you should pass comma separated list to -u, like module_a,module_b etc:

./odoo.py -d database_name_to_update  -u module_a,module_b,module_c

note that there should NOT be space between module names, just comma. Also, -d option is mandatory, you must specify (using -d) the database name you'd like to update modules in.

UPDATE:

if you can't use '-u' because module is not installed yet, then you can use '-i' instead (i.e. install), but if a target module is a fresh new module (usually custom one) and it is not yet added to module list, then I'm afraid to disappoint you but odoo currently does not support module list update from command line, however there is a pretty nice workaround (see the following thread: https://www.odoo.com/forum/help-1/question/how-to-update-module-list-from-command-line-84385). so you'll need to update module list first, and then use "-i" to install new modules.