Packaging:Alternatives

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Alternatives

What are alternatives

Alternatives provide means for parallel installation of packages which provide the same functionality by maintaining sets of symlinks (one per package) pointing to alternativized files like this: /path/original-file -> /etc/alternatives/packagename-original-file -> /path/original-file.suffix For more information, see update-alternatives(8) manpage.

Recommended usage

Alternatives can be used to allow parallel installation of software that can be used as a drop-in replacement and functions with sufficient similarity that users and other programs would, within reason, not need to know which variant is currently installed (for example: the various MTAs which all provide /usr/sbin/sendmail). Selection of which of the parallel-installed packages to use for a given alternativized file can only be done system-wide by a root-level user. Alternatives are not recommended to facilitate parallel installation of software whose selection should be done by users (for example: the various MPI environments).

How to use alternatives

If a package is using alternatives, the files which would otherwise conflict must be installed with an appropriate suffix (for example: %{_sbindir}/sendmail.postfix instead of %{_sbindir}/sendmail), the original locations must be touched (for example: touch %{_sbindir}/sendmail), the links set up by alternatives must be listed as %ghost in the file list and proper Requires: must be added, like in the examples below.

Putting the alternativized files in the file list ensures that they are owned by respective packages, which means that commands like:

  • rpm -qf /usr/bin/foo
  • yum install /usr/bin/foo
  • repoquery --whatprovides /usr/bin/foo

all work properly. Using %ghost for this purpose allows using globs and generated file lists.

Examples

Example from antlr.spec:

Requires(post): %{_sbindir}/update-alternatives
Requires(postun): %{_sbindir}/update-alternatives
...
%install
...
touch %{buildroot}%{_bindir}/antlr

%post
%{_sbindir}/update-alternatives --install %{_bindir}/antlr \
  %{name} %{_bindir}/antlr-java 10

%postun
if [ $1 -eq 0 ] ; then
  %{_sbindir}/update-alternatives --remove %{name} %{_bindir}/antlr-java
fi
...
%files
...
%ghost %{_bindir}/antlr
%{_bindir}/antlr-java

And a more complex example of alternatives invocation from sendmail.spec, slightly edited:

Requires(post): %{_sbindir}/update-alternatives
Requires(postun): %{_sbindir}/update-alternatives
Requires(preun): %{_sbindir}/update-alternatives
...
%install
...
# rename files for alternative usage
mv %{buildroot}%{_sbindir}/sendmail %{buildroot}%{_sbindir}/sendmail.sendmail
touch %{buildroot}%{_sbindir}/sendmail
for i in mailq newaliases rmail; do
	mv %{buildroot}%{_bindir}/$i %{buildroot}%{_bindir}/$i.sendmail
	touch %{buildroot}%{_bindir}/$i
done
mv %{buildroot}%{_mandir}/man1/mailq.1 %{buildroot}%{_mandir}/man1/mailq.sendmail.1
touch %{buildroot}%{_mandir}/man1/mailq.1
mv %{buildroot}%{_mandir}/man1/newaliases.1 %{buildroot}%{_mandir}/man1/newaliases.sendmail.1
touch %{buildroot}%{_mandir}/man1/newaliases.1
mv %{buildroot}%{_mandir}/man5/aliases.5 %{buildroot}%{_mandir}/man5/aliases.sendmail.5
touch %{buildroot}%{_mandir}/man5/aliases.5
mv %{buildroot}%{_mandir}/man8/sendmail.8 %{buildroot}%{_mandir}/man8/sendmail.sendmail.8
touch %{buildroot}%{_mandir}/man8/sendmail.8

%postun
if [ "$1" -ge "1" ]; then
	if [ "`readlink %{_sysconfdir}/alternatives/mta`" == "%{_sbindir}/sendmail.sendmail" ]; then
		%{_sbindir}/alternatives --set mta %{_sbindir}/sendmail.sendmail
	fi
fi

%post
# Set up the alternatives files for MTAs.
%{_sbindir}/update-alternatives --install %{_sbindir}/sendmail mta %{_sbindir}/sendmail.sendmail 90 \
	--slave %{_bindir}/mailq mta-mailq %{_bindir}/mailq.sendmail \
	--slave %{_bindir}/newaliases mta-newaliases %{_bindir}/newaliases.sendmail \
	--slave %{_bindir}/rmail mta-rmail %{_bindir}/rmail.sendmail \
	--slave /usr/lib/sendmail mta-sendmail /usr/lib/sendmail.sendmail \
	--slave %{_sysconfdir}/pam.d/smtp mta-pam %{_sysconfdir}/pam.d/smtp.sendmail \
	--slave %{_mandir}/man8/sendmail.8.gz mta-sendmailman %{_mandir}/man8/sendmail.sendmail.8.gz \
	--slave %{_mandir}/man1/mailq.1.gz mta-mailqman %{_mandir}/man1/mailq.sendmail.1.gz \
	--slave %{_mandir}/man1/newaliases.1.gz mta-newaliasesman %{_mandir}/man1/newaliases.sendmail.1.gz \
	--slave %{_mandir}/man5/aliases.5.gz mta-aliasesman %{_mandir}/man5/aliases.sendmail.5.gz \
	--initscript sendmail
...

%preun
if [ $1 = 0 ]; then
	%{_sbindir}/update-alternatives --remove mta %{_sbindir}/sendmail.sendmail
fi
...

%files
...
%ghost %{_sbindir}/sendmail
%ghost %{_bindir}/mailq
%ghost %{_bindir}/newaliases
%ghost %{_bindir}/rmail
%ghost /usr/lib/sendmail
%ghost %{_sysconfdir}/pam.d/smtp
%ghost %{_mandir}/man8/sendmail.8.gz
%ghost %{_mandir}/man1/mailq.1.gz
%ghost %{_mandir}/man1/newaliases.1.gz
%ghost %{_mandir}/man5/aliases.5.gz

%{_sbindir}/sendmail.sendmail
%{_bindir}/mailq.sendmail
%{_bindir}/newaliases.sendmail
%{_bindir}/rmail.sendmail
/usr/lib/sendmail.sendmail
%config(noreplace) %{_sysconfdir}/pam.d/smtp.sendmail
%{_mandir}/man8/sendmail.sendmail.8.gz
%{_mandir}/man1/mailq.sendmail.1.gz
%{_mandir}/man1/newaliases.sendmail.1.gz
%{_mandir}/man5/aliases.sendmail.5.gz

%attr(0755,root,root) %{_initrddir}/sendmail