Chapter 6. Package maintainer scripts and installation procedure

40

6.2 Maintainer scripts Idempotency

It is necessary for the error recovery procedures that the scripts be idempotent. This means

that if it is run successfully, and then it is called again, it doesn't bomb out or cause any harm,

but just ensures that everything is the way it ought to be. If the first call failed, or aborted

half way through for some reason, the second call should merely do the things that were left

undone the first time, if any, and exit with a success status if everything is OK.

1

6.3 Controlling terminal for maintainer scripts

The maintainer scripts are guaranteed to run with a controlling terminal and can interact with

the user. If they need to prompt for passwords, do full screen interaction or something simi 

lar you should do these things to and from

/dev/tty

, since

dpkg

will at some point redirect

scripts' standard input and output so that it can log the installation process. Likewise, because

these scripts may be executed with standard output redirected into a pipe for logging pur 

poses, Perl scripts should set unbuffered output by setting

$|=1

so that the output is printed

immediately rather than being buffered.

Each script should return a zero exit status for success, or a nonzero one for failure.

6.4 Summary of ways maintainer scripts are called

  new preinst

install

  new preinst

install

old version

  new preinst

upgrade

old version

  old preinst

abort upgrade

new version

  postinst

configure

most recently configured version

  old postinst

abort upgrade

new version

  conflictor's postinst

abort remove in favour

package new version

  deconfigured's postinst

abort deconfigure in favour

failed install package version

removing

conflicting package version

  prerm

remove

  old prerm

upgrade

new version

  new prerm

failed upgrade

old version

  conflictor's prerm

remove in favour

package new version

  deconfigured's prerm

deconfigure in favour

package being installed version

removing

conflicting package version

  postrm

remove

  postrm

purge

  old postrm

upgrade

new version

  new postrm

failed upgrade

old version

1

This is so that if an error occurs, the user interrupts

dpkg

or some other unforeseen circumstance happens you

don't leave the user with a badly broken package when

dpkg

attempts to repeat the action.