(debian-policy.info)Introduction to package maintainer scripts

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6.1 Introduction to package maintainer scripts
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It is possible to supply scripts as part of a package which the package
management system will run for you when your package is installed,
upgraded or removed.

These scripts are the control information files ‘preinst’, ‘postinst’,
‘prerm’ and ‘postrm’.  They must be proper executable files; if they are
scripts (which is recommended), they must start with the usual ‘#!’
convention.  They should be readable and executable by anyone, and must
not be world-writable.

The package management system looks at the exit status from these
scripts.  It is important that they exit with a non-zero status if there
is an error, so that the package management system can stop its
processing.  For shell scripts this means that you `almost always' need
to use ‘set -e’ (this is usually true when writing shell scripts, in
fact).  It is also important, of course, that they exit with a zero
status if everything went well.

Additionally, packages interacting with users using ‘debconf’ in the
‘postinst’ script should install a ‘config’ script as a control
information file.  See Note: Prompting in maintainer scripts. for
details.

When a package is upgraded a combination of the scripts from the old and
new packages is called during the upgrade procedure.  If your scripts
are going to be at all complicated you need to be aware of this, and may
need to check the arguments to your scripts.

Broadly speaking the ‘preinst’ is called before (a particular version
of) a package is unpacked, and the ‘postinst’ afterwards; the ‘prerm’
before (a version of) a package is removed and the ‘postrm’ afterwards.

Programs called from maintainer scripts should not normally have a path
prepended to them.  Before installation is started, the package
management system checks to see if the programs ‘ldconfig’,
‘start-stop-daemon’, and ‘update-rc.d’ can be found via the ‘PATH’
environment variable.  Those programs, and any other program that one
would expect to be in the ‘PATH’, should thus be invoked without an
absolute pathname.  Maintainer scripts should also not reset the ‘PATH’,
though they might choose to modify it by prepending or appending
package-specific directories.  These considerations really apply to all
shell scripts.