This Week’s Program: Jan 30 - Feb 3

Sometimes I feel confident that I know about computers. This week I realized how little I know about computers. An innocuous little thing scratched an itch in my brain and before too long I found myself deep in a rabbit hole.

This week I kept working on setting up a comfy development environment on my FreeBSD server with Ansible. I encountered one of those things that reaffirmed that doing something out of the typical Linux-world brings a host of interesting questions. I made a lot of small commits this week, so I’ll highlight some of the more interesting ones.

db1edadb6fb75d2136860d9183213d99aaac2126

I add authorized keys to my remote user from my GitHub public SSH keys. Then in another Ansible play, I clone my dotfiles, unpack them into my home directory with Stow, and then clone my emacs.d. Now it feels like home!

In other commits, I install Ruby with rbenv and OpenJDK and Leiningen.

9ad744071f8348d5553726a1ea36c93a77a30ece

In this commit, I break out my Ansible tasks into 3 distinct Roles: bootstrap, packages, and user. This helps organize the Playbook and also allows me to reuse things if I ever decide to spin up multiple FreeBSD systems.

a9286cf21e60cd04460c49ac51b4dc669b48b488

I made this commit yesterday, but really my entire week had led up to this. I put a .termcap file in my remote home directory with this content:

xterm-256color|xterm with clear screen capabilities:\
    :Co#256:pa#32767:\
    :AB=\E[48;5;%dm:AF=\E[38;5;%dm:\
    :tc=xterm-clear:

What is this file? Why does the file have this content? Why is it in my home directory? What is this syntax? What does it mean‽

Friends, I don’t know about computers.

Termcap

I did not know this was even a thing until this week.

Do you have a Mac, like I do? Open your Terminal. Do some stuff in that terminal session. Open your $PAGER or Vi and then close it. What happened? What happens on my Terminal is that the man page or Vi opens and fills the entire screen. When it closes, that screen disappears entirely putting your Terminal back into it’s previous state as though Vi had never been there.

If you use GNU Screen, maybe you’re familiar with the altscreen configuration command. In a terminal you can have an “alternate” or “secondary” screen where something like less or vi can open and when it closes it will put you back on the “primary” screen.

This wasn’t happening on my FreeBSD server. I would open less and when I quit out of it, the contents would just stay there. I found it a minor annoyance, different than what I expected. I wondered why this wasn’t working like I expected it to. I googled. Then I fell down the hole.

Try this on your Mac. Do some work on your terminal and then run:

tput smcup

Your terminal should now be in a nearly new state, with your prompt still there. Do some other stuff and then run:

tput rmcup

You should be back in the previous Terminal state. The tput command will perform a capability of your terminal. In this case the capabilities of smcup & rmcup. In the case of a Mac or Linux, these capabilities are defined in the terminfo database. tput looks at your $TERM environment variable, looks up that terminal in the terminfo database, sees how that terminal performs that capability and then executes that. With programs that use curses or equivalent (like less or vi), typically they instruct the terminal to carry out these capabilities.

From Wikipedia:

Terminfo is a library and database that enables programs to use display terminals in a device-independent manner. Mark Horton implemented the first terminfo library in 1981-1982 as an improvement over termcap.

…

Terminfo was included with UNIX System V Release 2 and soon became the preferred form of terminal descriptions in System V, rather than termcap (which BSD continued to use).

So Terminfo is a newer (i.e. 1980’s) implementation of the older (i.e. 1970’s) Termcap library and database which the BSD derivatives, including FreeBSD, continue to use.

The purposes of these databases is to generalize Terminal capabilities and match Terminal types to how they execute those capabilities. This is wild to me, because my entire experience of using a terminal has been through an xterm-like terminal emulator. But back when Termcap and Terminfo were devised, Terminals were a whole thing. When you look at a Termcap file, you’ll see a whole bunch of references to older terminals. The VT100 is there. So is the Infoton 400 and the Texas Instruments Silent 700 and the IBM 3101.

There’s a reference to the Philips-manufactured French Minitel, used for Videotex services and brought to the United States by US Videotel where my wife, Vera, used one of these terminals for her first experiences on the Internet.¹

All of these artifacts of computer history are there, in your termcap database. Terminals of every era and manufacture and their capabilities. All of them.

So why won’t my screen clear after I close a manpage?

Turns out that a lot of folks don’t like this functionality. A 2009 patch to FreeBSD removes this capability from xterm terminals, but adds an xterm-clear entry that you can use to make a similar terminal with the tc capability. My ~/.termcap file that I committed makes xterm-256color inherit this capability.

I had never encountered termcap before. I had never thought deeply about the capabilities of older Terminals. I had never thought much about xterm or what it means to emulate a terminal and I had certainly never thought that Unix systems carried references to legacy systems around with them in this manner. Now I know, and I know what I don’t know.

Now when I close vim, the screen resets to its previous state. It’s the little things.

💾 Mark