Parsing command-line arguments

A server app that runs a batch job often takes parameter values from the command line. How can we get these values in our program?

How to do it...

The obvious way to parse command-line arguments is as follows (see command_line_arguments.dart):

void
 main(List<String> args) {
  print("script arguments:");
 for(String arg in args)
    print(arg);
}

Now, the command dart command_line_arguments.dart param1 param2 param3 gives you the following output:

script arguments:

param1

param2

param3

However, you can also test this from within Dart Editor, open the menu Run, and select Manage Launches (Ctrl + Shift + M). Fill in the parameters in the Script arguments window:

Script arguments

What if your parameters are in the key:value form, as shown in the following code?

par1:value1 par2:value2 par3:value3

In this case, use the following code snippet:

for(String arg in args) {
 List<String> par = arg.split(':');
 var key = par[0];
 var value = par[1];
    print('Key is: $key - Value is: $value');
  }

The previous code snippet gives you the following output:

Key is: par1 - Value is: value1

Key is: par2 - Value is: value2

Key is: par3 - Value is: value3

The split method returned List<String> with a key and value for each parameter. A more sophisticated way to parse the parameters can be done as follows:

final parser = new ArgParser();
argResults = parser.parse(args);
List<String> pars = argResults.rest;
print(pars); // [par1:value1, par2:value2, par3:value3]

Again, use split to get the keys and values.

How it works...

The main() function can take an optional argument List<String> args to get parameters from the command line. It only takes a split of the parameter strings to get the keys and values.

The second option uses the args package from the pub repository, authored by the Dart team. Include args:any in the dependencies section of the pubspec.yaml file. Then, you can use the package by including import 'package:args/args.dart'; at the top of the script. The args package can be applied both in client and server apps. It can be used more specifically for the parsing of GNU and POSIX style options and is documented at https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/args/args.

See also

• Refer to the Searching in files recipe in Chapter 6, Working with Files and Streams, for an example of how to use the args package with a flag