Assume that I created an rest server endpoint:

http://localhost:3000/rest_api/user/login and I have an account with username: jimthunderbird and password 123456, here is how to log the user in using curl and the services module in Drupal 7

// Retrieve CSRF token
$curl_get = curl_init();
curl_setopt_array($curl_get, array(
    CURLOPT_RETURNTRANSFER => 1,
    CURLOPT_URL => 'http://localhost:3000/services/session/token',
));
$csrf_token = curl_exec($curl_get);
curl_close($curl_get);
$csrf_header = 'X-CSRF-Token: ' . $csrf_token;

// REST Server URL
$request_url = 'http://localhost:3000/rest_api/user/login';

// User data
$user_data = array(
  'username' => 'jimthunderbird',
  'password' => '123456',
);

// cURL
$curl = curl_init();
curl_setopt($curl, CURLOPT_URL, $request_url);
curl_setopt($curl, CURLOPT_POST, 1); // Do a regular HTTP POST
curl_setopt($curl, CURLOPT_HTTPHEADER, array('Content-Type: application/json', $csrf_header));
curl_setopt($curl, CURLOPT_POSTFIELDS, json_encode($user_data)); // Set POST data
curl_setopt($curl, CURLOPT_RETURNTRANSFER, TRUE);

$response = curl_exec($curl);

print $response;

$http_code = curl_getinfo($curl, CURLINFO_HTTP_CODE);
// Check if login was successful
if ($http_code == 200) {
  // Convert json response as array
  $logged_user = json_decode($response);
}
else {
  // Get error msg
  $http_message = curl_error($curl);
  die($http_message);
}

print_r($logged_user);