ros2 commandline service call

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What's the correct syntax for performing a service call from the command line in ROS2 ?

Consider the following service, defined in example_interfaces package

AddTwoInts.srv

int a
int b
---
int c

Assuming that the server is called "add_two_ints"

I managed to pass one argument to the service call in the following way

$ ros2 service call /add_two_ints example_interfaces/AddTwoInts "a: '1'"        
requester: making request: example_interfaces.srv.AddTwoInts_Request(a=1, b=0)

How to pass also the second argument ?

These are not working

$ ros2 service call /add_two_ints example_interfaces/AddTwoInts "a: '1'b:'2'"
$ ros2 service call /add_two_ints example_interfaces/AddTwoInts "a: '1' b:'2'"
$ ros2 service call /add_two_ints example_interfaces/AddTwoInts "a: '1', b:'2'"
$ ros2 service call /add_two_ints example_interfaces/AddTwoInts "a:'1'" "b:'2'"
$ ros2 service call /add_two_ints example_interfaces/AddTwoInts "a:'1'", "b:'2'"