path: root/doc/user/factories.xml

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  <para>

  &SCons; provides a number of platform-independent functions,
  called <literal>factories</literal>,
  that perform common file system manipulations
  like copying, moving or deleting files and directories,
  or making directories.
  These functions are <literal>factories</literal>
  because they don't perform the action
  at the time they're called,
  they each return an &Action; object
  that can be executed at the appropriate time.

  </para>

  <section>
  <title>Copying Files or Directories:  The &Copy; Factory</title>

    <para>

    Suppose you want to arrange to make a copy of a file,
    and don't have a suitable pre-existing builder.
    <footnote>
    <para>
    Unfortunately, in the early days of SCons design,
    we used the name &Copy; for the function that
    returns a copy of the environment,
    otherwise that would be the logical choice for
    a Builder that copies a file or directory tree
    to a target location.
    </para>
    </footnote>
    One way would be to use the &Copy; action factory
    in conjunction with the &Command; builder:

    </para>

    <programlisting>
        Command("file.out", "file.in", Copy("$TARGET", "$SOURCE"))
    </programlisting>

    <para>

    Notice that the action returned by the &Copy; factory
    will expand the &cv-link-TARGET; and &cv-link-SOURCE; strings
    at the time &file_out; is built,
    and that the order of the arguments
    is the same as that of a builder itself--that is,
    target first, followed by source:

    </para>

    <screen>
       % <userinput>scons -Q</userinput>
       Copy("file.out", "file.in")
    </screen>

    <para>

    You can, of course, name a file explicitly
    instead of using &cv-TARGET; or &cv-SOURCE;:

    </para>

    <programlisting>
      Command("file.out", [], Copy("$TARGET", "file.in"))
    </programlisting>

    <para>

    Which executes as:

    </para>

    <screen>
      % <userinput>scons -Q</userinput>
      Copy("file.out", "file.in")
    </screen>

    <para>

    The usefulness of the &Copy; factory
    becomes more apparent when
    you use it in a list of actions
    passed to the &Command; builder.
    For example, suppose you needed to run a
    file through a utility that only modifies files in-place,
    and can't "pipe" input to output.
    One solution is to copy the source file
    to a temporary file name,
    run the utility,
    and then copy the modified temporary file to the target,
    which the &Copy; factory makes extremely easy:

    </para>

    <programlisting>
      Command("file.out", "file.in",
              [
                Copy("tempfile", "$SOURCE"),
                "modify tempfile",
                Copy("$TARGET", "tempfile"),
              ])
    </programlisting>

    <para>

    The output then looks like:

    </para>

    <screen>
      % <userinput>scons -Q</userinput>
      Copy("tempfile", "file.in")
      modify tempfile
      Copy("file.out", "tempfile")
    </screen>

  </section>

  <section>
  <title>Deleting Files or Directories:  The &Delete; Factory</title>

    <para>

    If you need to delete a file,
    then the &Delete; factory
    can be used in much the same way as
    the &Copy; factory.
    For example, if we want to make sure that
    the temporary file
    in our last example doesn't exist before
    we copy to it,
    we could add &Delete; to the beginning
    of the command list:

    </para>

    <programlisting>
      Command("file.out", "file.in",
              [
                Delete("tempfile"),
                Copy("tempfile", "$SOURCE"),
                "modify tempfile",
                Copy("$TARGET", "tempfile"),
              ])
    </programlisting>

    <para>

    Which then executes as follows:

    </para>

    <screen>
      % <userinput>scons -Q</userinput>
      Delete("tempfile")
      Copy("tempfile", "file.in")
      modify tempfile
      Copy("file.out", "tempfile")
    </screen>

    <para>

    Of course, like all of these &Action; factories,
    the &Delete; factory also expands
    &cv-link-TARGET; and &cv-link-SOURCE; variables appropriately.
    For example:

    </para>

    <programlisting>
      Command("file.out", "file.in",
              [
                Delete("$TARGET"),
                Copy("$TARGET", "$SOURCE")
              ])
    </programlisting>

    <para>

    Executes as:

    </para>

    <screen>
      % <userinput>scons -Q</userinput>
      Delete("file.out")
      Copy("file.out", "file.in")
    </screen>

    <para>

    Note, however, that you typically don't need to
    call the &Delete; factory explicitly in this way;
    by default, &SCons; deletes its target(s)
    for you before executing any action.

    </para>

    <para>

    One word of caution about using the &Delete; factory:
    it has the same variable expansions available
    as any other factory, including the &cv-SOURCE; variable.
    Specifying <literal>Delete("$SOURCE")</literal>
    is not something you usually want to do!

    </para>

  </section>

  <section>
  <title>Moving (Renaming) Files or Directories:  The &Move; Factory</title>

    <para>

    The &Move; factory
    allows you to rename a file or directory.
    For example, if we don't want to copy the temporary file,
    we could use:

    </para>

    <programlisting>
      Command("file.out", "file.in",
              [
                Copy("tempfile", "$SOURCE"),
                "modify tempfile",
                Move("$TARGET", "tempfile"),
              ])
    </programlisting>

    <para>

    Which would execute as:

    </para>

    <screen>
      % <userinput>scons -Q</userinput>
      Copy("tempfile", "file.in")
      modify tempfile
      Move("file.out", "tempfile")
    </screen>

  </section>

  <section>
  <title>Updating the Modification Time of a File:  The &Touch; Factory</title>

    <para>

    If you just need to update the
    recorded modification time for a file,
    use the &Touch; factory:

    </para>

    <programlisting>
      Command("file.out", "file.in",
              [
                Copy("$TARGET", "$SOURCE"),
                Touch("$TARGET"),
              ])
    </programlisting>

    <para>

    Which executes as:

    </para>

    <screen>
      % <userinput>scons -Q</userinput>
      Copy("file.out", "file.in")
      Touch("file.out")
    </screen>

  </section>

  <section>
  <title>Creating a Directory:  The &Mkdir; Factory</title>

    <para>

    If you need to create a directory,
    use the &Mkdir; factory.
    For example, if we need to process
    a file in a temporary directory
    in which the processing tool
    will create other files that we don't care about, 
    you could use:

    </para>

    <programlisting>
      Command("file.out", "file.in",
              [
                Delete("tempdir"),
                Mkdir("tempdir"),
                Copy("tempdir/${SOURCE.file}", "$SOURCE"),
                "process tempdir",
                Move("$TARGET", "tempdir/output_file"),
                Delete("tempdir"),
              ])
    </programlisting>

    <para>

    Which executes as:

    </para>

    <screen>
      % <userinput>scons -Q</userinput>
      Delete("tempdir")
      Mkdir("tempdir")
      Copy("tempdir/file.in", "file.in")
      process tempdir
      Move("file.out", "tempdir/output_file")
      scons: *** [file.out] tempdir/output_file: No such file or directory
    </screen>

  </section>

  <section>
  <title>Changing File or Directory Permissions:  The &Chmod; Factory</title>

    <para>

    To change permissions on a file or directory,
    use the &Chmod; factory.
    The permission argument uses POSIX-style
    permission bits and should typically
    be expressed as an octal,
    not decimal, number:

    </para>

    <programlisting>
      Command("file.out", "file.in",
              [
                Copy("$TARGET", "$SOURCE"),
                Chmod("$TARGET", 0755),
              ])
    </programlisting>

    <para>

    Which executes:

    </para>

    <screen>
      % <userinput>scons -Q</userinput>
      Copy("file.out", "file.in")
      Chmod("file.out", 0755)
    </screen>

  </section>

  <section>
  <title>Executing an action immediately:  the &Execute; Function</title>

    <para>

    We've been showing you how to use &Action; factories
    in the &Command; function.
    You can also execute an &Action; returned by a factory
    (or actually, any &Action;)
    at the time the &SConscript; file is read
    by using the &Execute; function.
    For example, if we need to make sure that
    a directory exists before we build any targets,

    </para>

    <programlisting>
      Execute(Mkdir('/tmp/my_temp_directory'))
    </programlisting>

    <para>

    Notice that this will
    create the directory while
    the &SConscript; file is being read:

    </para>

    <screen>
      % <userinput>scons</userinput>
      scons: Reading SConscript files ...
      Mkdir("/tmp/my_temp_directory")
      scons: done reading SConscript files.
      scons: Building targets ...
      scons: `.' is up to date.
      scons: done building targets.
    </screen>

    <para>

    If you're familiar with Python,
    you may wonder why you would want to use this
    instead of just calling the native Python
    <function>os.mkdir()</function> function.
    The advantage here is that the &Mkdir;
    action will behave appropriately if the user
    specifies the &SCons; <option>-n</option> or
    <option>-q</option> options--that is,
    it will print the action but not actually
    make the directory when <option>-n</option> is specified,
    or make the directory but not print the action
    when <option>-q</option> is specified.

    </para>

    <para>

    The &Execute; function returns the exit status
    or return value of the underlying action being executed.
    It will also print an error message if the action
    fails and returns a non-zero value.
    &SCons; will <emphasis>not</emphasis>, however,
    actually stop the build if the action fails.
    If you want the build to stop
    in response to a failure in an action called by &Execute;,
    you must do so by explicitly
    checking the return value
    and calling the &Exit; function
    (or a Python equivalent):

    </para>

    <programlisting>
    if Execute(Mkdir('/tmp/my_temp_directory')):
        # A problem occurred while making the temp directory.
        Exit(1)
    </programlisting>

  </section>