SCons; run up to two build commands in parallel.
      To avoid having to type <userinput>-j 2</userinput> by hand
      every time,
      you can set the external environment variable
      &SCONSFLAGS; to a string containing
      <option>-j 2</option>, as well as any other
      command-line options that you want &SCons; to always use.
      &SCONSFLAGS; is an exception to the usual rule that
      &SCons; itself avoids looking at environment variables from the
      shell you are running.

      </para>

      <para>

      If, for example,
      you are using a POSIX shell such as <command>bash</command>
      or <command>zsh</command>
      and you always want &SCons; to use the
      <option>-Q</option> option,
      you can set the &SCONSFLAGS;
      environment as follows:

      </para>

      <scons_example name="commandline_SCONSFLAGS">
        <file name="SConstruct">
def b(target, source, env):
    pass

def s(target, source, env):
    return "    ... [build output] ..."

a = Action(b, strfunction=s)
env = Environment(BUILDERS={'A': Builder(action=a)})
env.A('foo.out', 'foo.in')
        </file>
        <file name="foo.in">
foo.in
        </file>
      </scons_example>

      <scons_output example="commandline_SCONSFLAGS" suffix="1">
        <scons_output_command>scons</scons_output_command>
        <scons_output_command>export SCONSFLAGS="-Q"</scons_output_command>
        <scons_output_command environment="SCONSFLAGS=-Q">scons</scons_output_command>
      </scons_output>

      <para>

      For &csh;-style shells on POSIX systems
      you can set the &SCONSFLAGS; environment variable as follows:

      </para>

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

      <para>

      For the Windows command shell (<command>cmd</command>)
      you can set the &SCONSFLAGS; environment variable as follows:

      </para>

      <screen>
C:\Users\foo> <userinput>set SCONSFLAGS="-Q"</userinput>
      </screen>

      <para>

      To set &SCONSFLAGS; more permanently you can add the
      setting to the shell's startup file on POSIX systems,
      and on Windows you can use the
      <literal>System Properties</literal> control panel applet
      to select <literal>Environment Variables</literal>
      and set it there.

      </para>

    </section>

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

      <para>

      The &f-link-GetOption; function
      lets you query the values set by the various command-line options.

      </para>

      <para>

      One use case for &GetOption; is to check the operation
      mode in order to bypass some steps,
      for example, checking whether
      the <option>-h</option> (or <option>--help</option>)
      option was given.
      Normally, &SCons; does not print its help text
      until after it has read all of the SConscript files,
      since any SConscript can make additions to the help text.
      Of course, reading all of the SConscript files
      takes extra time.
      If you know that your configuration does not define
      any additional help text in subsidiary SConscript files,
      you can speed up displaying the command-line help
      by using a &GetOption; query as a guard for whether
      to load the subsidiary SConscript files:

      </para>

      <sconstruct>
if not GetOption('help'):
    SConscript('src/SConscript', export='env')
      </sconstruct>

      <para>

      The same technique can be used to special-case the
      clean (<literal>GetOption('clean')</literal>)
      and no-execute (<literal>GetOption('no_exec')</literal>)
      modes.

      </para>

      <para>

      In general, the string that you pass to the
      &f-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 &f-GetOption; strings for fetching them,
      are available in the
      <xref linkend="sect-command-line-option-strings"></xref> section,
      below.

      </para>

      <para>

      &f-GetOption; can be used to retrieve the values of options
      defined by calls to &f-link-AddOption;. A &f-GetOption; call
      must appear after the &f-AddOption; call for that option
      (unlike the defining of build targets,
      this is a case where "order matters" in &SCons;).
      If the &f-AddOption; call supplied a <parameter>dest</parameter>
      keyword argument, a string with that name is what to pass
      as the argument to &f-GetOption;, otherwise it is a
      (possibly modified) version of the first long option name -
      see &f-link-AddOption;.

      </para>

    </section>

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

      <para>

      You can also set the values of certain (but not all) &SCons;
      command-line options from within the &SConscript; files
      by using the &f-link-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 <option>-j</option>
      or <option>--jobs</option> option,
      so that you 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 <option>-j</option> option is
      somewhat system-dependent.
      One rough guideline is that the more processors
      your system has,
      the higher you want to set the
      <option>-j</option> 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
      <envar>NUM_CPU</envar> 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
      provides the right level of flexibility:

      </para>

      <scons_example name="commandline_SetOption">
        <file name="SConstruct" printme="1">
import os

num_cpu = int(os.environ.get('NUM_CPU', 2))
SetOption('num_jobs', num_cpu)
print("running with -j %s" % GetOption('num_jobs'))
        </file>
        <file name="foo.in">
foo.in
        </file>
      </scons_example>

      <para>

      The above snippet of code
      sets the value of the <option>--jobs</option> 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 command-line option.
      The string for fetching or setting the <option>--jobs</option>
      value is <parameter>num_jobs</parameter>
      for historical reasons.)
      The code in this example prints the <parameter>num_jobs</parameter>
      value for illustrative purposes.
      It uses a default value of <literal>2</literal>
      to provide some minimal parallelism even on
      single-processor systems:

      </para>

      <scons_output example="commandline_SetOption" suffix="1">
        <scons_output_command>scons -Q</scons_output_command>
      </scons_output>

      <para>

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

      </para>

      <scons_output example="commandline_SetOption" suffix="2">
        <scons_output_command>export NUM_CPU="4"</scons_output_command>
        <scons_output_command environment="NUM_CPU=4">scons -Q</scons_output_command>
      </scons_output>

      <para>

      But any explicit
      <option>-j</option> or <option>--jobs</option>
      value you specify on the command line is used first,
      whether
      the <envar>NUM_CPU</envar> environment
      variable is set or not:

      </para>

      <scons_output example="commandline_SetOption" suffix="3">
        <scons_output_command>scons -Q -j 7</scons_output_command>
        <scons_output_command>export NUM_CPU="4"</scons_output_command>
        <scons_output_command environment="NUM_CPU=4">scons -Q -j 3</scons_output_command>
      </scons_output>

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

      </para>

      <para>

      &SetOption; works for options added with &AddOption;,
      but only if they were created with
      <parameter>settable=True</parameter> in the call to &AddOption;
      (only available in SCons 4.8.0 and later).

      </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_progress</literal></entry>
      <entry><option>-Q</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 id="sect-AddOption">
    <title>Adding Custom Command-Line Options:  the &AddOption; Function</title>

      <para>

      You can also define your own command-line options
      for the project with the &f-link-AddOption; function.
      The &AddOption; function takes the same arguments
      as the <function>add_option</function> method
      from the &Python; standard library module
      <systemitem>optparse</systemitem>
      <footnote>
      <para>
      The &AddOption; function is,
      in fact, implemented using a subclass
      of <classname>optparse.OptionParser</classname>.
      </para>
      </footnote>
      (see <ulink url="https://docs.python.org/3/library/optparse.html"/>).
      </para>

      <para>
      Once you add a custom command-line option
      with the &AddOption; function,
      the value of the option (if any) is immediately available
      using the &f-link-GetOption; function.
      The argument to &f-GetOption; must be the name of the
      variable which holds the option.
      If the <parameter>dest</parameter>
      keyword argument to &AddOption; is specified, the value is the
      variable name.
      given. If not given, it is the name
      (without the leading hyphens) of the first long option name
      given to &AddOption;
      after replacing any remaining hyphen characters
      with underscores, since hyphens are not legal in Python
      identifier names.

      </para>

      <para>

      &f-link-SetOption; works for options added with &AddOption;,
      but only if they were created with
      <parameter>settable=True</parameter> in the call to &AddOption;
      (only available in SCons 4.8.0 and later).

      </para>

      <para>

      One useful example of using this functionality
      is to provide a <option>--prefix</option> to help describe
      where to install files:

      </para>

      <scons_example name="commandline_AddOption">
        <file name="SConstruct" printme="1">
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)
        </file>
        <file name="foo.in">
foo.in
        </file>
      </scons_example>

      <para>

      The above code uses the &GetOption; function
      to set the <varname>$PREFIX</varname>
      &consvar; to a
      value you specify with a command-line
      option of <option>--prefix</option>.
      Because <varname>$PREFIX</varname>
      expands to a null string if it's not initialized,
      running &SCons; without the
      option of <option>--prefix</option>
      installs the file in the
      <filename>/usr/bin/</filename> directory:

      </para>

      <scons_output example="commandline_AddOption" suffix="1">
        <scons_output_command>scons -Q -n</scons_output_command>
      </scons_output>

      <para>

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

      </para>

      <scons_output example="commandline_AddOption" suffix="2">
        <scons_output_command>scons -Q -n --prefix=/tmp/install</scons_output_command>
      </scons_output>

      <note>
      <para>
      The <systemitem>optparse</systemitem> parser which &SCons; uses
      allows option-arguments to follow their options after either
      an <literal>=</literal> or space separator,
      however the latter form does not work well in &SCons; for
      added options and should be avoided.
      &SCons; does not place an ordering constraint on the
      types of command-line arguments,
      so while <option>--input=ARG</option> is unambiguous,
      for <option>--input ARG</option>
      it is not possible to tell without instructions whether
      <parameter>ARG</parameter> is an argument belonging to the
      <parameter>input</parameter> option or a standalone word.
      &SCons; considers words on the command line which do not
      begin with hyphen as either command-line build variables
      or command-line targets,
      both of which are made available for use in an &SConscript;
      (see the immediately following sections for details).
      Thus, they must be collected before &SConscript; processing
      takes place. &AddOption; calls do provide the
      necessary instructions to resolve the ambiguity,
      but as they appear in &SConscript; files,
      &SCons; does not have the information early enough,
      and unexpected things may happen,
      such as option-arguments appearing in the list of targets,
      and processing exceptions due to missing option-arguments.
      </para>
      <para>
      As a result,
      this usage style should be avoided when invoking &scons;.
      For single-argument options,
      tell your users to use the <option>--input=ARG</option>
      form on the command line.
      For multiple-argument options
      (<parameter>nargs</parameter> value greater than one),
      set <parameter>nargs</parameter> to one in the
      &AddOption; call and either: combine the option-arguments into one word
      with a separator, and parse the result in your own code
      (see the built-in <option>--debug</option> option, which
      allows specifying multiple arguments as a single comma-separated
      word, for an example of such usage); or allow the option to
      be specified multiple times by setting
      <literal>action='append'</literal>. Both methods can be
      supported at the same time.
      </para>
      </note>

    </section>

  </section>

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

    <para>

    You may want to control various aspects
    of your build by allowing
    <varname>variable</varname>=<replaceable>value</replaceable>
    pairs to be specified on the command line.
    For example, suppose you want 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>=<replaceable>value</replaceable>
    assignments from the command line.
    This allows you to modify
    aspects of your build in response
    to specifications on the command line.

    </para>

    <para>

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

    </para>

    <scons_example name="commandline_ARGUMENTS">
       <file name="SConstruct" printme="1">
env = Environment()
debug = ARGUMENTS.get('debug', 0)
if int(debug):
    env.Append(CCFLAGS='-g')
env.Program('prog.c')
       </file>
       <file name="prog.c">
prog.c
       </file>
    </scons_example>

    <para>

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

    </para>

    <scons_output example="commandline_ARGUMENTS" suffix="1">
       <scons_output_command>scons -Q debug=0</scons_output_command>
       <scons_output_command>scons -Q debug=0</scons_output_command>
       <scons_output_command>scons -Q debug=1</scons_output_command>
       <scons_output_command>scons -Q debug=1</scons_output_command>
    </scons_output>

    <note><para>

    Two usage notes (both shown in the example above):

    <itemizedlist>
      <listitem>
        <para>
          No matter how you intend to use them, the values read from
          a command line (i.e., external to the program) are
          always strings. You may need to do type conversion.
        </para>
      </listitem>
      <listitem>
        <para>
          When you retrieve from the &ARGUMENTS; dictionary,
          it is useful to use the &Python; dictionary
          <methodname>get</methodname> method,
          so you can supply a default value if the variable is
          not given on the command line. Otherwise, the build
          will fail with a &KeyError;
          if the variable is not set.
        </para>
      </listitem>
    </itemizedlist>



    </para></note>

    <para>

    &SCons; keeps track of the precise build command used to build each object file,
    and as a result can determine that the object and executable files
    need rebuilding when the value of the <parameter>debug</parameter>
    argument has changed.

    </para>

    <para>

    The &ARGUMENTS; dictionary has two minor drawbacks.
    First, because it is a dictionary,
    it can only map each keyword to one value,
    and thus only "remembers" the last setting
    for each keyword on the command line.
    This makes the &ARGUMENTS; dictionary
    less than ideal if you want to allow
    specifying 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
    (&Python; versions since 3.6 now maintain dictionaries in
    insertion order, so this problem is mitigated).

    </para>

    <para>

    To accommodate these requirements,
    &SCons; also provides an &ARGLIST; variable
    that gives you direct access to build variable
    settings from 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 lets you
    add to the &CPPDEFINES; &consvar;
    by specifying multiple
    <varname>define=</varname>
    settings on the command line:

    </para>

    <scons_example name="commandline_ARGLIST">
       <file name="SConstruct" printme="1">
cppdefines = []
for key, value in ARGLIST:
    if key == 'define':
        cppdefines.append(value)
env = Environment(CPPDEFINES=cppdefines)
env.Object('prog.c')
       </file>
       <file name="prog.c">
prog.c
       </file>
    </scons_example>

    <para>

    Yields the following output:

    </para>

    <scons_output example="commandline_ARGLIST" suffix="1">
       <scons_output_command>scons -Q define=FOO</scons_output_command>
       <scons_output_command>scons -Q define=FOO define=BAR</scons_output_command>
    </scons_output>

    <para>

    Note that the &ARGLIST; and &ARGUMENTS;
    variables do not interfere with each other,
    but rather provide slightly different views
    into how you specified
    <varname>variable</varname>=<replaceable>value</replaceable>
    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 &Python; dictionary access),
    and the &ARGLIST; list
    is more flexible
    (since you can examine the
    specific order in which
    the command-line variable settings were given).

    </para>

    <section id="sect-controlling-cli-variables">
    <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 &consvar;.
      To help with this,
      &SCons; provides a &Variables; container class to
      hold definitions of such build variables,
      and a mechanism to apply the
      build variables to a &consenv;.
      This allows you to control how the build variables affect
      &consenvs;.

      </para>

      <para>

      For example, suppose that you want to set
      a &RELEASE; &consvar; 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 build command
      with the appropriate define
      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; &consvar;:

      </para>

      <scons_example name="commandline_Variables1">
        <file name="SConstruct" printme="1">
vars = Variables(None, ARGUMENTS)
vars.Add('RELEASE', default=0)
env = Environment(variables=vars, CPPDEFINES={'RELEASE_BUILD': '${RELEASE}'})
env.Program(['foo.c', 'bar.c'])
        </file>
        <file name="foo.c">
foo.c
        </file>
        <file name="bar.c">
bar.c
        </file>
      </scons_example>

      <para>

      This &SConstruct; snippet first creates a &Variables; object which
      uses the values from the command-line variables dictionary &ARGUMENTS;.
      It then uses the object's &Add;
      method to indicate that the &RELEASE;
      variable can be set on the command line, and that
      if not set the default value is <literal>0</literal>.
      The newly created &Variables; object
      is passed to the &Environment; call
      used to create the &consenv;
      using a &variables; keyword argument.
      This then allows you 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>

      <scons_output example="commandline_Variables1" suffix="1">
        <scons_output_command>scons -Q RELEASE=1</scons_output_command>
      </scons_output>

      <para>

      The <literal>Variables()</literal> call in this example looks
      a little awkward. The function takes two optional arguments:
      a script name and a dictionary.  In order to specify the
      dictionary as the second argument, you must provide the
      script argument as the first; since there's actually no script,
      use <constant>None</constant> as a sentinel value.
      However, if you omit all the arguments,
      the default behavior is to read from the &ARGUMENTS; dictionary anyway,
      which is what we want.  The example shows it this way because the arguments
      were introduced in this order, but you should feel free to just
      leave off the arguments if the default behavior is what you want.

      </para>

      <para>

      Historical note:  In old &SCons; (prior to 0.98.1 from 2008),
      these build variables were known as "command-line build options."
      At that time, the class was named &Options;
      and the predefined functions to construct options were named
      &BoolOption;, &EnumOption;, &ListOption;,
      &PathOption;, &PackageOption; and &AddOptions; (contrast
      with the current names in
      <xref linkend="sect-build-variable-functions"></xref>, below).
      Because the Internet has a very long memory,
      you may encounter these names in older
      &SConscript; files, wiki pages, blog entries, StackExchange
      articles, etc.
      These old names no longer work, but a mental substitution
      of <quote>Variable</quote> for <quote>Option</quote>
      allows the concepts to transfer to current usage models.

      </para>

    </section>

    <section id="sect-variables-help">
    <title>Providing Help for Command-Line Build Variables</title>

      <para>

      To make command-line build variables more useful,
      you may want to provide
      some help text to describe the available variables
      when you ask for help (run <userinput>scons -h</userinput>).
      You can write this text by hand,
      but &SCons; provides some assistance.
      Variables objects provide a
      &GenerateHelpText; method to
      generate text that describes
      the various variables that
      have been added to it. The default text includes
      the help string itself plus other information
      such as allowed values.
      (The generated text can also be customized by
      replacing the <methodname>FormatVariableHelpText</methodname>
      method).
      You then pass the output from this method to
      the &Help; function:

      </para>

      <scons_example name="commandline_Variables_Help">
        <file name="SConstruct" printme="1">
vars = Variables()
vars.Add('RELEASE', help='Set to 1 to build for release', default=0)
env = Environment(variables=vars)
Help(vars.GenerateHelpText(env))
        </file>
      </scons_example>

      <para>

      &scons; now displays some useful text
      when the <option>-h</option> option is used:

      </para>

      <scons_output example="commandline_Variables_Help" suffix="1">
        <scons_output_command>scons -Q -h</scons_output_command>
      </scons_output>

      <para>

      You can see the help output shows the default value
      as well as the current actual value of the build variable.

      </para>

    </section>

    <section id="sect-variables-file">
    <title>Reading Build Variables From a File</title>

      <para>

      Being able to specify the
      value of a build variable on the command line
      is useful,
      but can still become tedious
      if you have to specify the variable
      every time you run &SCons;.
      To make this easier,
      you can provide customized build variable settings
      in a &Python; script by providing a file name when the
      &Variables; object is created:

      </para>

      <scons_example name="commandline_Variables_custom_py_1">
        <file name="SConstruct" printme="1">
vars = Variables('custom.py')
vars.Add('RELEASE', help='Set to 1 to build for release', default=0)
env = Environment(variables=vars, CPPDEFINES={'RELEASE_BUILD': '${RELEASE}'})
env.Program(['foo.c', 'bar.c'])
Help(vars.GenerateHelpText(env))
        </file>
        <file name="foo.c">
foo.c
        </file>
        <file name="bar.c">
bar.c
        </file>
        <file name="custom.py">
RELEASE = 1
        </file>
      </scons_example>

      <para>

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

      </para>

      <scons_example_file example="commandline_Variables_custom_py_1" name="custom.py"></scons_example_file>

      <para>

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

      </para>

      <scons_output example="commandline_Variables_custom_py_1" suffix="1">
        <scons_output_command>scons -Q</scons_output_command>
      </scons_output>

      <para>

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

      </para>

      <scons_example name="commandline_Variables_custom_py_2">
        <file name="SConstruct">
vars = Variables('custom.py')
vars.Add('RELEASE', help='Set to 1 to build for release', default=0)
env = Environment(variables=vars, CPPDEFINES={'RELEASE_BUILD': '${RELEASE}'})
env.Program(['foo.c', 'bar.c'])
Help(vars.GenerateHelpText(env))
        </file>
        <file name="foo.c">
foo.c
        </file>
        <file name="bar.c">
bar.c
        </file>
        <file name="custom.py" printme="1">
RELEASE = 0
        </file>
      </scons_example>

      <para>

      The object files are rebuilt appropriately
      with the new variable:

      </para>

      <scons_output example="commandline_Variables_custom_py_2" suffix="1">
        <scons_output_command>scons -Q</scons_output_command>
      </scons_output>

      <para>

      Finally, you can combine both methods with:

      </para>

      <screen>
vars = Variables('custom.py', ARGUMENTS)
      </screen>

      <para>

      If both a variables script and a dictionary are supplied,
      the dictionary is evaluated last, so values from the
      command line "win" if there are any duplicate keys.
      This rule allows you to move some common settings
      to a variables script, but still be able to override those
      for a given build without changing the script.

      </para>

      <!-- TODO: maybe show a variable used to select between variable scripts? -->

    </section>

    <section id="sect-build-variable-functions">
    <title>Pre-Defined Build Variable Functions</title>

      <para>

      &SCons; provides a number of convenience functions
      that provide behavior definitions
      for various types of command-line build variables.
      These functions all return a tuple which is ready
      to be passed to the &Add; or &AddVariables; method call.
      You are of course free to define your own behaviors
      as well.

      </para>

      <section id="sect-variables-bool">
      <title>True/False Values:  the &BoolVariable; Build Variable Function</title>

        <para>

        It is 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 accommodate
        different preferences for how to represent
        &true; or &false; values.
        The &BoolVariable; function
        makes it easy to accommodate 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>

        <scons_example name="commandline_BoolVariable">
          <file name="SConstruct" printme="1">
vars = Variables('custom.py')
vars.Add(BoolVariable('RELEASE', help='Set to build for release', default=False))
env = Environment(variables=vars, CPPDEFINES={'RELEASE_BUILD': '${RELEASE}'})
env.Program('foo.c')
          </file>
          <file name="foo.c">
foo.c
          </file>
        </scons_example>

        <para>

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

        </para>

        <scons_output example="commandline_BoolVariable" suffix="1">
          <scons_output_command>scons -Q RELEASE=yes foo.o</scons_output_command>
        </scons_output>

        <scons_output example="commandline_BoolVariable" suffix="2">
          <scons_output_command>scons -Q RELEASE=t foo.o</scons_output_command>
        </scons_output>

        <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>

        <scons_output example="commandline_BoolVariable" suffix="3">
          <scons_output_command>scons -Q RELEASE=no foo.o</scons_output_command>
        </scons_output>

        <scons_output example="commandline_BoolVariable" suffix="4">
          <scons_output_command>scons -Q RELEASE=f foo.o</scons_output_command>
        </scons_output>

        <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 you try to specify
        any other value,
        &SCons; supplies an appropriate error message:

        </para>

        <scons_output example="commandline_BoolVariable" suffix="5">
          <scons_output_command>scons -Q RELEASE=bad_value foo.o</scons_output_command>
        </scons_output>

      </section>

      <section id="sect-variables-enum">
      <title>Single Value From a Selection:  the &EnumVariable; Build Variable Function</title>

        <para>

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

        </para>

        <scons_example name="commandline_EnumVariable">
          <file name="SConstruct" printme="1">
vars = Variables('custom.py')
vars.Add(
    EnumVariable(
        'COLOR',
        help='Set background color',
        default='red',
        allowed_values=('red', 'green', 'blue'),
    )
)
env = Environment(variables=vars, CPPDEFINES={'COLOR': '"${COLOR}"'})
env.Program('foo.c')
Help(vars.GenerateHelpText(env))
          </file>
          <file name="foo.c">
foo.c
          </file>
        </scons_example>

        <para>

        You can now explicitly set the &COLOR; build variable
        to any of the specified allowed values:

        </para>

        <scons_output example="commandline_EnumVariable" suffix="1">
          <scons_output_command>scons -Q COLOR=red foo.o</scons_output_command>
          <scons_output_command>scons -Q COLOR=blue foo.o</scons_output_command>
          <scons_output_command>scons -Q COLOR=green foo.o</scons_output_command>
        </scons_output>

        <para>

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

        </para>

        <scons_output example="commandline_EnumVariable" suffix="2">
          <scons_output_command>scons -Q COLOR=magenta foo.o</scons_output_command>
        </scons_output>

        <para>

        This example can also serve to further illustrate help
        generation: the help message here picks up not only the
        <parameter>help</parameter> text, but augments it with
        information gathered from <parameter>allowed_values</parameter>
        and <parameter>default</parameter>:

        </para>

        <scons_output example="commandline_EnumVariable" suffix="3">
          <scons_output_command>scons -Q -h</scons_output_command>
        </scons_output>

        <para>

        The &EnumVariable; function also provides a way
        to map alternate names to allowed values.
        Suppose, for example, you want to allow
        the word <literal>navy</literal> to be used as a synonym for
        <literal>blue</literal>.
        You do this by adding a &map; dictionary
        that maps its key values
        to the desired allowed value:

        </para>

        <scons_example name="EnumVariable_map">
          <file name="SConstruct" printme="1">
vars = Variables('custom.py')
vars.Add(
    EnumVariable(
        'COLOR',
        help='Set background color',
        default='red',
        allowed_values=('red', 'green', 'blue'),
        map={'navy': 'blue'},
    )
)
env = Environment(variables=vars, CPPDEFINES={'COLOR': '"${COLOR}"'})
env.Program('foo.c')
          </file>
          <file name="foo.c">
foo.c
          </file>
        </scons_example>

        <para>

        Now you can supply
        <literal>navy</literal> on the command line,
        and &SCons; translates that into <literal>blue</literal>
        when it comes time to use the &COLOR;
        variable to build a target:

        </para>

        <scons_output example="EnumVariable_map" suffix="1">
          <scons_output_command>scons -Q COLOR=navy foo.o</scons_output_command>
        </scons_output>

        <para>

        By default, when using the &EnumVariable; function,
        the allowed values are case-sensitive:

        </para>

        <scons_output example="commandline_EnumVariable" suffix="4">
          <scons_output_command>scons -Q COLOR=Red foo.o</scons_output_command>
          <scons_output_command>scons -Q COLOR=BLUE foo.o</scons_output_command>
          <scons_output_command>scons -Q COLOR=nAvY foo.o</scons_output_command>
        </scons_output>

        <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>

        <scons_example name="commandline_EnumVariable_ic1">
          <file name="SConstruct" printme="1">
vars = Variables('custom.py')
vars.Add(
    EnumVariable(
        'COLOR',
        help='Set background color',
        default='red',
        allowed_values=('red', 'green', 'blue'),
        map={'navy': 'blue'},
        ignorecase=1,
    )
)
env = Environment(variables=vars, CPPDEFINES={'COLOR': '"${COLOR}"'})
env.Program('foo.c')
          </file>
          <file name="foo.c">
foo.c
          </file>
        </scons_example>

        <para>

        Which yields the output:

        </para>

        <scons_output example="commandline_EnumVariable_ic1" suffix="1">
          <scons_output_command>scons -Q COLOR=Red foo.o</scons_output_command>
          <scons_output_command>scons -Q COLOR=BLUE foo.o</scons_output_command>
          <scons_output_command>scons -Q COLOR=nAvY foo.o</scons_output_command>
          <scons_output_command>scons -Q COLOR=green foo.o</scons_output_command>
        </scons_output>

        <para>

        Notice that an &ignorecase; value of <literal>1</literal>
        preserves the case-spelling supplied,
        only ignoring the case for matching.
        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>

        <scons_example name="commandline_EnumVariable_ic2">
          <file name="SConstruct" printme="1">
vars = Variables('custom.py')
vars.Add(
    EnumVariable(
        'COLOR',
        help='Set background color',
        default='red',
        allowed_values=('red', 'green', 'blue'),
        map={'navy': 'blue'},
        ignorecase=2,
    )
)
env = Environment(variables=vars, CPPDEFINES={'COLOR': '"${COLOR}"'})
env.Program('foo.c')
          </file>
          <file name="foo.c">
foo.c
          </file>
        </scons_example>

        <para>

        Now &SCons; uses values of
        <literal>red</literal>,
        <literal>green</literal> or
        <literal>blue</literal>
        regardless of how those values are spelled
        on the command line:

        </para>

        <scons_output example="commandline_EnumVariable_ic2" suffix="1">
          <scons_output_command>scons -Q COLOR=Red foo.o</scons_output_command>
          <scons_output_command>scons -Q COLOR=nAvY foo.o</scons_output_command>
          <scons_output_command>scons -Q COLOR=GREEN foo.o</scons_output_command>
        </scons_output>

      </section>

      <section id="sect-variables-list">
      <title>Multiple Values From a List:  the &ListVariable; Build Variable Function</title>

        <para>

        Another way in which you might want to control a build variable is to
        specify a list of allowed values, of which one or more can be chosen
        (where &EnumVariable; allows exactly one value to be chosen).
        &SCons; provides this through the &ListVariable; function.
        If, for example, you want to be able to set a
        &COLORS; variable to one or more of the allowed values:

        </para>

        <scons_example name="commandline_ListVariable">
          <file name="SConstruct" printme="1">
vars = Variables('custom.py')
vars.Add(
    ListVariable(
        'COLORS', help='List of colors', default=0, names=['red', 'green', 'blue']
    )
)
env = Environment(variables=vars, CPPDEFINES={'COLORS': '"${COLORS}"'})
env.Program('foo.c')
          </file>
          <file name="foo.c">
foo.c
          </file>
        </scons_example>

        <para>

        You can now specify a comma-separated list of allowed values,
        which get translated into a space-separated
        list for passing to the build commands:

        </para>

        <scons_output example="commandline_ListVariable" suffix="1">
          <scons_output_command>scons -Q COLORS=red,blue foo.o</scons_output_command>
          <scons_output_command>scons -Q COLORS=blue,green,red foo.o</scons_output_command>
        </scons_output>

        <para>

        In addition, the &ListVariable; function
        lets you specify explicit keywords of
        &all; or &none;
        to select all of the allowed values,
        or none of them, respectively:

        </para>

        <scons_output example="commandline_ListVariable" suffix="2">
          <scons_output_command>scons -Q COLORS=all foo.o</scons_output_command>
          <scons_output_command>scons -Q COLORS=none foo.o</scons_output_command>
        </scons_output>

        <para>

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

        </para>

        <scons_output example="commandline_ListVariable" suffix="3">
          <scons_output_command>scons -Q COLORS=magenta foo.o</scons_output_command>
        </scons_output>

        <para>

        You can use this last characteristic as a way to enforce at least
        one of your valid options being chosen by specifying the valid
        values with the <parameter>names</parameter> parameter and then
        giving a value not in that list as the <parameter>default</parameter>
        parameter - that way if no value is given on the command line,
        the default is chosen, &SCons; errors out as this is invalid.
        The example is, in fact, set up that way by using
        <literal>0</literal> as the default:

        </para>

        <scons_output example="commandline_ListVariable" suffix="4">
          <scons_output_command>scons -Q foo.o</scons_output_command>
        </scons_output>

        <para>

        This technique works for &EnumVariable; as well.

        </para>

      </section>

      <section id="sect-variables-path">
      <title>Path Names:  the &PathVariable; Build Variable Function</title>

        <para>

        &SCons; provides 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 preprocessor macro
        that controls the location of a
        configuration file:

        </para>

        <scons_example name="commandline_PathVariable">
          <file name="SConstruct" printme="1">
vars = Variables('custom.py')
vars.Add(
    PathVariable(
        'CONFIG', help='Path to configuration file', default='__ROOT__/etc/my_config'
    )
)
env = Environment(variables=vars, CPPDEFINES={'CONFIG_FILE': '"$CONFIG"'})
env.Program('foo.c')
          </file>
          <file name="foo.c">
foo.c
          </file>
          <file name="__ROOT__/etc/my_config">
/opt/location
          </file>
          <file name="__ROOT__/usr/local/etc/other_config">
/opt/location
          </file>
        </scons_example>

        <para>

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

        </para>

        <scons_output example="commandline_PathVariable" suffix="1">
          <scons_output_command>scons -Q foo.o</scons_output_command>
          <scons_output_command>scons -Q CONFIG=__ROOT__/usr/local/etc/other_config foo.o</scons_output_command>
        </scons_output>

        <para>

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

        </para>

        <scons_output example="commandline_PathVariable" suffix="2">
          <scons_output_command>scons -Q CONFIG=__ROOT__/does/not/exist foo.o</scons_output_command>
        </scons_output>

        <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 as the validation function:

        </para>

        <scons_example name="commandline_PathIsFile">
          <file name="SConstruct" printme="1">
vars = Variables('custom.py')
vars.Add(
    PathVariable(
        'CONFIG',
        help='Path to configuration file',
        default='__ROOT__/etc/my_config',
        validator=PathVariable.PathIsFile,
    )
)
env = Environment(variables=vars, CPPDEFINES={'CONFIG_FILE': '"$CONFIG"'})
env.Program('foo.c')
          </file>
          <file name="foo.c">
foo.c
          </file>
          <file name="__ROOT__/etc/my_config">
/opt/location
          </file>
        </scons_example>

        <para>

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

        </para>

        <scons_example name="commandline_PathIsDir">
          <file name="SConstruct" printme="1">
vars = Variables('custom.py')
vars.Add(
    PathVariable(
        'DBDIR',
        help='Path to database directory',
        default='__ROOT__/var/my_dbdir',
        validator=PathVariable.PathIsDir,
    )
)
env = Environment(variables=vars, CPPDEFINES={'DBDIR': '"$DBDIR"'})
env.Program('foo.c')
          </file>
          <file name="foo.c">
foo.c
          </file>
          <file name="__ROOT__/var/my_dbdir">
/opt/location
          </file>
        </scons_example>

        <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 as the validation function:

        </para>

        <scons_example name="commandline_PathIsDirCreate">
          <file name="SConstruct" printme="1">
vars = Variables('custom.py')
vars.Add(
    PathVariable(
        'DBDIR',
        help='Path to database directory',
        default='__ROOT__/var/my_dbdir',
        validator=PathVariable.PathIsDirCreate,
    )
)
env = Environment(variables=vars, CPPDEFINES={'DBDIR': '"$DBDIR"'})
env.Program('foo.c')
          </file>
          <file name="foo.c">
foo.c
          </file>
          <file name="__ROOT__/var/my_dbdir">
/opt/location
          </file>
        </scons_example>

        <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 you supply:

        </para>

        <scons_example name="commandline_PathAccept">
          <file name="SConstruct" printme="1">
vars = Variables('custom.py')
vars.Add(
    PathVariable(
        'OUTPUT',
        help='Path to output file or directory',
        default=None,
        validator=PathVariable.PathAccept,
    )
)
env = Environment(variables=vars, CPPDEFINES={'OUTPUT': '"$OUTPUT"'})
env.Program('foo.c')
          </file>
          <file name="foo.c">
foo.c
          </file>
        </scons_example>

      </section>

      <section id="sect-variables-package">
      <title>Enabled/Disabled Path Names: the &PackageVariable; Build Variable Function</title>

        <para>

        Sometimes you want to give
        even more control over a path name variable,
        allowing them to be explicitly enabled or disabled
        by using <literal>yes</literal> or <literal>no</literal> keywords,
        in addition to allowing supplying an explicit path name.
        &SCons; provides the &PackageVariable;
        function to support this:

        </para>

        <scons_example name="commandline_PackageVariable">
          <file name="SConstruct" printme="1">
vars = Variables("custom.py")
vars.Add(
    PackageVariable("PACKAGE", help="Location package", default="__ROOT__/opt/location")
)
env = Environment(variables=vars, CPPDEFINES={"PACKAGE": '"$PACKAGE"'})
env.Program("foo.c")
          </file>
          <file name="foo.c">
foo.c
          </file>
          <file name="__ROOT__/opt/location">
/opt/location
          </file>
          <file name="__ROOT__/usr/local/location">
/opt/location
          </file>
        </scons_example>

        <para>

        When the &SConscript; file uses the &PackageVariable; function,
        you can still use the default
        or supply an overriding path name,
        but you 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>

        <scons_output example="commandline_PackageVariable" suffix="1">
          <scons_output_command>scons -Q foo.o</scons_output_command>
          <scons_output_command>scons -Q PACKAGE=__ROOT__/usr/local/location foo.o</scons_output_command>
          <scons_output_command>scons -Q PACKAGE=yes foo.o</scons_output_command>
          <scons_output_command>scons -Q PACKAGE=no foo.o</scons_output_command>
        </scons_output>

      </section>

    </section>

    <section id="sect-variables-multiple">
    <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 the build variables to be added to the object.
      Each build variable is specified
      as either a tuple of arguments,
      or as a call to one of the pre-defined
      functions for pre-packaged command-line build variables,
      which returns such a tuple. Note that an individual tuple
      cannot take keyword arguments in the way that a call to
      &Add; or one of the build variable functions can.
      The order of variables given to &AddVariables; does not
      matter.

      </para>

      <scons_example name="commandline_AddVariables_1">
        <file name="SConstruct" printme="1">
vars = Variables()
vars.AddVariables(
    ('RELEASE', 'Set to 1 to build for release', 0),
    ('CONFIG', 'Configuration file', '/etc/my_config'),
    BoolVariable('warnings', help='compilation with -Wall and similar', default=True),
    EnumVariable(
        'debug',
        help='debug output and symbols',
        default='no',
        allowed_values=('yes', 'no', 'full'),
        map={},
        ignorecase=0,
    ),
    ListVariable(
        'shared',
        help='libraries to build as shared libraries',
        default='all',
        names=list_of_libs,
    ),
    PackageVariable(
        'x11', help='use X11 installed here (yes = search some places)', default='yes'
    ),
    PathVariable('qtdir', help='where the root of Qt is installed', default=qtdir),
)
        </file>
      </scons_example>

      <para>
      </para>

    </section>

    <section id="sect-variables-unknown">
    <title>Handling Unknown Command-Line Build Variables:  the &UnknownVariables; Function</title>

      <para>

      Humans, of course,
      occasionally misspell variable names in their command-line settings.
      &SCons; does not generate an error or warning
      for any unknown variables specified on the command line,
      because it can not reliably tell
      whether a given "misspelled" variable is
      really unknown and a potential problem or not.
      After all, you might be processing arguments directly
      using &ARGUMENTS; or &ARGLIST; with some Python
      code in your &SConscript; file.

      </para>

      <para>

      If, however, you are using a &Variables; object to
      define a specific set of command-line build variables
      that you expect to be able to set,
      you may want to provide an error
      message or warning of your own
      if a variable setting is specified
      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 to get the
      settings &Variables; did not recognize:

      </para>

      <scons_example name="commandline_UnknownVariables">
        <file name="SConstruct" printme="1">
vars = Variables(None)
vars.Add('RELEASE', help='Set to 1 to build for release', default=0)
env = Environment(variables=vars, CPPDEFINES={'RELEASE_BUILD': '${RELEASE}'})
unknown = vars.UnknownVariables()
if unknown:
    print("Unknown variables: %s" % " ".join(unknown.keys()))
    Exit(1)
env.Program('foo.c')
        </file>
        <file name="foo.c">
foo.c
        </file>
      </scons_example>

      <para>

      The &UnknownVariables; method returns a dictionary
      containing the keywords and values
      of any variables 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).
      The example above,
      checks whether the dictionary
      returned by &UnknownVariables; is non-empty,
      and if so prints the Python list
      containing the names of the unknown variables
      and then calls the &Exit; function
      to terminate &SCons;:

      </para>

      <scons_output example="commandline_UnknownVariables" suffix="1">
        <scons_output_command>scons -Q NOT_KNOWN=foo</scons_output_command>
      </scons_output>

      <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 &consenv;
      with the <parameter>variables=</parameter>
      keyword argument of an &Environment; call: the variables
      in the object are not fully processed until this has happened.

      </para>

    </section>

  </section>

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

    <section id="sect-var-COMMAND-LINE-TARGETS">
    <title>Fetching Command-Line Targets: the &COMMAND_LINE_TARGETS; Variable</title>

      <para>

      &SCons; provides a &COMMAND_LINE_TARGETS; variable
      that lets you fetch the list of targets that were
      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
      whenever a specific program is built.
      You can do this by checking for the
      target in the &COMMAND_LINE_TARGETS; list:

      </para>

      <scons_example name="commandline_COMMAND_LINE_TARGETS">
        <file name="SConstruct" printme="1">
if 'bar' in COMMAND_LINE_TARGETS:
    print("Don't forget to copy `bar' to the archive!")
Default(Program('foo.c'))
Program('bar.c')
        </file>
        <file name="foo.c">
foo.c
        </file>
        <file name="bar.c">
foo.c
        </file>
      </scons_example>

      <para>

      Now, running &SCons; with the default target
      works as usual,
      but explicitly specifying the &bar; target
      on the command line generates the warning message:

      </para>

      <scons_output example="commandline_COMMAND_LINE_TARGETS" suffix="1">
        <scons_output_command>scons -Q</scons_output_command>
        <scons_output_command>scons -Q bar</scons_output_command>
      </scons_output>

      <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 id="sect-default-targets-function">
    <title>Controlling the Default Targets:  the &Default; Function</title>

      <para>

      You can control
      which targets &SCons; builds by default - that is,
      when there are no targets specified on the command line.
      As mentioned previously,
      &SCons; normally builds every target
      in or below the current directory unless you
      explicitly specify one or more targets
      on the command line.
      Sometimes, however, you may want
      to specify that only
      certain programs, or programs in certain directories,
      should be built by default.
      You do this with the &Default; function:

      </para>

      <scons_example name="commandline_Default1">
         <file name="SConstruct" printme="1">
env = Environment()
hello = env.Program('hello.c')
env.Program('goodbye.c')
Default(hello)
         </file>
         <file name="hello.c">
hello.c
         </file>
         <file name="goodbye.c">
goodbye.c
         </file>
      </scons_example>

      <para>

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

      </para>

      <scons_output example="commandline_Default1" suffix="1">
         <scons_output_command>scons -Q</scons_output_command>
         <scons_output_command>scons -Q</scons_output_command>
         <scons_output_command>scons -Q goodbye</scons_output_command>
      </scons_output>

      <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>

      <scons_output example="commandline_Default1" suffix="2">
         <scons_output_command>scons -Q .</scons_output_command>
      </scons_output>

      <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>

      <scons_example name="commandline_Default2">
         <file name="SConstruct" printme="1">
env = Environment()
prog1 = env.Program('prog1.c')
Default(prog1)
prog2 = env.Program('prog2.c')
prog3 = env.Program('prog3.c')
Default(prog3)
         </file>
         <file name="prog1.c">
prog1.c
         </file>
         <file name="prog2.c">
prog2.c
         </file>
         <file name="prog3.c">
prog3.c
         </file>
      </scons_example>

      <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
      build only the
      <application>prog1</application>
      and
      <application>prog3</application>
      programs by default:

      </para>

      <scons_output example="commandline_Default2" suffix="1">
         <scons_output_command>scons -Q</scons_output_command>
         <scons_output_command>scons -Q .</scons_output_command>
      </scons_output>

      <para>

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

      </para>

      <scons_example name="commandline_Default3">
         <file name="SConstruct" printme="1">
env = Environment()
env.Program(['prog1/main.c', 'prog1/foo.c'])
env.Program(['prog2/main.c', 'prog2/bar.c'])
Default('prog1')
         </file>
         <directory name="prog1"></directory>
         <directory name="prog2"></directory>
         <file name="prog1/main.c">
int main() { printf("prog1/main.c\n"); }
         </file>
         <file name="prog1/foo.c">
int foo() { printf("prog1/foo.c\n"); }
         </file>
         <file name="prog2/main.c">
int main() { printf("prog2/main.c\n"); }
         </file>
         <file name="prog2/bar.c">
int bar() { printf("prog2/bar.c\n"); }
         </file>
      </scons_example>

      <para>

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

      </para>

      <scons_output example="commandline_Default3" suffix="1">
         <scons_output_command>scons -Q</scons_output_command>
         <scons_output_command>scons -Q</scons_output_command>
         <scons_output_command>scons -Q .</scons_output_command>
      </scons_output>

      <para>

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

      </para>

      <scons_example name="commandline_Default4">
         <file name="SConstruct" printme="1">
env = Environment()
prog1 = env.Program('prog1.c')
prog2 = env.Program('prog2.c')
Default(None)
         </file>
         <file name="prog1.c">
prog1.c
         </file>
         <file name="prog2.c">
prog2.c
         </file>
      </scons_example>

      <para>

      Which would produce build output like:

      </para>

      <scons_output example="commandline_Default4" suffix="1">
         <scons_output_command>scons -Q</scons_output_command>
         <scons_output_command>scons -Q .</scons_output_command>
      </scons_output>

      <section id="sect-var-DEFAULT-TARGETS">
      <title>Fetching the List of Default Targets: the &DEFAULT_TARGETS; Variable</title>

        <para>

        &SCons; provides a &DEFAULT_TARGETS; variable
        that lets you get at the current list of default targets
        specified by calls to the &Default; function or method.
        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.
        You can do this easily by calling the <function>str</function>
        on the elements in a list comprehension:

        </para>

        <scons_example name="commandline_DEFAULT_TARGETS_1">
           <file name="SConstruct" printme="1">
prog1 = Program('prog1.c')
Default(prog1)
print("DEFAULT_TARGETS is %s" % [str(t) for t in DEFAULT_TARGETS])
           </file>
           <file name="prog1.c">
prog1.c
           </file>
        </scons_example>

        <para>

        (Keep in mind that 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
        you leave off the <option>-Q</option> flag when you run &SCons;:)

        </para>

        <scons_output example="commandline_DEFAULT_TARGETS_1" suffix="1">
           <scons_output_command>scons</scons_output_command>
        </scons_output>

        <para>

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

        </para>

        <scons_example name="commandline_DEFAULT_TARGETS_2">
           <file name="SConstruct" printme="1">
prog1 = Program('prog1.c')
Default(prog1)
print("DEFAULT_TARGETS is now %s" % [str(t) for t in DEFAULT_TARGETS])
prog2 = Program('prog2.c')
Default(prog2)
print("DEFAULT_TARGETS is now %s" % [str(t) for t in DEFAULT_TARGETS])
           </file>
           <file name="prog1.c">
prog1.c
           </file>
           <file name="prog2.c">
prog2.c
           </file>
        </scons_example>

        <para>

        Which yields the output:

        </para>

        <scons_output example="commandline_DEFAULT_TARGETS_2" suffix="1">
           <scons_output_command>scons</scons_output_command>
        </scons_output>

        <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 have added the default targets
        you expect to find in it.

        </para>

      </section>

    </section>

    <section id="sect-var-BUILD-TARGETS">
    <title>Fetching the List of Build Targets, Regardless of Origin: the &BUILD_TARGETS; Variable</title>

      <para>

      You have already seen 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; tries 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>

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

      <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>

      <scons_example name="commandline_BUILD_TARGETS_1">
        <file name="SConstruct" printme="1">
prog1 = Program('prog1.c')
Program('prog2.c')
Default(prog1)
print("BUILD_TARGETS is %s" % [str(t) for t in BUILD_TARGETS])
        </file>
        <file name="prog1.c">
prog1.c
        </file>
        <file name="prog2.c">
prog2.c
        </file>
      </scons_example>

      <para>

      Notice how the value of &BUILD_TARGETS;
      changes depending on whether a target is
      specified on the command line - &BUILD_TARGETS;
      takes from  &DEFAULT_TARGETS;
      only if there are no &COMMAND_LINE_TARGETS;:

      </para>

      <scons_output example="commandline_BUILD_TARGETS_1" suffix="1">
        <scons_output_command>scons -Q</scons_output_command>
        <scons_output_command>scons -Q prog2</scons_output_command>
        <scons_output_command>scons -Q -c .</scons_output_command>
      </scons_output>

    </section>

  </section>

</chapter>
