[lttng-dev] Machine Interface

[simon.marchi@polymtl.ca (Simon Marchi)]

On 6 February 2014 12:07, Woegerer, Paul <Paul_Woegerer at mentor.com> wrote:
> Hi Simon,
>
> On 02/06/2014 02:32 PM, Simon Marchi wrote:
>> script) is easier than XML, but it's still not bulletproof. Consider
>> this eventual yaml structure I just made up for the output of the
>> session list.
>>
>> sessions:
>>   - name: my_session
>>     domain: kernel
>>     events:
>>       - name: sched_switch
>>         filter: prev_pid == 1234
>>       - name: sched_migrate_task
>>         otherproperty: "something"
>>   - name: another_session
>>     domain: ust
>>     events:
>>       - name: event_hello
>>       - name: event_bonjour
>>         loglevel: 3
>>
>> If you want to get the list of sessions, you'll probably want to grep
>> the "name: " entries, but then event names would match as well. Of
>> course, names could be different, or you could grep for the
>> appropriate amount of spaces/tabs at the beginning of the line. But
>> then it also depends if the name dictionary entry is the first, then
>> it will have a "-" in front... It's possible, but it will also be very
>> easy to break scripts.
>
> The following simple script below would e.g. give you the list of
> sessions for you example above. To demonstrate the flexibility it
> also generates the event list for session my_session. This is just
> a quick prototype but it should make clear that YAML is considerably
> easier to parse from a shell script than any other option (and also
> reasonably robust). Also notice that no external helpers (like grep)
> are needed.

This is impressive scripting! Although I can see this growing a little
bit out of control for more complex yaml trees. I agree with you that
yaml is the easiest of them to parse in a pure shell script since the
formatting is more restricted. But if you're on a machine that can
take it, using a solution based on an XPath tool (or equivalent) will
make it so much more maintainable.

> list_of_sessions=''
> current_session=''
> my_session_events=''
>
> while IFS= read -r; do
>         line=$REPLY
>         case "${line}" in
>                 "      - "*)
>                         ;&
>                 "        "*)
>                         L2_key=${line:8}
>                         L2_key=${L2_key%:*}
>                         let vpos=2+8+${#L2_key}
>                         L2_value=${line:${vpos}}
>                         ;;
>                 "  - "*)
>                         # New L1 list elem -> reset L2 state
>                         L2_key=''
>                         L2_value=''
>                         ;&
>                 "    "*)
>                         L1_key=${line:4}
>                         L1_key=${L1_key%:*}
>                         let vpos=2+4+${#L1_key}
>                         L1_value=${line:${vpos}}
>                         ;;
>         esac
>         echo "($L1_key,$L1_value), ($L2_key,$L2_value)"
>
>         # Collect the session names
>         if [ "$L1_key" = "name" ]; then
>                 current_session="$L1_value"
>                 list_of_sessions="$list_of_sessions $current_session"
>         fi
>         # Which events are in session my_session ?
>         if [ "$current_session" = "my_session" ] && [ "$L1_key" = "events" ] && [ "$L2_key" = "name" ]; then
>                 my_session_events="$my_session_events $L2_value"
>         fi
> done
> echo
> echo List of sessions: $list_of_sessions
> echo my_session events: $my_session_events
> echo
>
>
> Thanks,
> Paul
>
> --
> Paul Woegerer, SW Development Engineer
> Sourcery Analyzer <http://go.mentor.com/sourceryanalyzer>
> Mentor Graphics, Embedded Software Division