I generally like to use english keyboard layout for programming, makes it easier to get to the special characters. However, since I'm a hungarian, from time-to-time would like to use accented characters as well. E.g. á, é, ű, ú, ő...

In GNU/Linux to change keyboard layout isn't the easiest to do. As far as I remember (from about 10 years ago) in Windows there is a graphical app that helps you with this. In GNU/Linux this is all via text files.

Here are the steps I've done:

1. add a new section in the us symbol file - /usr/share/X11/xkb/symbols/us:
   

   partial alphanumeric_keys
   xkb_symbols "euro-hu" {
   
       include "us(basic)"
       name[Group1]= "English (US, with Hun)";
   
       include "eurosign(5)"
   
       include "level3(ralt_switch)"
   
       key <AD07> { [            u,            U,        uacute,           Uacute ] };
       key <AD03> { [            e,            E,        eacute,           Eacute ] };
       key <AD08> { [            i,            I,        iacute,           Iacute ] };
       key <AD09> { [            o,            O,        oacute,           Oacute ] };
       key <AD10> { [            p,            P,    odiaeresis,       Odiaeresis ] };
       key <AC01> { [            a,            A,        aacute,           Aacute ] };
       key <AE10> { [            0,   parenright,    odiaeresis,       Odiaeresis ] }; // 0 ) ö Ö
       key <AE11> { [        minus,   underscore,    udiaeresis,       Udiaeresis ] }; // - _ ü Ü
       key <AE12> { [        equal,         plus,        oacute,           Oacute ] }; // = + ó Ó
       key <AD11> { [  bracketleft,    braceleft,  odoubleacute,     Odoubleacute ] }; // [ { ő Ő
       key <AD12> { [ bracketright,   braceright,        uacute,           Uacute ] }; // ] } ú Ú
       key <AC10> { [    semicolon,        colon,        eacute,           Eacute ] }; // ; : é É
       key <AC11> { [   apostrophe,     quotedbl,        aacute,           Aacute ] }; // ' " á Á
       key <BKSL> { [    backslash,          bar,  udoubleacute,     Udoubleacute ] }; // \ | ű Ű
       key <LSGT> { [    backslash,          bar,  udoubleacute,     Udoubleacute ] }; // \ | í Í
   };
   

2. the new keyboard variant should also be noted in the following four files as
   well:

   1. /usr/share/X11/xkb/rules/base.xml:
      

      <variant>
        <configItem>
          <name>euro</name>
          <description>English (US, euro on 5)</description>
        </configItem>
      </variant>
      <variant>
        <configItem>
          <name>euro-hu</name>
          <description>English (US, with Hun)</description>
        </configItem>
      </variant>
      <variant>
        <configItem>
          <name>intl</name>
          <description>English (US, intl., with dead keys)</description>
        </configItem>
      </variant>
      

   2. /usr/share/X11/xkb/rules/base.lst:
      

      euro            us: English (US, euro on 5)
      euro-hu         us: English (US, with Hun)
      intl            us: English (US, intl., with dead keys)
      

   3. /usr/share/X11/xkb/rules/evdev.xml - same way as base.xml

   4. /usr/share/X11/xkb/rules/evdev.lst - same as base.lst

1. Restart the system and the new keyboard layout should be available - for gnome - in the Regional & Language setting's Input Sources section.

If you wish to dig deeper in the topic there is a good documentation about this over on a polish programmer's site Michał Kosmulski's "Creating custom keyboard layouts for X11 using KB" article.

There is also an option to make keyboard layout modifications with Xmodmap, but I've had some bad experiences with it. Namely it was slow to load the config, so rather do it via XKB. Possible that I was doing something bad with Xmodmap... but we will never no that now. :D

Originally I've posted this on https://szab.it/posts/custom-keyboard-layout-in-gnu_linux/