Learning Nim - Argument Parsing

Nim is a statically typed language with python-like syntax and the ability to add metaprogramming. It was designed to used for systems and embedded programming. It can be directly compiled to c, c++, objective-c, or javascript depending on the use case.

From the cyber security side of things, nim has had a number of offensive application interest in it. Mostly due to the ease of programming at the systems level, unfamiliarity of the compiled programs it generates with AV, the inherent "obfuscation" of windows api calls, and that reverse engineering tools /plugins arent readily built to handle the nim compiled binaries.

Coming from a strong python and minor c, it feel familiar and definitely has its quirks, like argument parsing. I didn't find a lot of great sources on argument parsing, and the exmamples I did find, while would work, felt lacking in the explanation of what was going on and why thing were named the way they were.

Hopefully, today (or whenever you are reading this) you will be a little more understanding of argument parsing in nim. Additionally, please note, this isn't meant to be the guide on production system argument parsing in nim. Just a cyber security guy trying to write quick programs, if I find a new, safer or more robust, way of parsing later, I'll hopefully edit this with a link.

Also, reading this will have code snipets that explain the code in the source block below. To get the whole gist, click here.

Some overview notes:

• see documentation page for slightly more details: https://nim-lang.org/docs/parseopt.html
• each argument token has a kind, a key, and val.
  • kind being the type of argument, cmdEnd signaling the end of the command line
  • key being the name
  • val being the key's value provided or empty string
• in nim, argument values must be passed with = or :

This is putting all of the information above along with some comments, and the discard to handle the cmdArgument kind (if needed for your program).

Note: The gist has syntax highlighting for easier reading.

# default variables that could be modified from cmd arguments (if needed /desired)
var portNum: int = 5454

# CLI Arguments for the Server
#  in nim argument values must be passed with = or :

# Brief Basic Arugment Parsing in Nim, probably not for production arg parsing
# see documentation page for slightly more details: https://nim-lang.org/docs/parseopt.html
# each argument token has a kind, a key, and val. 
# ... kind being the type of argument, cmdEnd signaling the end of the command line
# ... key being the name
# ... val being the key's value provided or empty string
# getopt: is for iterating over the arguments with it returning the tuple with kind, key, val 
# ... https://nim-lang.org/docs/parseopt.html#getopt
for kind, key, value in getopt(): # we do this because it is a tuple of three returned

  # kind (CmdLineKind): this is the type of argument seen, cmdArgument, cmdLongOption, or cmdShortOption
  # our first case switch because it dictates how we process the argument (since cmdArgument doesnt have a value)
  case kind
  # cmdArgument is without the "-" in the arguments, 
  # like nim c ./file.nim, the c is a cmdArgument
  of cmdArgument:
    # echo "Got arg with key: ", key
    discard #(should discard this iteration of the loop and move to the next one)

  # cmdLongOption, cmdShortOption: 
  of cmdLongOption, cmdShortOption:
    case key
    # -p is to specify port number as in -p=443 or -p:443 but not -p 443
    of "p":
      # echo "Got arg 'p' with value: ", value
      # change the port value string, to an int & store in portNum variable defined above
      discard parseInt(value, portNum, 0) 
  # getopt tells us this is not needed to check for (but may for other procs with arguments parsing)
  of cmdEnd:
    discard # we shouldn't need to discard this and move on to after the loop now