xkb config file for mapping caps to ctrl

90-caps2ctrl.conf

# This is a config file that maps caps-lock key to yet another ctrl key.
# It works pretty much on any Linux system with XKb, the X Keyboard extention,
# so anything that has Xorg would probably work.
# The file goes to /etc/X11/xorg.conf.d/ folder.

Section "InputClass"
    Identifier "keyboard defaults"
    MatchIsKeyboard "on"

    Option "XKbOptions" "caps:ctrl_modifier"
EndSection

# This is equivalent to executing `$ setxkbmap -option 'caps:ctrl_modifier'` on start every time.
# If you want to do something else I highly recommend to read `$ man xkeyboard-config`,
# it contains many more options for what you can do with caps and other keys.