exchange caps_key into ctrl using xmodmap

!
! Swap Caps_Lock and Control_L
!
remove Lock = Caps_Lock
remove Control = Control_L
keysym Control_L = Caps_Lock
keysym Caps_Lock = Control_L
add Lock = Caps_Lock
add Control = Control_L

you had better save .xmodmap in ~/
at first, in command window, xmodmap .xmodmap