External commands, e.g. compilers, can be executed using the External template:

external 'javac ' id '.java' execute

Again, all output between external and execute keywords goes to building up the command to be executed. For example, the command above would run a Java compiler on a .java file named after the current object, e.g. ‘javac Stopwatch.java’.

There are different flavors for how the eternal command is executed: see below.

Commands are executed immediately they are encountered, and the generator continues executing without waiting for the command to return. Execute forks a command processor, which will open a command prompt window (shell terminal) for batch commands, or the GUI program for GUI executables, documents and locations.

To block generator execution during the external command, replace the final execute command with an executeBlocking command, i.e.

external 'javac ' id '.java' executeBlocking

Note that executeBlocking can only work for programs, i.e. something that exists as an executable (e.g. .exe, .com or .bat on Windows); execute also works with URLs, documents, and internal DOS commands such as mkdir. To use executeBlocking for non-programs, you can make a command that invokes the platform’s command processor or a script like metarun. E.g. the following would work in Windows:

external 'start "" cmd /x /c mkdir NEW' executeBlocking

A useful resource for command processors, batch files and shell scripts is www.ss64.com.

If your generator needs the output from an external command, and you do not want a visible command prompt while the external command runs, replace the final execute command with a read command, i.e.

external 'javac ' id '.java' read

The command’s output will be placed on the generator output stream, or you can save it in a variable:

$javacOutput = __(external 'javac ' id '.java' read)

external..read will block like executeBlocking, as the generator needs to wait for the output. It is however like external..execute in terms of the commands it accepts on Windows: there is no need to prepend cmd.exe /u /c, as that is already done internally. On Windows, if the argument is a document rather than a command, it will open that document and wait for the user to close it before returning: prepend start "" to have it return immediately, leaving the document open.

Note that the call will only return when the external command has finished, so an invocation with just cmd, or anything else that waits for input, will start a command processor that never returns. You can interrupt in MERL with the progress gauge Break button, and kill these cmd.exe instances from Task Manager, possibly resulting in an error and/or opening the results in MetaEdit+.

On non-Windows platforms, external..read does not use metarun or metahelp (the MetaEdit+ helper scripts that allow you to ‘run’ a document or URL), but you can of course explicitly call them.