#------------------------------------------------------------------------------
#
#  Copy.py
#  Demonstration of how to adjust program operation so as to depend on command
#  line options. Copies a list of files to stdout or to another file, depending 
#  what is found on the command line. See examples below.
#  
#  $ python3 Copy.py file1 file2 ..              # copies files to stdout
#  $ python3 Copy.py -o outfile file1 file2 ..   # copies files to outfile
#
#  The -o option will overwrite outfile if it exists, and otherwise creates
#  outfile. All other command line combinations cause the program to print
#  a usage message to stderr, then quit.
#  
#------------------------------------------------------------------------------
import sys

# usage()
# Prints error messages to stderr
def usage():
   print("Usage: $ python3 Copy.py [-o <output file>] file1 file2 ..", file=sys.stderr)
   exit()
# end


def main():

   if len(sys.argv)==1:
      usage()
   elif sys.argv[1]=="-o" and len(sys.argv)==2:
      usage()
   elif sys.argv[1]=="-o":
      out = open(sys.argv[2], 'w')  # open file in mode 'w' write
      files = sys.argv[3:]
   else:
      out = sys.stdout
      files = sys.argv[1:]
   # end

   s = ""
   for name in files:
      try:
         f = open(name)  # default open mode is 'r' read
      except FileNotFoundError as e:
         print(e, file=sys.stderr)
         usage()
      # end

      content = f.read()  # reads entire file as one string
      s += content
   # end

   # now print everything to output file or to stdout
   print(s, file=out)

# end

#------------------------------------------------------------------------------
if __name__=='__main__':

   main()

# end