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