Chapter 6. Package maintainer scripts and installation procedure

42

3

1 If the package is being upgraded, call:

new preinst

upgrade

old version

new postrm

abort upgrade

old version

If that too fails, then

old postinst

abort upgrade

old version

is called.

2 Otherwise, if the package had some configuration files from a previous version in 

stalled (i.e., it is in the  configuration files only  state):

new preinst

install

old version

Error unwind:

new postrm

abort install

old version

3 Otherwise (i.e., the package was completely purged):

new preinst

install

Error unwind:

new postrm

abort install

4 The new package's files are unpacked, overwriting any that may be on the system al 

ready, for example any from the old version of the same package or from another pack 

age. Backups of the old files are kept temporarily, and if anything goes wrong the pack 

age management system will attempt to put them back as part of the error unwind.

It is an error for a package to contain files which are on the system in another package,

unless

Replaces

is used (see `Overwriting files and replacing packages  

Replaces

' on

page

49

).

It is a more serious error for a package to contain a plain file or other kind of non directory

where another package has a directory (again, unless

Replaces

is used). This error can

be overridden if desired using

  force overwrite dir

, but this is not advisable.

Packages which overwrite each other's files produce behavior which, though determin 

istic, is hard for the system administrator to understand. It can easily lead to  missing 

programs if, for example, a package is installed which overwrites a file from another

package, and is then removed again.

2

A directory will never be replaced by a symbolic link to a directory or vice versa; instead,

the existing state (symlink or not) will be left alone and

dpkg

will follow the symlink if

there is one.

5

1 If the package is being upgraded, call

2

Part of the problem is due to what is arguably a bug in

dpkg

.