lttng-add-trigger(1) ===================== :revdate: 27 May 2020 NAME ---- lttng-add-trigger - Create LTTng triggers SYNOPSIS -------- [verse] *lttng* ['linkgenoptions:(GENERAL OPTIONS)'] *add-trigger* [--id ID] [--fire-every N] [--fire-once-after N] --condition CONDITION-NAME CONDITION-ARGS --action ACTION-NAME ACTION-ARGS [--action ACTION-NAME ACTION-ARGS]... DESCRIPTION ----------- The `lttng add-trigger` command is used to create triggers. A trigger is an association between a *condition* and one or more *actions*. When the condition associated to a trigger is met, the actions associated to that trigger are executed. The tracing does not have to be active for the conditions to be met, and triggers are independent from tracing sessions. By default, a trigger fires every time its condition is met. The '--fire-every' and '--fire-once-after' options can be used to change this mode. The syntax by which conditions and actions are specified is described below. [[conditions]] Conditions ~~~~~~~~~~ Conditions are specified with the `--condition` option, followed by a condition name, and possibly some more arguments, depending on the specific condition. There must be exactly one condition given in the `lttng add-trigger` invocation. The available conditions are: Event rule: `on-event [event rule arguments]`:: This type of condition is met when the tracer encounters an event matching the given even rule. The arguments describing the event rule are the same as those describing the event rules of the man:lttng-enable-event(1) command, with these exceptions: - It is not possible to use filter operands that use values from the context. + Fields to capture can be specified with the option:--capture option, followed by a capture expression. Zero or more captures can be configured. See the <> section below for more information. [[actions]] Actions ~~~~~~~ Actions are specified with the `--action` option, followed by an action name, and possibly some more arguments, depending on the specific action. There must be at least one action given in the `lttng add-trigger` invocation. The available actions are: Notify: *notify*:: This action causes the LTTng session daemon to send a notification, through its notification mechanism (see man:lttng-sessiond(8)). Some details about the condition evaluation are sent along with the notification. Start session: *start-session* session-name:: This action causes the LTTng session daemon to start tracing for the session with the given name. If no session with the given name exist at the time the condition is met, nothing is done. Stop session: *stop-session* session-name:: This action causes the LTTng session daemon to stop tracing for the session with the given name. If no session with the given name exist at the time the condition is met, nothing is done. Rotate session: *rotate-session* session-name:: This action causes the LTTng session daemon to rotate the session with the given name. See man:lttng-rotate(1) for more information about the session rotation concept. If no session with the given name exist at the time the condition is met, nothing is done. Snapshot session: *snapshot-session* session-name:: This action causes the LTTng session daemon to take a snapshot of the session with the given name. See man:lttng-snapshot(1) for more information about the session snapshot concept. If no session with the given name exist at the time the condition is met, nothing is done. [[capture-expr]] Capture expression ~~~~~~~~~~~~~~~~~~ A capture expression can be specified with the option:--capture option when creating a new on-event condition. If the capture expression corresponds with an event's field when tracing, the runtime dynamic value corresponding to the capture expression is captured. NOTE: Make sure to **single-quote** the capture expression when running the command from a shell, as capture expressions typically include characters having a special meaning for most shells. * Supported field types: - integer, - unsigned integer, - floating point value, - fixed-size array of integers, - variable-size array of integers (sequence), - enumeration, - text string, - element of any allowing previous type. * The dynamic value of an event field is captured by using its name as a C identifier. + The square bracket notation is available, like in the C language, to access array/sequence field. Only a constant, positive integer number can be used within square brackets. If the index is out of bounds, the capture expression evaluates to `unavailable`. + An enumeration field's value is an integer. + When the capture's field does not exist, the capture expression evaluates to `unavailable`. + Examples: `my_field`, `target_cpu`, `seq[7]` * The dynamic value of a statically-known context field is captured by prefixing its name with `$ctx.`. See man:lttng-add-context(1) to get a list of available contexts. + When the expression's statically-known context field does not exist, the capture expression evaluates to `unavailable`. + Examples: `$ctx.prio`, `$ctx.preemptible`, `$ctx.perf:cpu:stalled-cycles-frontend`. + NOTE: The statically-known context field does NOT need to be added using the man:lttng-add-context(1) command. The statically-known context fields are always available in the context of triggers. * The dynamic value of an application-specific context field is captured by prefixing its name with `$app.` (follows the format used to add such a context field with the man:lttng-add-context(1) command). + When the expression's application-specific context field does not exist, the capture expression evaluates to `unavailable`. + Example: `$app.server:cur_user`. + NOTE: The application-specific context field does NOT need to be added using the man:lttng-add-context(1) command. The application-specific context fields fields are always available in the context of triggers. OPTIONS ------- option:--condition:: Define the condition for the trigger. See the <> section for more details. option:--action:: Define an action for the trigger. See the <> section for more details. option:--id='ID':: Set the id of the trigger to 'ID'. If omitted, an id will automatically be assigned to the trigger by the session daemon. + If a trigger with the specified 'ID' already exists, the trigger creation will fail. option:--fire-every 'N':: Execute the trigger's actions every 'N' times the condition is met. option:--fire-once-after 'N':: Execute the trigger's actions once after 'N' times the condition is met, then never after that. include::common-cmd-help-options.txt[] include::common-cmd-footer.txt[] SEE ALSO -------- man:lttng-list-triggers(1), man:lttng-remove-trigger(1), man:lttng(1)