Você precisa usar a expressão eval
#!/bin/bash
VER_URXVT='urxvt -help 2>&1 | head -n 2'
echo $VER_URXVT
eval $VER_URXVT
Da página de manual eval
The args are read and concatenated together into a single com-
mand. This command is then read and executed by the shell, and
its exit status is returned as the value of eval. If there are
no args, or only null arguments, eval returns 0.