PLEXIL Executive

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11 May 2015


The PLEXIL Executive (formerly called the Universal Executive, or UE), is an implementation of the PLEXIL language. It is also a runtime environment that provides or facilitates the following capabilities:

  • loading and execution of PLEXIL plans and libraries
  • commanding and querying state of external systems
  • notifications about execution status:

PLEXIL Application

The PLEXIL Executive, often referred to simply as the "executive", is, strictly speaking, a software library written in C++. To use the executive, it must run as part of a PLEXIL application. PLEXIL applications consist of an executive, one or more interface adapters, and one or more external systems or simulators. External interfacing is discussed at length elsewhere. This chapter describes how to operate the executive itself within the context of a PLEXIL application; namely, how to start the executive and specify its application configuration, operating parameters, and textual debugging output.

If you are new to PLEXIL, we suggest you start with the simulators described in Simulating Plan Execution.

Running the Executive

There are two ways to start the PLEXIL Executive: the plexilexec script, and the universalExec executable.

The plexilexec script

The most flexible way to start the PLEXIL Executive is by running the plexilexec shell script. plexilexec can launch a graphical user interface, run various checks on a plan file, and actually run the plan.

Let's assume you have a Plexil plan filed as foo.plx. The most basic invocation of the script at Unix command line is as follows. (The second line is an abbreviated form of the first).

  plexilexec -plan foo.plx
  plexilexec -p foo.plx

An interface configuration file is required to run the Plexil executive, and defaults to a "dummy" configuration filed as plexil/examples/dummy-config.xml. You can specify an interface configuration file (call it my-config.xml) as follows.

  plexilexec -plan foo.plx -config my-config.xml
  plexilexec -p foo.plx -c my-config.xml

If your plan uses libraries (discussed in Plexil Reference), they may be specified as well. In the following example, assume two library files named lib1.plx and lib2.plx.

  plexilexec -plan foo.plx -config my-config.xml -library lib1.plx -library lib2.plx
  plexilexec -p foo.plx -c my-config.xml -l lib1.plx -l lib2.plx

There are many more command line options available; type plexilexec -help to see a listing. We provide a few pointers here:

  • The -check (or -ch) option runs Plexil's static type checker on the plan prior to having it loaded.
  • The -debug (or -d) option specifies a Debug Configuration file (see next section).
  • The -quiet (or -q) option suppresses a leading printed summary and default debug messages during execution.

The PLEXIL Viewer chapter describes additional options that launch the plan viewer, which allows monitoring of plan execution down to the microstep level.

The universalExec executable

The universalExec executable is the actual PLEXIL Executive program. Unlike the plexilexec script, all it does is run plans.

universalExec accepts the following command line options:

  • -c config_file - interface configuration file; if not supplied, a basic configuration is auto-generated
  • -d debug_file - debug configuration file; default is Debug.cfg in the current directory
  • -l library_name_or_file - library to preload; this option can appear multiple times
  • -L library_directory - directory to search for libraries; this option can appear multiple times
  • -p plan_file - the plan file to run
  • -v - connect to a PLEXIL Viewer; suboptions:
    • -b - allow Viewer to block execution (e.g. for breakpoints); default is non-blocking
    • -h hostname_or_ip_addr - the host running the Viewer; default is localhost
    • -n port_number - the TCP port number of the Viewer; default is 49100

Using the examples above, typical invocations might look like:

  universalExec -p foo.plx -c my-config.xml
  universalExec -p foo.plx -c my-config.xml -l lib1.plx -l lib2.plx

Output and the Debug Configuration File

The PLEXIL Executive can generate optional text output. This output can be used to debug a plan or simply study its behavior. To have such output printed as the executive runs, a debug configuration file (or debug file for short) is needed. This file must exist in the directory from which the executive is started (which is not necessarily the same directory as that of the plan). By default, a file named Debug.cfg, if it exists, is used. A debug file with another name can be specified using the -debug or -d command line option to plexilexec (described in previous section), or the similar commands plexiltest and plexilsim described in the simulator chapter.

The debug file is a text file. Lines should start with either a comment character ('#') or a colon, after which should be the tag. A tag is an ad-hoc string, consisting of words separated by colons, with no spaces. Debugging messages encoded in the executive that match any of the tags in the file will be printed to the standard output (i.e. the terminal in which the executive runs) as the plan is executed.

The following are some useful entries to place in this file.


This prints the final outcome of every node.


This prints the start and end times (along with outcome) of every node.


This prints the state transitions of every node. It can be quite verbose.

Unfortunately, the debug file possibilities are not well documented. An incomplete, though fairly current, listing of possible debug tags is found in the file plexil/doc/CompleteDebugFlags.cfg. A somewhat less complete listing of debug tag explanations is found in plexil/doc/DebugFlagDefinitions.txt.

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