Author: FCM team

# FCM: User Guide: Annex: The FCM 1 Extract System

## Introduction

The FCM 1 extract system is deprecated. The documentation for the current extract system can be found at FCM Make.

The extract system provides an interface between the revision control system (currently Subversion) and the build system. Where appropriate, it extracts code from the repository and other user-defined locations to a directory tree suitable for feeding into the build system. In this chapter, we shall use many examples to explain how to use the extract system. At the end of this chapter, you will be able to extract code from the local file system as well as from different branches of different repository URLs. You will also learn how to mirror code to an alternate destination. Finally, you will be given an introduction on how to specify configurations for the build system via the extract configuration file. (For further information on the build system, please see the next chapter The Build System.) The last section of the chapter tells you what you can do in the case when Subversion is not available.

## The Extract Command

To invoke the extract system, simply issue the command:

    fcm extract

By default, the extract system searches for an extract configuration file `ext.cfg` in `$PWD` and then `$PWD/cfg`. If an extract configuration file is not found in these directories, the command fails with an error. If an extract configuration file is found, the system will use the configuration specified in the file to perform the current extract.

If the destination of the extract does not exist, the system performs a new full extract to the destination. If a previous extract already exists at the destination, the system performs an incremental extract, updating any modifications if necessary. If a full (fresh) extract is required for whatever reason, you can invoke the extract system using the `-f` option, (i.e. the command becomes `fcm extract -f`). If you simply want to remove all the items generated by a previous extract in the destination, you can invoke the extract system using the `--clean` option.

For further information on the extract command, please see FCM Command Reference > fcm extract.

## Simple Usage

The extract configuration file is the main user interface of the extract system. It is a line based text file. For a complete set of extract configuration file declarations, please refer to the Annex: Declarations in FCM extract configuration file.

### Extract from a local path

A simple example of a basic extract configuration file is given below:

    # Example 1
    # ----------------------------------------------------------------------
    cfg::type         ext       # line 1
    cfg::version      1.0       # line 2
                                # line 3
    dest              $PWD      # line 4
                                # line 5
    repos::var::user  $HOME/var # line 6
                                # line 7
    expsrc::var::user code      # line 8

The above demonstrates how to use the extract system to extract code from a local user directory. Here is an explanation of what each line does:

- line 1: the label `CFG::TYPE` declares the type of the configuration file. The value `ext` tells the system that it is an extract configuration file.
- line 2: the label `CFG::VERSION` declares the version of the extract configuration file. The current default is `1.0`. Although it is not currently used, if we have to change the format of the configuration file at a later stage, we shall be able to use this number to determine whether we are reading a file with an older format or one with a newer format.
- line 3: a blank line or a line beginning with a `#` is a comment, and is ignored by the interpreter.
- line 4: the label `DEST` declares the destination root directory of this extract. The value `$PWD` expands to the current working directory.
- line 5: comment line, ignored.
- line 6: the label `REPOS::<pck>::<branch>` declares the top level URL or path of a repository. The package name of the repository is given by <pck>. In our example, we choose `var` as the name of the package. (You can choose any name you like, however, it is usually sensible to use a package name that matches the name of the project or system you are working with.) The branch name in the repository is given by <branch>. (Again, you can choose any name you like, however, it is usually sensible to use a name such as `base`, `user` or something that matches your branch name.) In our example, the word `user` is normally used to denote a local user directory. Hence the statement declares that the repository path for the `var` package in the `user` branch can be found at `$HOME/var`.
- line 7: comment line, ignored.
- line 8: the label `EXPSRC::<pck>::<branch>` declares an expandable source directory for the package <pck> in the branch <branch>. In our example, the package name is `var`, and the branch name is `user`. These match the package and the branch names of the repository declaration in line 6. It means that the source directory declaration is associated with the path `$HOME/var`. The value of the declaration `code` is therefore a sub-directory under `$HOME/var`. By declaring a source directory using an `EXPSRC` label, the system automatically searches for all sub-directories (recursively) under the declared source directory.

Invoking the extract system using the above configuration file will extract all sub-directories under `$HOME/var/code` to `$PWD/src/var/code`. Note: the extract system ignores symbolic links and hidden files, (i.e. file names beginning with a `.`). It will write a build configuration file to `$PWD/cfg/bld.cfg`. The configuration used for this extract will be written to the configuration file at `$PWD/cfg/ext.cfg`.

Note - incremental extract
  Suppose you have already performed an extract using the above configuration file. At a later time, you have made some changes to some of the files in the source directory. Re-running the extract system on the same configuration will trigger an incremental extract. In an incremental extract, the system will update only those files that are modified. If the last modified time (or last commit revision) of a source file in the current extract differs from that in the previous extract, the system will attempt a content comparison. The system updates the destination only if the content and/or file access permission of the source differs from that of the destination.

### Extract from a Subversion URL

The next example demonstrates how to extract from a Subversion repository URL:

    # Example 2
    # ----------------------------------------------------------------------
    cfg::type           ext                    # line 1
    cfg::version        1.0                    # line 2
                                               # line 3
    dest                $PWD                   # line 4
                                               # line 5
    repos::var::base    svn://server/var/trunk # line 6
    revision::var::base 1234                   # line 7
                                               # line 8
    expsrc::var::base   code                   # line 9

- line 1-5: same as example 1.
- line 6: the line declares the repository location of the `base` branch of the `var` package to be the Subversion URL `svn://server/var/trunk`.
- line 7: the label `REVISION::<pck>::<branch>` declares the revision of the repository associated with the package <pck> in the branch <branch>. The current line tells the extract system to use revision 1234 of `svn://server/var/trunk`. It is worth noting that the declared revision must be a revision when the declared branch exists. The actual revision used is the last changed revision of the declared one. If the revision is not declared, the default is to use the last changed revision at the HEAD of the branch.
- line 8: comment line, ignored.
- line 9: the line declares an expandable source directory in the repository `svn://server/var/trunk`.

Invoking the extract system using the above configuration file will extract all sub-directories under `svn://server/var/trunk/code` to `$PWD/src/var/code`. It will write a build configuration file to `$PWD/cfg/bld.cfg`. The configuration used for this extract will be written to the configuration file at `$PWD/cfg/ext.cfg`.

EXPSRC or SRC?

  So far, we have only declared source directories using the `EXPSRC` statement, which stands for expandable source directory. A source directory declared using this statement will trigger the system to search recursively for any sub-directories under the declared one. Any sub-directories containing regular source files will be included in the extract. Symbolic links, hidden files and empty directories (or those containing only symbolic links and/or hidden files) are ignored.

  If you do not want the system to search for sub-directories underneath your declared source directory, you can declare your source directory using the `SRC` statement. The `SRC` statement is essentially the same as `EXPSRC` except that it does not trigger the automatic recursive search for source directories. In fact, the system implements the `EXPSRC` statement by expanding it into a list of `SRC` statements.

Package and sub-package

  The second field of a repository, revision or source directory declaration label is the name of the container package. It is a name selected by the user to identify the system or project he/she is working on. (Therefore, it is often sensible to choose an identifier that matches the name of the project or system.) The package name provides a unique namespace for a file container. Source directories are automatically arranged into sub-packages, using the names of the sub-directories as the names of the sub-packages. For example, the declaration at line 9 in example 2 will put the source directory in the `var/code` sub-package automatically.

  Note that, in additional to slash `/`, double colon `::` and double underscore `__` (internal only) also act as delimiters for package names. Please avoid using them for naming your files and directories.

  You can declare a sub-package name explicitly in your source directory statement. For example, the following two lines are equivalent:

      src::var::base                      code/VarMod_Surface
      src::var/code/VarMod_Surface::base  code/VarMod_Surface

  Explicit sub-package declaration should not be used normally, as it requires a lot more typing (although there are some situations where it can be useful, e.g. if you need to re-define the package name).

  Currently, the extract system only supports non-space characters in the package name, as the space character is used as a delimiter between the declaration label and its value. If there are spaces in the path name to a file or directory, you should explicity re-define the package name of that path to a package name with no space using the above method. However, we recommend that only non-space characters are used for naming directories and files to make life simpler.

The expanded extract configuration file

  At the end of a successful extract, the configuration used by the current extract is written in `cfg/ext.cfg` under the extract destination root. This file is an expanded version of the original, with changes in the following declarations:

  - All revision keywords are converted into revision numbers.
  - If a revision is not defined for a repository, it is set to the corresponding revision number of the HEAD revision.
  - All URL keywords are converted into the full URLs.
  - All `EXPSRC` declarations are expanded into `SRC` declarations.
  - All other variables are expanded.

  With this file, it should be possible for a later extract to re-create the current configuration even if the contents of the repository have changed. (This applies only to code stored in the repository.)

### Mirror code to an alternate location

The next example demonstrates how to extract from a repository and mirror the code to an alternate location. It is essentially the same as example 2, except that it has three new lines to describe how the system can mirror the extracted code to an alternate location.

    # Example 3
    # ----------------------------------------------------------------------
    cfg::type           ext
    cfg::version        1.0

    dest                $PWD

    rdest::machine      tx01                           # line 6
    rdest::logname      frva                           # line 7
    rdest               /scratch/frva/extract/example3 # line 8

    repos::var::base    svn://server/var/trunk
    revision::var::base 1234

    expsrc::var::base   code

Here is an explanation of what each line does:

- line 6: `RDEST::MACHINE` declares the target machine to which the code will be mirrored. The example mirrors the code to the machine named `tx01`.
- line 7: `RDEST::LOGNAME` declares the user name of the target machine, to which the user has login access. If this is not declared, the system uses the login name of the current user on the local machine.
- line 8: `RDEST` declares the root directory of the alternate destination, where the mirror version of the extract will be sent.

Invoking the extract system on the above configuration will trigger an extract similar to that given in example 2, but it will also attempt to mirror the contents at `$PWD/src/var/code` to `/scratch/frva/extract/example3/src` on the alternate destination. It will also mirror the expanded extract configuration file `$PWD/cfg/ext.cfg` to `/scratch/frva/extract/example3/cfg/ext.cfg` and `$PWD/cfg/bld.cfg` to `/scratch/frva/extract/example3/cfg/bld.cfg`. It is also worth noting that the content of the build configuration file will be slightly different, since it will include directory names appropriate for the alternate destination.

Note - mirroring command

  The extract system currently supports `rdist` and `rsync` as its mirroring tool. The default is `rsync`. To use `rdist` instead of `rsync`, add the following line to your extract configuration file:

      rdest::mirror_cmd  rdist

  If `rsync` is used to mirror an extract, the system needs to issue a separate remote shell command to create the container directory of the mirror destination. The default is to issue a shell command in the form `ssh -n -oBatchMode=yes LOGNAME@MACHINE mkdir -p DEST`. The following declarations can be used to modify the command:

      # Examples using the default settings:
      rdest::rsh_mkdir_rsh         ssh
      rdest::rsh_mkdir_rshflags    -n -oBatchMode=yes
      rdest::rsh_mkdir_mkdir       mkdir
      rdest::rsh_mkdir_mkdirflags  -p

  In addition, the default `rsync` shell command is `rsync -a --exclude='.*' --delete-excluded --timeout=900 --rsh='ssh -oBatchMode=yes' SOURCE DEST`. The following declarations can be used to modify the command:

      # Examples using the default settings:
      rdest::rsync       rsync
      rdest::rsyncflags  -a --exclude='.*' --delete-excluded --timeout=900 \
                         --rsh='ssh -oBatchMode=yes'

## Advanced Usage

### Extract from multiple repositories

So far, we have only extracted from a single location. The extract system is not much use if that is the only thing it can do. In fact, the extract system supports extract of multiple source directories from multiple branches in multiple repositories. The following configuration file is an example of how to extract from multiple repositories:

    # Example 4
    # ----------------------------------------------------------------------
    cfg::type           ext
    cfg::version        1.0

    dest                $PWD

    repos::var::base    fcm:var_tr              # line 6
    repos::ops::base    fcm:ops_tr              # line 7
    repos::gen::base    fcm:gen_tr              # line 8

    revision::gen::base 2468                    # line 10

    expsrc::var::base   src/code                    # line 12
    expsrc::var::base   src/scripts                 # line 13
    expsrc::ops::base   src/code                    # line 14
    src::gen::base      src/code/GenMod_Constants   # line 15
    src::gen::base      src/code/GenMod_Control     # line 16
    src::gen::base      src/code/GenMod_FortranIO   # line 17
    src::gen::base      src/code/GenMod_GetEnv      # line 18
    src::gen::base      src/code/GenMod_ModelIO     # line 19
    src::gen::base      src/code/GenMod_ObsInfo     # line 20
    src::gen::base      src/code/GenMod_Platform    # line 21
    src::gen::base      src/code/GenMod_Reporting   # line 22
    src::gen::base      src/code/GenMod_Trace       # line 23
    src::gen::base      src/code/GenMod_UMConstants # line 24
    src::gen::base      src/code/GenMod_Utilities   # line 25

Here is an explanation of what each line does:

- line 6-8: these lines declare the repositories for the `base` branches of the `var`, `ops` and `gen` packages respectively. It is worth noting that the values of the declarations are no longer Subversion URLs but are FCM URL keywords. These keywords are normally declared in the central configuration file of the FCM system, and will be expanded into the corresponding Subversion URLs by the FCM system. For further information on URL keywords, please see Code Management System > Using Subversion > Basic Command Line Usage > Repository & Revision Keywords.
- line 10: this line declares the revision number for the `base` branch of the `gen` package, i.e. for the `fcm:gen_tr` repository. It is worth noting that the revision numbers for the `var` and `ops` packages have not been declared. By default, their revision numbers will be set to the last changed revision at the HEAD.
- line 12-14: these line declares the source directories for the `base` branches of the `var` and `ops` packages. For the `var` package, we are extracting everything from the `code` and the `scripts` sub-directory. For the `ops` package, we are extracting everything from the `code` directory.
- line 15-25: these line declares the source directories for the `base` branch of the `gen` package. The source directories declared will not be searched for sub-directories underneath the declared directories.

We shall end up with a directory tree such as:

    $PWD
       |
       |--- cfg
       |      |
       |      |--- bld.cfg
       |      |--- ext.cfg
       |
       |--- src
              |
              |--- gen
              |      |
              |      |--- code
              |              |
              |              |--- GenMod_Constants
              |              |--- GenMod_Control
              |              |--- GenMod_FortranIO
              |              |--- GenMod_GetEnv
              |              |--- GenMod_ModelIO
              |              |--- GenMod_ObsInfo
              |              |--- GenMod_Platform
              |              |--- GenMod_Reporting
              |              |--- GenMod_Trace
              |              |--- GenMod_UMConstants
              |              |--- GenMod_Utilities
              |
              |--- ops
              |      |
              |      |--- code
              |              |
              |              |--- ...
              |
              |--- var
                     |
                     |--- code
                     |       |
                     |       |--- ...
                     |
                     |--- scripts
                             |
                             |--- ...

Note - revision number

  As seen in the above example, if a revision number is not specified for a repository URL, it defaults to the last changed revision at the HEAD of the branch. The revision number can also be declared in other ways:

  - Any revision arguments acceptable by Subversion are allowed. You can use a valid revision number, a date between a pair of curly brackets (e.g. `{2005-05-01T12:00}`) or the keyword HEAD. However, please do not use the keywords BASE, COMMITTED or PREV as these are reserved for working copy only.
  - FCM revision keywords are allowed. These must be defined for the corresponding repository URLs in either the central or the user FCM configuration file. For further information on revision keywords, please see Code Management > Using Subversion > Basic Command Line Usage > Repository & Revision Keywords.
  - Do not use the keyword USER, as it is used internally by the extract system.

  If a revision number is specified for a branch, the actual revision used by the extract system is the last changed revision of the branch, which may differ from the declared revision. While this behaviour is useful in most situations, some users may find it confusing to work with. It is possible to alter this behaviour so that extract will fail if the declared revision does not correspond to a changeset of the declared branch. Make the following declaration to switch on this checking:

      revmatch  true

### Extract from multiple branches

We have so far dealt with a single branch in any package. The extract system can be used to combine changes from different branches of a package. An example is given below:

    # Example 5
    # ----------------------------------------------------------------------
    cfg::type               ext
    cfg::version            1.0

    dest                    $PWD

    repos::var::base        fcm:var_tr
    repos::ops::base        fcm:ops_tr
    repos::gen::base        fcm:gen_tr

    revision::gen::base     2468

    expsrc::var::base       src/code
    expsrc::var::base       src/scripts
    expsrc::ops::base       src/code
    src::gen::base          src/code/GenMod_Constants
    src::gen::base          src/code/GenMod_Control
    src::gen::base          src/code/GenMod_FortranIO
    src::gen::base          src/code/GenMod_GetEnv
    src::gen::base          src/code/GenMod_ModelIO
    src::gen::base          src/code/GenMod_ObsInfo
    src::gen::base          src/code/GenMod_Platform
    src::gen::base          src/code/GenMod_Reporting
    src::gen::base          src/code/GenMod_Trace
    src::gen::base          src/code/GenMod_UMConstants
    src::gen::base          src/code/GenMod_Utilities

    repos::var::branch1     fcm:var_br/frva/r1234_new_stuff   # line 27
    repos::var::branch2     fcm:var_br/frva/r1516_bug_fix     # line 28
    repos::ops::branch1     fcm:ops_br/opsrc/r3188_good_stuff # line 29

The configuration file in example 5 is similar to that of example 4 except for the last three lines. Here is an explanation of what they do:

- line 27: this line declares a repository URL for the `branch1` branch of the `var` package. From the URL of the branch, we know that the branch was created by the user `frva` based on the trunk at revision at 1234. The description of the branch is `branch1`. The following points are worth noting:
  - By declaring a new branch with the same package name to a previously declared branch, it is assumed that both branches reside in the same Subversion repository.
  - No revision is declared for this URL, so the default is used which is the last changed revision at the HEAD of the branch.
  - No source directory is declared for this URL. By default, if no source directory is declared for a branch repository, it will attempt to use the same set of source directories as the first declared branch of the package. In this case, the source directories declared for the `base` branch of the `var` package will be used.
- line 28: this line declares another branch called `branch2` for the `var` package. No source directory is declared for this URL either, so it will use the same set of source directories declared for the `base` branch.
- line 29: this line declares a branch called `branch1` for the `ops` package. It will use the same set of source directories declared for the `ops` package `base` branch.

When we invoke the extract system, it will attempt to extract from the first declared branch of a package, if the last changed revision of the source directory is the same in all the branches. However, if the last changed revision of the source directory differs for different branches, the system will attempt to obtain an extract priority list for each source directory, using the following logic:

1. The system looks for source directory packages from the first declared branch to the last declared branch.
2. The branch in which a source directory package is first declared is the `base` branch of the source directory package.
3. The last changed revision of a source directory package in a subsequently declared repository branch is compared with that of the base branch. If the last changed revision is the same as that of the base branch, the source directory of this branch is discarded. Otherwise, it is placed at the end of the extract priority list.

For the `var` package in the above example, let us assume that we have three source directory packages X, Y and Z under `code`, and their last changed revisions under `base` are 100. Let's say we have committed some changes to X and Z in the `branch1` branch at revision 102, and other changes to Y and Z in the `branch2` branch at revision 104, the extract priority lists for X, Y and Z will look like:

- X: base (100, base), branch1 (102), branch2 (100, discarded)
- Y: base (100, base), branch1 (100, discarded), branch2 (104)
- Z: base (100, base), branch1 (102), branch2 (104)

Once we have an extract priority list for a source directory, we can begin extracting source files in the source directory. The source directory of the base branch is extracted first, followed by that in the subsequent branches. If a source file in a subsequent branch has the same content as the that in the base branch, it is discarded. Otherwise, the following logic determines the branch to use:

1. If a source file is modified in only one subsequent branch, the source file in that branch is extracted.
2. If a source file is modified in two or more subsequent branches, but their modifications are the same, then the source file in the first modification is used.
3. If a source file is modified in two or more subsequent branches and their modifications differ, then the behaviour depends on the "conflict mode" setting, which can be `fail`, `merge` (default) and `override`. If the conflict mode is `fail`, the extract fails. If the conflict mode is `merge`, the system will attempt to merge the changes using a tool such as `diff3`. The result of the merge will be used to update the destination. The extract fails only if there are unresolved conflicts in the merge. (In which case, the conflict should be resolved using the version control system before re-running the extract system.) If the conflict mode is `override`, the change in the latest declared branch takes precedence, and the changes in all other branches will be ignored. The conflict mode can be changed using the `CONFLICT` declaration in the extract configuration file. E.g:

       conflict  fail

Once the system has established which source files to use, it determines whether the destination file is out of date or not. The destination file is out of date if it does not exist or if its content differs from the version of the source file we are using. The system only updates the destination if it is considered to be out of date.

The extract system can also combine changes from branches in the Subversion repository and the local file system. This is demonstrated in the next example.

    # Example 6
    # ----------------------------------------------------------------------
    cfg::type               ext
    cfg::version            1.0

    dest                    $PWD

    repos::var::base        fcm:var_tr
    repos::ops::base        fcm:ops_tr
    repos::gen::base        fcm:gen_tr

    revision::gen::base     2468

    expsrc::var::base       src/code
    expsrc::var::base       src/scripts
    expsrc::ops::base       src/code
    src::gen::base          src/code/GenMod_Constants
    src::gen::base          src/code/GenMod_Control
    src::gen::base          src/code/GenMod_FortranIO
    src::gen::base          src/code/GenMod_GetEnv
    src::gen::base          src/code/GenMod_ModelIO
    src::gen::base          src/code/GenMod_ObsInfo
    src::gen::base          src/code/GenMod_Platform
    src::gen::base          src/code/GenMod_Reporting
    src::gen::base          src/code/GenMod_Trace
    src::gen::base          src/code/GenMod_UMConstants
    src::gen::base          src/code/GenMod_Utilities

    repos::var::branch1     fcm:var_br/frva/r1234_new_stuff
    repos::var::branch2     fcm:var_br/frva/r1516_bug_fix
    repos::ops::branch1     fcm:ops_br/opsrc/r3188_good_stuff

    repos::var::user        $HOME/var                         # line 31
    repos::gen::user        $HOME/gen                         # line 32

Example 6 is similar to example 5 except that it is also extracting from local directories. Here is an explanation of the lines:

- line 31-32: these line declare the repositories for the `user` branches of the `var` and `gen` packages respectively. Both are local paths at the local file system. There are no declarations for source directories for the `user` branches, so they use the same set of source directories of the first declared branches, the `base` branches in both cases.

Note - the INC declaration
  You have probably realised that the above examples have many repeated lines. To avoid having repeated lines in multiple extract configuration files, you can use `INC` declarations to include other extract configuration files. For example, if the configuration file of example 5 is stored in the file `$HOME/example5/ext.cfg`, line 1 to 29 of example 6 can be replaced with an `INC` declaration. Example 6 can then be written as:

      inc                     $HOME/example5/ext.cfg

      repos::var::user        $HOME/var
      repos::gen::user        $HOME/gen

  Note: the `INC` declaration supports the special environment variable $HERE. If this variable is already set in the environment, it acts as a normal environment variable. However, if it is not set, it will be expanded into the container directory of the current extract configuration file. This feature is particularly useful if you are including a hierarchy of extract configurations from files in the same container directory in a repository.

### Inherit from a previous extract

All the examples above dealt with standalone extract, that is, the current extract is independent of any other extract. If a previous extract exists in another location, the extract system can inherit from this previous extract in your current extract. This works like a normal incremental extract, except that your extract will only contain the changes you have specified (compared with the inherited extract) instead of the full source directory tree. This type of incremental extract is useful in several ways. For instance:

- It is fast, because you only have to extract and mirror files that you have changed.
- The subsequent build will also be fast, since it will use incremental build.
- You do not need write access to the original extract. A system administrator can set up a stable version in a central account, which developers can then inherit from.
- You want an incremental extract, but you need to leave the original extract unmodified.

The following example is based on example 4 and example 6. The assumption is that an extract has already been performed at the directory `~frva/var/vn22.0` based on the configuration file in example 4.

    # Example 7
    # ----------------------------------------------------------------------
    cfg::type               ext
    cfg::version            1.0

    dest                    $PWD

    use                     ~frva/var/vn22.0                  # line 6

    repos::var::branch1     fcm:var_br/frva/r1234_new_stuff   # line 8
    repos::var::branch2     fcm:var_br/frva/r1516_bug_fix     # line 9
    repos::ops::branch1     fcm:ops_br/opsrc/r3188_good_stuff # line 10

    repos::var::user        $HOME/var                         # line 12
    repos::gen::user        $HOME/gen                         # line 13

- line 6: this line replaces line 1 to 25 of example 6. It declares that the current extract should inherit from the previous extract located at `~frva/var/vn22.0`.

Running the extract system using the above configuration will trigger an incremental extract, as if you are running an incremental extract having modified the configuration file in example 4 to that of example 6. The only difference is that the original extract using the example 4 configuration will be left untouched at `~frva/var/vn22.0`, and the new extract will contain only the changes in the branches declared from line 8 to 13.

Note: extract inheritance allows you to add more branches to a package, but you should not redefine the `REPOS`, `REVISION`, `EXPSRC` or `SRC` declarations of a branch that is already declared (and already extracted) in the inherited extract. Although the system will not stop you from doing so, you may end up with an extract that does not quite do what it is supposed to do. For example, if the `base` branch in the `foo` package (`repos::foo::base`) is already defined and extracted in an extract you are inheriting from, you should not redefine any of the `*::foo::base` declarations in your current extract. However, you are free to add more branches for the same package with new labels (e.g. `repos::foo::b1`), and indeed new packages that are not already defined in the inherited extract (e.g. `repos::bar::base`).

If you are setting up an extract to be inherited, you do not have to perform a build. If you don't you will still gain the benefit of incremental file extract, but you will be performing a full build of the code.

Note - inherit and mirror

  It is worth bearing in mind that `rdest::*` settings are not inherited. If mirroring is required in the inheriting extract, it will require its own set of `rdest::*` declarations.

  The system will, however, assume that a mirrored version of the inherited extract is available for inheritance from the mirrored destination of the current extract.

  E.g.: Consider an extract at `/path/to/inherited/` and an inheriting extract at `/path/to/current/`. If the former does not have a mirror, the latter should not have one either. If the former mirrors to `machine@/path/to/inherited/mirror/` and the latter mirrors to `machine@/path/to/current/mirror/`, the system will assume that the subsequent build at `machine@/path/to/current/mirror/` can inherit from the build at `machine@/path/to/inherited/mirror/`. This is illustrated below:

      /path/to/current/       => at machine: /path/to/current/mirror/
      use /path/to/inherited/ => at machine: use /path/to/inherited/mirror/

### Extract - Build Configuration

Configuration settings for feeding into the build system can be declared through the extract configuration file using the `BLD::` prefix. Any line in an extract configuration containing a label with such a prefix will be considered a build system variable. At the end of a successful extract, the system strips out the `BLD::` prefix before writing these variables to the build configuration file. Some example entries are given between line 17 and 22 in the following configuration file:

    # Example 8
    # ----------------------------------------------------------------------
    cfg::type           ext
    cfg::version        1.0

    dest                $PWD

    repos::var::base    fcm:var_tr
    repos::ops::base    fcm:ops_tr
    repos::gen::base    fcm:gen_tr

    revision::gen::base 2468

    expsrc::var::base   src/code
    expsrc::var::base   src/scripts
    expsrc::ops::base   src/code
    src::gen::base      src/code/GenMod_Constants
    src::gen::base      src/code/GenMod_Control
    src::gen::base      src/code/GenMod_FortranIO
    src::gen::base      src/code/GenMod_GetEnv
    src::gen::base      src/code/GenMod_ModelIO
    src::gen::base      src/code/GenMod_ObsInfo
    src::gen::base      src/code/GenMod_Platform
    src::gen::base      src/code/GenMod_Reporting
    src::gen::base      src/code/GenMod_Trace
    src::gen::base      src/code/GenMod_UMConstants
    src::gen::base      src/code/GenMod_Utilities

    bld::target         VarProg_AnalysePF.exe   # line 27

    bld::tool::fc       sxmpif90                # line 29
    bld::tool::cc       sxmpic++                # line 30
    bld::tool::ld       sxmpif90                # line 31

The above example is essentially the same as example 4, apart from the additional build configuration. The following is a simple explanation of what the lines represent: (For detail of the build system, please see the next chapter on The Build System.)

- line 27: the line declares a default target of the build.
- line 29-31: the lines declare the Fortran compiler, the C compiler and the linker respectively.

Note - use of variables

  When you start using the extract system to define compiler flags for the build system, you may end up having to make a lot of long and repetitive declarations. In this case, you may want to define variables to replace the repetitive parts of the declarations.

  Environment variables whose names contain only upper case latin alphabets, numbers and underscores can be referenced in a declaration value via the syntax `$NAME` or `${NAME}`. For example:

      repos::um::base    ${HOME}/svn-wc/um
      bld::tool::fflags  $MY_FFLAGS

  You can define a user variable by making a declaration with a label that begins with a percent sign `%`. The value of a user variable remains in memory until the end of the current file is reached. You can reference a user variable in a declaration value via the syntax `%NAME` or `%{NAME}`. For example:

      # Declare a variable %fred
      %fred                     -Cdebug -eC -Wf,-init heap=nan stack=nan

      bld::tool::fflags         %fred
      # bld::tool::fflags       -Cdebug -eC -Wf,-init heap=nan stack=nan

      bld::tool::fflags::foo    %fred -f0
      # bld::tool::fflags::foo  -Cdebug -eC -Wf,-init heap=nan stack=nan -f0

      bld::tool::fflags::bar    -w %fred
      # bld::tool::fflags::bar  -w -Cdebug -eC -Wf,-init heap=nan stack=nan

  Further to this, each declaration results in an internal variable of the same name and you can also refer to any of these internal variables in the same way. So, the example given above could also be written as follows:

      bld::tool::fflags         -Cdebug -eC -Wf,-init heap=nan stack=nan
      bld::tool::fflags::foo    %bld::tool::fflags -f0
      bld::tool::fflags::bar    -w %bld::tool::fflags

Note - as-parsed configuration

  If you use a hierarchy of `INC` declarations or variables, you may end up with a configuration file that is difficult to understand. To help you with this, the extract system generates an as-parsed configuration file at `cfg/parsed_ext.cfg` of the destination. The content of the as-parsed configuration file is what the extract system actually reads. It should contain everything in your original extract configuration file, except that all `INC` declarations, environment variables and user/internal variables are expanded.

## Diagnostic verbose level

The amount of diagnostic messages generated by the extract system is normally set to a level suitable for normal everyday operation. This is the default diagnostic verbose level 1. If you want a minimum amount of diagnostic messages, you should set the verbose level to 0. If you want more diagnostic messages, you can set the verbose level to 2 or 3. You can modify the verbose level in two ways. The first way is to set the environment variable FCM_VERBOSE to the desired verbose level. The second way is to invoke the extract system with the `-v <level>` option. (If set, the command line option overrides the environment variable.)

The following is a list of diagnostic output at each verbose level:

Level 0
  - Report the time taken to extract the code.
  - Report the time taken to mirror the code.
  - If `rdist` is used to mirror the code, run the command with the `-q` option.
Level 1
  - Everything at verbose level 0.
  - Report the name of the extract configuration file.
  - Report the location of the extract destination.
  - Report date/time at the beginning of the extract step.
  - If the revision specified for a repository branch is not its last changed revision, print an information statement to inform the user of the last changed revision of the branch.
  - Summarises the destination status and the source status.
  - Report date/time at the beginning of the mirror step.
  - Report the location of the alternate destination.
  - Report total time.
Level 2
  - Everything at verbose level 1.
  - If the revision specified for a repository branch is not current (i.e. the specified revision number is less than the revision number of the last commit revision), print an information statement to inform the user of the last commit revision of the branch.
  - Report the detail of each change in the destination.
  - If `rdist` is used to mirror the code, run the command without the `-q` option.
Level 3
  - Everything at verbose level 2.
  - Report all shell commands invoked by the extract system with timestamp.
  - If `rdist` is used to mirror the code, print the `distfile` supplied to the command.
  - If `rsync` is used to mirror the code, invoke the command with the `-v` option.

## When Subversion Is Not Available

The extract system can still be used if Subversion is not available. Clearly, you can only use local repositories. However, you can still do incremental extract, mirror an extract to an alternate location, or combine code from multiple local repositories.

If you are using Subversion but your server is down then clearly there is little you can do. However, if you already have an extract then you can re-run `fcm extract` as long as the extract configuration file only refers to fixed revisions. If this is not the case then you can always use the expanded extract configuration file which can be found in `cfg/ext.cfg` under the extract destination root. This means that you can continue to makes changes to local code and do incremental extracts even whilst your Subversion server is down.