The
FTAM Virtual File Store (VFS) is defined by the FTAM ISO
standard, and provides a common file system abstraction for all
FTAM implementations to use. Each vendor independently maps the
FTAM VFS to their real file system.
The FTAM VFS defines several file attributes—including
access control mechanisms—that are not native to HP-UX.
(See Chapter 5 “FTAM File Protection”) To implement
these attributes and mechanisms, HP-UX FTAM uses supplemental files
called shadow files.
When FTAM creates or modifies an HP-UX file, it creates a
shadow file for the HP-UX file. The shadow file has the same name
as the HP-UX file, prefixed by a period and underscore ("._").
The shadow file is located in the same directory as the HP-UX file.
For example, suppose you use the fcp command to copy a file
named "mydata" from a remote host to your home
directory. If you list the whole directory (use $ ls -a), you will
find both of the following files:
The file named ._mydata
is the FTAM shadow file for the file named mydata.
FTAM must be able to handle new files, which have no shadow
file, as well as existing files.
If FTAM attempts to create or modify
a file for which there is a shadow file, it applies the attributes
and access control settings noted in the shadow file.
If FTAM attempts to create or modify a file for
which there is no shadow file, it applies the default attributes
and access control settings noted in Chapter 7 “FTAM File Details ” FTAM creates a new shadow file, which
contains these defaults.
Precautionary Notes about Shadow Files |
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An HP-UX file together with its FTAM
shadow file forms a logical unit, which should be maintained. This
logical unit can be thought of as "an FTAM file,"
and is distinct from an ordinary HP-UX file. The two HP-UX files
are paired, based on their related names. However, the underlying
HP-UX system does not enforce this logical matching.
As a result, common HP-UX utilities like cp, mv, rm, or chmod
can inadvertently cause inconsistencies when applied to FTAM files.
For example, consider the previous case, which discussed two
files: one called mydata,
and its shadow file, ._mydata.
If you use mv to rename mydata
to junedata,
the contents of the file are unchanged. However, the complementary
shadow file ._mydata
is not automatically renamed to become ._junedata.
Consequently, the FTAM shadow file information it contains is no
longer linked to the matching data file.
Of course, FTAM can still be used with the junedata
file, but it will apply default attribute information
as noted previously. If the shadow file ._mydata
contained other (non-default) information, such as access
control, that information is effectively lost. It is possible, in
this example, to use mv to also rename ._mydata
to ._junedata.
This restores the logical pairing between the shadow file and the
data file.
However, the best approach is to use ftam or fcattr-n to rename
the file. These FTAM utilities will automatically update any existing
shadow files. Likewise, you will want to use FTAM utilities like
fcp and fmv to copy or move FTAM files.
You should also use fdel or ftam to delete FTAM files. If
you use rm instead, the shadow file will continue to exist. Over
time, you could accumulate a clutter of unused shadow files. In
the worst case, you could create a new file with the same name as
an outdated shadow file. The information in the shadow file may
not match the real attributes or access control you expect or desire
for the new file.
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