Command Line Arguments

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.
• argv[0] is the name of the program
  • We will see how to set this later.
  • But it is sometimes used when two programs are the same thing.
  • ls -i /bin/*++
• envp should be null terminated.
• The environment
  • This is not reallly in the section but...
  • LC_COLLATE is an environment variable used by sorting functions in some programs.
    • If the value is C, ascii order is used
    • If the value is en_US.UTF-8 us english is used.
      • en and US are apparent
      • UTF-8 says use 8 bit Unicode.
  • To see this I would
    • Create some files where the English and ASCII sort order would be different. (touch a)
    • Start a new shell (bash)
    • export LC_COLLATE=C
    • ls
    • export LC_COLLATE=en_US.UTF-8
    • ls
    • Exit the shell to return to normal environment (exit)
• Please see args.cpp
• It will be nice to use this in a program
  • Let's write a simple greeting program that
    • By default prints "Hello World!"
    • If run with -G phrase will print the phrase.
    • If run with --greeting phrase will print the phrase
    • If run with --help or -h will print a help message.
    • Otherwise complains about the argument and continues
  • I do the following
    • first set variables to default values

      • string greeting{"Hello World"}
        string ending{"!"} 

    • Next I loop over the arguments.

      • int i{1};
        while (i < argc) {
        } 

      • Start at 1, because the first arg is the name of the program.
    • The arguments are c style strings.
      • I really don't want to convert then to strings (unless I need to), so I use strcmp
        • See man strcmp
        • or this page
          • man7.org is Kerrisk's pages.
        • int strcmp(const char *s1, const char *s2);
        • Returns 0 if the two strings are the same
        • Returns a negative if the first string is less than the second
        • Returns a positive otherwise.
        • My usage

        • if (!strcmp(argv[i], "-G") or !strcmp(argv[i], "--greeting")) { 
          }
                              

    • The body of the if
      • If it is a single flag argument (-h, --help)
        • Increment i
        • Set a value.
      • For values that follow
        • Increment i
        • Check to see if argc < i
          • If so, grab the argument and increment
          • If not, complain.
    • Additional statements are else if (!strcmp...)
    • The if ends in an else that we hit if no flags match.