Command Line Arguments

• This material is from chapter 6.
• As we have seen, many systems programs use command line arguments.
• It is easy to implement command line arguments.
• First the big lie
  • We have lied to you for a while now about main in c/c++
  • Main has no arguments int main()
  • In fact main can have 0 or two arguments.
  • int main(int argc, char * argv[])
  • or int main (int argc, char ** argv)
• argv
  • this is an array of c style strings.
  • Each is null terminated.
  • The last entery is a nullptr as well.
  • The first entry is special, it is the name of the program that is being executed.
• argc
  • this is the number of entries in argv
  • argv[argc] = nullptr
• Look at args.C
• Using argv and argc
  • argv[0] can be used to determin the behavior of a program.
    • ls -il /usr/bin/g++ /usr/bin/c++
  • State variables
    • Set a variable with a default value.
    • Search for the corresponding flag
    • If the flag is set, change the variable to the new value.
    • Execute the program with behavior based on the variables.
    • If an unknown flag is given, a required flag is not given or an invalid value is given
      • Complain to the user about the problem.
      • If you can recover, or use a resonable default continue
      • If you can not recover, exit.
• Types of command line arguments
  • Flags : turn an option on or off
  • Values Setting: normally take at least one argument
  • Look at the manpage for sort.
  • -l
  • --word
  • -l value
  • --word=value
  • -lvalue is also acceptable.
  • The keys are strange
  • sort this file

  • sort -Vrk2,2  -k 1,1 --key=3nr people
    	 

• Note
  • Multiple single character flags can be groupd
  • -Vr
  • A flag needing an argument can end the list
  • -Vrk2,2
  • A single argument character can have a space or not to the next argument.
  • -- is used for words (readability)
  • -- seem to require --flag=value
• really old commands do not rquire the - (see tar)
• You can simplify this by not allowing combination of flags.
  • strcmp to compare c style strings.
  • Write a program that will
    • print help with -h or --help
    • change the greeting string -gstring, -g string, --greeting=string, --greeting string
    • say hello n times with -m n
    • reverse the greeting string with -r or --reverse
    • print letters a through b of the greeting -x a,b , --range=a,b
  • Look at args2.C
• Appendix B of the book discusses getopt a tool for parsing command line arguments.