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What Shells Do (cont)

The "transformations" applied to input lines include:
  • history substitution ... e.g.   !20 !awk
  • file name expansion ... e.g.   *.c enr.07s?
  • variable evaluation ... e.g.   $PWD ${x-20}
To "execute that command" the shell needs to:
  • find file containing named program   (PATH)
  • start new process for execution of program