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6. Triggers
Triggers provide a well-defined method for packages to interact with one
another at package install and uninstall time. They are an extension
of the normal installation scripts (i.e. %pre) which allows one package
(the ``source'' of the trigger package, which I often think of as the
``triggered package'') to execute an action when the installation status
of another package (the ``target'' of the trigger) changes.
6.1 How do you implement a trigger?
Say the package ``mymailer'' needs an
/etc/mymailer/mailer
symlink which points
to the mail transport agent to use. If sendmail is installed, the link should
point to
/usr/bin/sendmail
, but it vmail is installed, the link should
instead point to
/usr/bin/vmail
. If both packages are present, we don't care
where the link points (realisticaly, sendmail and vmail should conflict
with one another), while if neither package is installed the link should
not exist at all.
This can be accomplished by mymailer providing trigger scripts which
move the symlink when any of the following occurs:
sendmail is installed
vmail is installed
sendmail is removed
vmail is removed
The first two of these scripts would look like this:
%triggerin -- sendmail
ln -sf /usr/bin/sendmail /etc/mymailer/mailer
%triggerin -- vmail
ln -sf /usr/bin/vmail /etc/mymailer/mailer
These are two installation triggers, triggered by one of sendmail or vmail.
They are run when:
mymailer is already installed, and sendmail is installed or
upgraded
mymailer is already installed, and vmail is installed or
upgraded
sendmail is already installed, and mymailer is installed or
upgraded
vmail is already installed, and mymailer is installed or
upgraded
For the upgrading, the strategy is a little different. Rather then
setting the link to point to the trigger, the link is set to point to
the
other
mailer (if it exists), as follows:
%triggerun -- sendmail
[ $2 = 0 ] || exit 0
if [ -f /usr/bin/vmail ]; then
ln -sf /usr/bin/vmail /etc/mymailer/mailer
else
rm -f /etc/mymailer/mailer
fi
%triggerun -- vmail
[ $2 = 0 ] || exit 0
if [ -f /usr/bin/sendmail ]; then
ln -sf /usr/bin/sendmail /etc/mymailer/mailer
else
rm -f /etc/mymailer/mailer
fi
%postun
[ $1 = 0 ] && rm -f /etc/mymailer/mailer
These trigger scripts get run when:
sendmail is installed, and mymailer is removed
vmail is installed, and mymailer is removed
mymailer is installed, and sendmail gets removed
mymailer is installed, and vmail gets removed
The
%postun
insures that
/etc/mymailer/mailer
is removed when mymailer
is removed (triggers get run at the same time as
%preun
scripts, so
doing this in the
%postun
is safe). Note that the triggers are testing
$2
to see if any action should occur. Recall that the
$1
passed to regular
scripts contains the number of instances of the package which will be
installed when the operation has completed.
$1
for triggers is exactly
the same -- it is the number of instances of the source (or triggered)
package which will remain when the trigger has completed. Similarly,
$2
is the number of instances of the target package which will remain. In
this case, if any of the targets will remain after the uninstall, the
trigger doesn't do anything (as it's probably being triggered by an
upgrade).
6.2 Trigger Syntax
Trigger specifications are of the form:
%trigger{un|in|postun} [[-n] <subpackage>] [-p <program>] -- <trigger>
The
-n
and
-p
arguments are the same as for
%post
scripts.  The
<trigger%gt;
portion is syntactically equivalent to a ``Requires''
specification (version numbers may be used). If multiple items are
given (comma separated), the trigger is run when
any
of those
conditions becomes true (the , can be read as ``or''). For example:
%triggerin -n package -p /usr/bin/perl -- fileutils > 3.0, perl < 1.2
print "I'm in my trigger!\n";
Will put a trigger in package 'package' which runs when the installation
status of either
fileutils > 3.0
or
perl < 1.2
is changed. The script will
be run through
/usr/bin/perl
rather then
/bin/sh
(which is the default).
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