A hidden options file trick

I was listening today to the OurSQL Episode 36: It's Not Our (De)fault! Part 1. As usual, Sheeri and Sarah are very informational and entertaining while explaining the innards of MySQL and their best practices.
Being a DBA oriented show, there was an omission in this podcast. There was no mention of custom groups that you can have for your my.cnf. This is mostly useful for developers. If your application requires some specific settings, instead of using a separated configuration file, you can use a different group, and then instruct your client applications to use that group.
By default, all client applications read the "[client]" group.
But you can tell your client to read a group that you can call whatever you like.
For example, with this configuration file,

[client]
user=common_user
password=common_password

[logrotation]
user=log_rotation_daemon
password=specific_password

You can have a Perl script that takes care of your particular log rotation needs. Instead of the normal credentials, it will use the ones listed in the [logrotation] group.

use strict;
use warnings;
use DBI;

my $dsn =   "DBI:mysql:test;"
            . "mysql_read_default_group=logrotation;"
            . "mysql_read_default_file=$ENV{HOME}/./my.cnf";
my $dbh = DBI->connect($dsn);

Notice that, for this option to work, the [logrotation] group must come after the [client] group, or the directives in the [client] group will override the ones in [logrotation]. That's why, in the options file, you find the directives for [mysqldump] at the bottom of the file.

So far, so good. This was a trick for developers, and probably many developers know it already. But there is another, related trick, that can be used by non-developers as well.
If you knew about these customized groups, you may have realized that you can't use them with the mysql standard command line client. Or, to say it better, there is no clearly documented way of doing so. There is, in fact, a cute trick that you can use.
Let's say that, from time to time, you want to use a different prompt, but you don't want to edit your $HOME/.my.cnf to change it. You just want your prompt to be there in the option file, and be able to recall it when the situation calls for it.
The mysql internal help does not tell anything about groups. However, a careful search of the manual gives this cryptic entry:

• --defaults-group-suffix=suffix, -g suffix
  
  In addition to the groups named on the command line, read groups that have the given suffix.
  

When I found it, I stared at this puzzling statement for a while. I could not understand which are the groups that are named in the command line.
Eventually, I figured out why there is a group-suffix and not simply a group. It means that if you add a suffix to a default group name, and you tell mysql to look for this suffix, then you will be able to use the appropriate group on demand.
For example, this options file will not work.

# wrong
[pinocchio]
prompt='I have a long nose  =======> '

[master]
prompt='master [\h] {\u} (\d) > '

[slave]
prompt='slave [\h] {\u} (\d) > '

But this one will work:

[mysqlpinocchio]
prompt='I have a long nose  =======> '

[mysqlmaster]
prompt='master [\h] {\u} (\d) > '

[mysqlslave]
prompt='slave [\h] {\u} (\d) > '

Here is a test run:

$ mysql --defaults-group-suffix=pinocchio
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 22
Server version: 5.1.54 MySQL Community Server (GPL)

Copyright (c) 2000, 2010, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

I have a long nose  =======> 

The meaning of the suffix part is that mysql will read the default groups (which are [client], and [mysql]), and it will also read any groups that are named "mysqlSUFFIX" or "clientSUFFIX". I have named the group "mysqlpinocchio" and therefore it has been used. It would have worked the same if I had called it "clientpinocchio".