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                            hterm Developer Guide


hterm is a JavaScript based terminal emulator that can be embedded in Chrome web applications. It almost works in Firefox, but depends on a small number of changes that haven't been merged yet.

See /HACK.md for general information about working with the source control setup.

If you want to hack on hterm itself, and not just integrate it into your own project, see the internals documentation.

Building the distributables

The ./bin/mkdist script can be used to generate the ./dist directory, which contains the hterm library source concatenated into ./dist/js/hterm_all.js. This is the file you should copy into your own projects.

Coding Style

See the libapps hacking document for details.


The ./bin/load_tests script can be used to launch a new instance of Chrome in an isolated profile, with the necessary command line arguments, and load the hterm test cases. Test results will appear in the JavaScript console.

You can re-run the tests by reloading the web page as long as you haven't made changes to hterm/js/deps_resources.shim.js. If you do change resources, run ./bin/mkdist to re-create them.

Debugging escape sequences

The ./bin/vtscope.py script can be used to step through a pre-recorded VT session on multiple terminals. This is extremely useful for finding and debugging how hterm responds to terminal escape sequences.

The idea is that you record (using the script utility on your Unix-like system) a terminal session that doesn't seem to be working right. You can then play that recording back through vtscope.py. Vtscope.py has the ability to play back simultaneously into two or more terminals.

When the two terminals start to diverge (say, the cursor moved to 0,0 in xterm, but somewhere else in hterm) you know where the trouble is. You can also say what should have happened based on what xterm did.

You can try it out with some of the pre-recorded test data.

First start vtscope.py...

$ cd libapps/hterm/bin
$ ./vtscope.py

Tell vtscope to wait for two clients...

vtscope> accept 2

Then open Secure Shell, and log in to the machine with the hterm source. Start the netcat utility with nc 8383. (If you don't have netcat, get it.)

Next, launch some other terminal (say, xterm) on the same machine. Start netcat again with the same command line.

Now you can load a recorded terminal session in vtscope...

vtscope> open ../test_data/vttest-01.log

And start stepping through the escape sequences...

vtscope> step
vtscope> step
vtscope> step

You should see the two connected terminals changing in lock-step as they receive the escape sequences.

If you‘re going to hand-edit your test data in emacs, don’t forget to add...

 # -*- coding: no-conversion -*-

...as the first line of the file (using vi, of course). Otherwise emacs will likely munge your escape sequences the first time you save.

Check out the comments in ./bin/vtscope.py for some more tricks.