125

Appendix G

Diversions   overriding a package's

version of a file (from old Packaging

Manual)

It is possible to have

dpkg

not overwrite a file when it reinstalls the package it belongs to, and

to have it put the file from the package somewhere else instead.

This can be used locally to override a package's version of a file, or by one package to override

another's version (or provide a wrapper for it).

Before deciding to use a diversion, read `Alternative versions of an interface  

update alternatives

(from old Packaging Manual)' on page

123

to see if you really want

a diversion rather than several alternative versions of a program.

There is a diversion list, which is read by

dpkg

, and updated by a special program

dpkg divert

. Please see

dpkg divert(8)

for full details of its operation.

When a package wishes to divert a file from another, it should call

dpkg divert

in its

preinst to add the diversion and rename the existing file. For example, supposing that a

smailwrapper

package wishes to install a wrapper around

/usr/sbin/smail

:

if [ install = "$1"

]; then

dpkg divert   package smailwrapper   add   rename \

  divert /usr/sbin/smail.real /usr/sbin/smail

fi

Testing

$1

is necessary so that the script doesn't try to add the diversion again

when

smailwrapper

is upgraded.

The

  package smailwrapper

ensures that

smailwrapper

's copy of

/usr/sbin/smail

can bypass the diversion and get installed as

the true version.

The postrm has to do the reverse: