The Compose key <http://wiki.wlug.org.nz/ComposeKey> is probably the
best way of typing less common characters available on any OS. It lets
you use mnemonic sequences that actually bear some relationship to the
character you are trying to enter, instead of having to remember
numeric codes. You will find there a list of the characters that I
found most useful, though the complete set will be in the Compose file
for your chosen locale.
You can even add your own compose sequences. This tutorial for KDE
<https://userbase.kde.org/Tutorials/ComposeKey/en> covers the procedure.
Basically, you create a file “.XCompose” in your home directory,
include a system Compose file to inherit the standard sequences, and put
in extra compose-key definitions using the same syntax as the system
I had got this far, but then was baffled as to why KDE seemed to pay no
attention whatsoever to my custom compose-key sequences. It turns out
you need to define additional environment variables to get Qt/KDE- and
GTK-based apps to use the X11 input method that actually reads
the .XCompose file. Put these in your .bashrc:
and when you logout and login again, it should all work. For example,
in my .XCompose, I have the line
<Multi_key> <p> <i>
which lets me type compose-p-i to produce “π”.