if( zHome==0 ){
    if( isOptional ) return 0;
    fossil_panic("cannot locate home directory - please set the "
                 "FOSSIL_HOME, LOCALAPPDATA, APPDATA, USERPROFILE, "
                 "or HOMEDRIVE / HOMEPATH environment variables");
  }
#else
  if( zHome==0 ){
    zHome = fossil_getenv("XDG_CONFIG_HOME");
  }
  if( zHome==0 ){
    zHome = fossil_getenv("HOME");
  }
  if( zHome==0 ){
    if( isOptional ) return 0;
    fossil_panic("cannot locate home directory - please set one of the "
                 "FOSSIL_HOME, XDG_CONFIG_HOME, or HOME environment "
                 "variables");
  }
#endif
  if( file_isdir(zHome, ExtFILE)!=1 ){
    if( isOptional ) return 0;
    fossil_panic("invalid home directory: %s", zHome);
  }
#if defined(_WIN32) || defined(__CYGWIN__)

`--vfs VFSNAME`: Load the named VFS into SQLite.


Environment Variables
---------------------

On most platforms, the location of the user’s account-wide `.fossil`
file is either `FOSSIL_HOME`, `XDG_CONFIG_HOME`, or `HOME`, in that order. 
This ordering lets you put this file somewhere other than at the top 
of your user’s home directory by defining `FOSSIL_HOME` to mask 
the always-defined `HOME`.  The `XDG_CONFIG_HOME` setting is defined
by some desktop environments to be the preferred "modern" directory
in which to store configuration files.

For native Windows builds and for Cygwin builds, the file is called
`_fossil` instead of `.fossil` to avoid problems with old programs that 
assume file names cannot begin with a dot, as was true in old versions 
of Windows and in MS-DOS. (Newer Microsoft OSes and file systems don’t have a
problem with such files, but still we take the safe path in case you’re
on a system with software that can’t cope.) We start our search with
`FOSSIL_HOME` again, but instead of falling back to `HOME`, we instead
try `USERPROFILE`, then `LOCALAPPDATA`, then `APPDATA`, and finally we
concatenate `HOMEDRIVE` + `HOMEPATH`.

`EDITOR`: Name the editor to use for check-in and stash comments.

local (or remote) testing of the moderation subsystem and its impact
on the contents and status of wiki pages.


`FOSSIL_HOME`: Location of the `~/.fossil` file. The first environment
variable found in the environment from the list `FOSSIL_HOME`,
`LOCALAPPDATA` (Windows), `APPDATA` (Windows), `HOMEDRIVE` and
`HOMEPATH` (Windows, used together), `XDG_CONFIG_HOME` (unix) and 
`HOME` is used as the
location of the `~/.fossil` file.


`FOSSIL_USE_SEE_TEXTKEY`: If set, treat the encryption key string for
SEE as text to be hashed into the actual encryption key.  This has no
effect if Fossil was not compiled with SEE support enabled.

`GATEWAY_INTERFACE`: If present and the `--nocgi` option is not, assume
fossil is invoked from a web server as a CGI command, and act
accordingly.

`HOME`: Location of the `~/.fossil` file. The first environment
variable found in the environment from the list `FOSSIL_HOME`,
`LOCALAPPDATA` (Windows), `APPDATA` (Windows), `HOMEDRIVE` and
`HOMEPATH` (Windows, used together), `XDG_CONFIG_HOME` (unix)
and `HOME` is used as the location of the `~/.fossil` file.

`HOMEDRIVE`, `HOMEPATH`: (Windows) Location of the `~/.fossil` file.
The first environment variable found in the environment from the list
`FOSSIL_HOME`, `LOCALAPPDATA` (Windows), `APPDATA` (Windows),
`HOMEDRIVE` and `HOMEPATH` (Windows, used together), and `HOME` is
used as the location of the `~/.fossil` file.

in the clone even before any users have been created, and in that case
it will be the new admin user. If `default-user` is not set, then the
first found environment variable from the list `FOSSIL_USER`, `USER`,
`LOGNAME`, and `USERNAME`, is the user name. As a final fallback, if
none of those are set, then the default user name is "root".


### Configuration Directory (often the Home Directory)

Fossil keeps some information interesting to each user in the user's
configuration file directory. This includes the global settings and the list of
repositories and checkouts used by `fossil all`.  On many,
but not all, systems the configuration file directory is the home directory

The user's configuration file directory is specified by the first environment
variable found in the environment from the list `FOSSIL_HOME`,
`LOCALAPPDATA` (Windows), `APPDATA` (Windows), `HOMEDRIVE` and
`HOMEPATH` (Windows, used together), `XDG_CONFIG_HOME` (unix), and `HOME`.

SQLite has its own notion of the user's configuration file directory, 
which is only exposed if the interactive SQL shell is run with the "fossil
sqlite3" command. Being a separate library, SQLite uses many of the
same variables to find the home directory, but uses them in a
different order, and does not use the `FOSSIL_HOME` nor
`XDG_CONFIG_HOME` variables.


### SQLite VFS to use

See [the SQLite documentation](http://www.sqlite.org/vfs.html) for an
explanation of what a VFS actually is and what it does.

than having to change the setting individually on each repository.

The configuration database also maintains a list of repositories.  This
list is used by the [/help/all | fossil all] command in order to run various
operations such as "sync" or "rebuild" on all repositories managed by a user.

On Unix systems, the configuration database is named ".fossil" and is
located in the user's configuration file directory, which on most older
unix systems is the user's home directory but on some "desktop" linux
environments is the "~/.config" directory.  On Windows, the configuration
database is named "_fossil" (using an underscore as the first character
instead of a dot) and is located in the directory specified by the
LOCALAPPDATA, APPDATA, or HOMEPATH environment variables, in that order.
You can run the [/help?cmd=info|fossil info] command from an open
check-out to see the location of the configuration database.

You can override this default location by defining the environment
variable FOSSIL_HOME pointing to an appropriate (writable) directory.


<h3>2.2 Repository Databases</h3>

The repository database is the file that is commonly referred to as
"the repository".  This is because the repository database contains,
among other things, the complete revision, ticket, and wiki history for
a project.  It is customary to name the repository database after then