Create an executable file that copies stdin to stdout, or else a script that does so through the invocation of an interpreter at the command line.

/*REXX pgm copies data from STDIN──►STDOUT (default input stream──►default output stream*/
 
  do while chars()\==0                           /*repeat loop until no more characters.*/
  call charin  , x                               /*read  a char from the  input stream. */
  call charout , x                               /*write "   "    "   "   output   "    */
  end   /*while*/                                /*stick a fork in it,  we're all done. */