Parsing Command Line Arguments

• This is somewhat Appendix B
• As you have seen, many programs take command line arguments.
  • gcc -o file -o2 -g .... file.cpp
  • ls -alrt
  • cp oldfile newfile
• These are passed to c/c++ programs as parameters to main.
  • There are three possible parameters to main
  • int argc the number of command line arguments.
  • char * argv[] or char ** argc the array of argument strings.
  • char * envp[] or char ** envp the array of environment variables.
• envp should be null terminated.
• Please see args.cpp
• They are also available in python.
  • see the sys pages for python
  • Please see pyArgs.pythonfile
• And in bash
  • The variable $# is the number of arguments
  • The variable $@ contains all of the arguments.
  • The variable $0 is argv[0],
  • The variable $1 is argv[1]
  • Shift shifts $1 to $n to the left and decrements $#
  • Please see args.bashfile
• A few comments (Hopefully we covered these while looking at the scripts but ...)
  • The extensions should be .py and .sh but
    • I don't know how to keep the web server from executing these files with this name.
    • That may change later.
  • Command line arguments are subject to shell expansion
    • * will be replaced with all files in the directory
    • ? will be replaced by any single character
    • ~ will be replaced with the path to your home directory
    • ~user will be replaced with the path to a user's home directory
    • Strings will be broken on space unless inside quotes.
    • |, ; && ||, io redirect ...
    • There are others but these are the big ones.
  • We will see later that to make a script file executable we need to
    • Make the first line a structured comment in the form
      • #!/path/to/interpreter
    • We also can run it two ways
      • interpreter script args
        • bash args.bashfile args
        • python pyArgs.pythonfile args
      • Or we can make it executable and run it as a command
        • chmod u+x file
        • ./file