GetPot - File
The following shows you some example code, that you could write in your configuration files. GetPot provides very powerful tools to read these kind of files. As for command line arguments you can define flags, options, variables and nominus arguments. To make things easier variables can be sorted into sections and subsections. The real marvel, though, since the 1.0 Version is the dollar bracket language. It is a lisp-like language that is easy enough to use in order to significantly cut down the time for writing config-files. On the other hand it is cryptic enough to prevent users from programming mischievous configuration files. Click here for an example of dollar bracket code. 
# -*- getpot -*- GetPot mode activation for emacs
#
# Example input file to be parsed by 'GetPot'
#
# (C) 2017 Frank R. Schaefer
# License Terms: MIT
############################################################################

# (*) --------------------------------------------------------------------------
#     examples dealing with input file parsing (input-file.py/.cpp/.java etc.)
#-------------------------------------------------------------------------------
clicks       = 231   # [1/s]
acceleration = 1.231 # [m/s^2]

[vehicle]
wheel-base  = 2.65            # [m]
initial-xyz = '100. 0.1  5.0' # [m]


   [./tires]     # i.e. vehicle/tires/ 
   # Coefficients for Pacejka's Magic Formula 
   # Reference: Bakker, Nyborg, Pacejka: 
   #            "Modelling for Use in Vehicle Dynamics Studies", 
   #             SAE Technical Paper Series 870421, 1988
   B = 3.7976    # [1]
   C = 1.25      # [1]
   E = -0.5      # [1]
   D = 64322.404 # [N]  

   [../chassis]  # i.e. vehicle/chassis/
   Roh = 1.21    # [kg/m^3] density of air 
   S   = 5.14    # [m^2]    reference surface
   Cd  = 0.45    # [1]      air drag coefficient

      [./doors]  # i.e. vehicle/chassis/doors/
      number = 777
      locks  = 'in place'


# back to the root name space
[] 
webpage = http://getpot.sourceforge.net/index.html

# some words about quotes and backslashes
# (1) whitespace requires quotes
user     = 'Frank Schaefer'                   
# (2) backslashed quote => quote  
latex-formula = '\\kappa\' = \\frac{d}{d s} \\kappa' 
# (3) double backslash in quotes = backslash
dos-file = 'C:\\Program Files\\Applications'

# (*) --------------------------------------------------------------------------
#     examples using the prefix filter (filter.py/.cpp/.java etc.)
#-------------------------------------------------------------------------------
[group]
# playing with pseudo command line arguments:
#   note that 
#               '-x', '3134' and '--rudimental' 
#   will be known as 
#               'flags/-x', 'flags/3234' and 'flags/rudimental'.
#
#   Therefore 3134, for example, is **NOT** a number as long,
#   as no prefix filter "flags/" is set.
-x 3134 
--rudimental 12 777

[other]
# this section will be skipped by the 'next' function when the prefix
# is set
nonsense


[user-variables]
# This section contains variables that the user defined himself.
# The GetPot interpreter uses 'get_variable_names()' in order to
# get to know their names.
preview-coefficient   = '12 cm'
lateral-side-distance = '2.1 m/s' 
control-interval      = '0.1 s'
compensation-ratio    = '0.4e34 rad/(m*s)'

[pseudo-function-calls]
# The following code shows how GetPot can be used to emulate
# trivial function calls. Please, note that no syntax checking
# can be provided by this method.
LE-DEBUT
smiley sad
new-line 1
# draw rectangle width = 40, height = 10
rectangle 40 10
new-line 1
smiley happy
# draw circle radius = 15
circle 15
new-line 1
smiley happy
new-line 2
LA-FIN