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Commit Scripts: Commit scripts are run as part of the standard Junos commit process and allow a user to modify the configuration, display warnings, cancel the commit and so on. For more information, see Commit Script Overview.
Op Scripts: Op scripts are essentially custom CLI commands. They execute Junos OS operational commands and inspect the resulting output. After inspection, op scripts can automatically correct errors within the device running Junos OS based on this output. Op scripts are invoked from the CLI by using the op command. For more information, see How Op Scripts Work
Event Scripts: Event scripts are run automatically in response to a Junos event. Their uses range from gathering data for troubleshooting a problem to changing the configuration in response to various events. For more information, see Event Scripts Overview.
You can use the XSLT or SLAX scripting languages for all the above script types.