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Take command of your command line.

Commandeer gets data from the command line to your variables and exits gracefully if there is any issue.

It does this little thing well and lets you deal with the rest.


In code

## myCLApp.nim

import commandeer

  argument integer, int
  argument floatingPoint, float
  argument character, char
  arguments strings, string
  option optionalInteger, int, "int", "i", -1
  option testing, bool, "testing", "t"
  exitoption "help", "h",
             "Usage: myCLApp [--testing|--int=<int>|--help] " &
             "<int> <float> <char> <string>..."
  errormsg "You made a mistake!"

echo("integer = ", integer)
echo("floatingPoint = ", floatingPoint)
echo("character = ", character)
echo("strings (one or more) = ", strings)

if optionalInteger != 0:
  echo("optionalInteger = ", optionalInteger)

if testing:
  echo("Testing enabled")

On the command line

$ myCLApp --testing 4 8.0 a one two -i:100
integer = 4
floatingPoint = 8.0
character = a
strings (one or more) = @[one, two]
optionalInteger = 100
Testing enabled
$ myCLApp 10 --help
Usage: myCLApp [--testing|--int=<int>|--help] <int> <float> <char> <string>...

When you have commandeer installed, try passing an incorrect set of command line arguments for fun!

See the tests folder for other examples.

It doesn't seek to do too much; it just does what's needed.


There are 2 ways to install commandeer:


Install nimble. Then do:

$ nimble install commandeer

This will install the latest tagged version of commandeer.


Copy the commandeer.nim file to your project and import it.

When I go this way for Nim libraries, I like to create a libs/ folder in my project and put third-party files in it. I then add the line path = "libs" to my nim.cfg file so that the libs/ directory is looked into at compile time.



commandline is used to delimit the space where you define the command line arguments and options you expect. All other commandeer constructs (described below) are placed under it. They are all optional - although you probably want to use at least one, right?

subcommand identifier, name[, alias1, alias2...]

subcommand declares identifier to be a variable of type bool that is true if the first command line argument passed is name or one of the aliases (alias1, alias2, etc.) and is false otherwise. Under it, you define the subcommand arguments and options you expect. All other commandeer constructs (described below) can be placed under it.

For example:

  subcommand add, "add", "a":
    arguments filenames, string
    option force, bool, "force", "f"
  option globalOption, bool, "global", "g"

if add:
  echo "Adding", filenames
if globalOption:
  echo "Global option activated"

See tests/testSubcommands.nim for a larger example.

argument identifier, type

argument declares a variable named identifier of type type initialized with the value of the corresponding command line argument converted to type type.

Correspondence works as follows: the first occurrence of argument corresponds to the first argument, the second to the second argument and so on. Note that if a subcommand is declared then 1) any top-level occurrence of argument is ignored, 2) the first subcommand argument corresponds to the first command line argument after the subcommand, the second to the second argument after the subcommand and so on.

arguments identifier, type [, atLeast1]

arguments declares a variable named identifier of type seq[type] initialized with the value of the sequential command line arguments that can be converted to type type. By default atLeast1 is true which means there must be at least one argument of type type or else an error is thrown. Passing false there allows for 0 or more arguments of the same type to be stored at identifier.

Warning: arguments myListOfStrings, string will eat all arguments on the command line. The same applies to other situations where one type is a supertype of another type in terms of conversion e.g., floats eat ints.

option identifier, type, long name, short name [, default]

option declares a variable named identifier of type type initialized with the value of the corresponding command line option --long name or -short name converted to type type if it is present. The -- and - are added by commandeer for your convenience. If the option is not present, identifier is initialized to its default type value or the passed default value.

The command line option syntax follows Nim's one and adds space (!) i.e., --times=3, --times:3, -t=3, -t:3, --times 3 and -t 3 are all valid.

Syntactic sugar is provided for boolean options such that only the presence of the option is needed to give a true value.

exitoption long name, short name, exit message

exitoption declares a long and short option string for which the application will immediately output exit message and exit. This can be used for subcommand specific exit messages too:

  subcommand add, "add":
    arguments filenames, string
    exitoption "help", "h", "add help"
  exitoption "help", "h", "general help"

This is mostly used for printing the version or the help message.

errormsg custom error message

errormsg sets a string custom error message that will be displayed after the other error messages if the command line arguments or options are invalid.

Valid types for type are:

  • int, float, string, bool, char


  • Keep as much logic out of the module and into the hands of the developer as possible
  • No magical variables should be made implicitly available. All created variables should be explicitly chosen by the developer.
  • Keep it simple and streamlined. Command line parsers can do a lot for you, but I prefer to be in adequate control.
  • Test in context. Tests are run on the installed package because that is what people get.


Run the test suite:

nimble tests

TODO and Contribution

  • Use and see what needs to be added