In clisp, the following code works:
(defun hit-history () (shell "tail ssqHitNum.txt"))
However, in Clozure CL, the shell function is not supported!
No, there is no standard way, but there are libraries which provide this functionality for the important implementations. For example, there's trivial-shell available in Quicklisp, which provides shell-command. (I didn't actually test it, but its among the recommended libraries on CLiki.) There is also external-program. Update: inferior-shell seems to be prefered these days, as Ehvince points out in a comment and his own answer.
You could also use read-time conditionals to make different implementations use their respective functionality to do this.
CCL has ccl:run-program, for example:
CL-USER> (run-program "whoami" '() :output *standard-output*)
foobar
#<EXTERNAL-PROCESS (whoami)[NIL] (EXITED : 0) #xC695EA6>