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(am-utils.info)Variable Expansion

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Variable Expansion
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   To allow generic location specifications Amd does variable expansion
on each location and also on some of the option strings.  Any option or
selector appearing in the form `$"var"' is replaced by the current
value of that option or selector.  For example, if the value of
`${key}' was `bin', `${autodir}' was `/a' and `${fs}' was
`${autodir}/local/${key}' then after expansion `${fs}' would have the
value `/a/local/bin'.  Any environment variable can be accessed in a
similar way.

   Two pathname operators are available when expanding a variable.  If
the variable name begins with `/' then only the last component of the
pathname is substituted.  For example, if `${path}' was `/foo/bar' then
`${/path}' would be expanded to `bar'.  Similarly, if the variable name
ends with `/' then all but the last component of the pathname is
substituted.  In the previous example, `${path/}' would be expanded to
`/foo'.

   Two domain name operators are also provided.  If the variable name
begins with `.' then only the domain part of the name is substituted.
For example, if `${rhost}' was `swan.doc.ic.ac.uk' then `${.rhost}'
would be expanded to `doc.ic.ac.uk'.  Similarly, if the variable name
ends with `.' then only the host component is substituted.  In the
previous example, `${rhost.}' would be expanded to `swan'.

   Variable expansion is a two phase process.  Before a location is
parsed, all references to selectors, eg `${path}', are expanded.  The
location is then parsed, selections are evaluated and option assignments
recorded.  If there were no selections or they all succeeded the
location is used and the values of the following options are expanded in
the order given: SUBLINK, RFS, FS, OPTS, REMOPTS, MOUNT and UNMOUNT.

   Note that expansion of option values is done after "all" assignments
have been completed and not in a purely left to right order as is done
by the shell.  This generally has the desired effect but care must be
taken if one of the options references another, in which case the
ordering can become significant.

   There are two special cases concerning variable expansion:

  1. before a map is consulted, any selectors in the name received from
     the kernel are expanded.  For example, if the request from the
     kernel was for `${arch}.bin' and the machine architecture was
     `vax', the value given to `${key}' would be `vax.bin'.

  2. the value of `${rhost}' is expanded and normalized before the
     other options are expanded.  The normalization process strips any
     local sub-domain components.  For example, if `${domain}' was
     `Berkeley.EDU' and `${rhost}' was initially `snow.Berkeley.EDU',
     after the normalization it would simply be `snow'.  Hostname
     normalization is currently done in a _case-dependent_ manner.


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