Tim Hatch

Equivalent of os.path.abspath in bash

Mon, 08 Mar 2010 13:00:40 -0800

I don’t know how many times I’ve written a script that takes a directory argument, but I want to store that as an absolute path because I’ll be changing directories a lot. I had tens of lines of code to tack on $PWD with lots of conditions, but last week it finally hit me:

fullpath="$(cd "$1"; pwd)"

It will also give you the path with ../ removed.

OpenSSH return codes

Thu, 18 Feb 2010 19:51:39 -0800

I’ve used openssh for many years, and dropbear for about two. Today I ran into a problem that was intermittently failing. I have an upgrade script that essentially boils down to:

ssh device do_stuff
ssh webservice change_something
ssh device udpate_config

At the top of my script, I want to make sure that an ssh agent is set up and the device has my public key, so I also include:

ssh -o 'batchmode yes' device 'exit 0'

With the versions I’m using (OpenSSH_5.2p1 to a dropbear v0.50 server), this is fundamentally broken, returning 255 most of the time and 0 every now and then. A workaround, in case you hit this too:

ssh -o 'batchmode yes' device 'echo Works' | grep -q Works

This preserves the $? being 0 for success, and nonzero otherwise.

Eagle freeware upgrade

Tue, 26 Jan 2010 15:47:16 -0800

If you upgrade your version of Eagle freeware on OS X, and it continually asks for a license key, the file you need to remove is /Applications/EAGLE/bin/EAGLE.key. Other prefs (say, if windows get saved offscreen) are stored in ~/Library/Preferences/EAGLE.plist.

Grepping for Processes

Wed, 21 Oct 2009 17:04:07 -0700

I learned something new today. Instead of doing

ps auxw | grep procname | grep -v grep

you can just do

ps auxw | grep [p]rocname

since the grep process itself will show up with punctuation in ps.

— via github.

rrdtool graphing script

Tue, 20 Oct 2009 21:48:13 -0700

If you’d like a copy of a pretty straightforward rrdtool frontend script, I’ve put online the one that I use at home and work for your enjoyment. It's called rrdgraphs.

Open Source and Crashing

Tue, 20 Oct 2009 21:31:04 -0700

At $JOB we use a lot of open source technology. Our servers run Linux, our dev machines (generally) run Linux, our core products are written with Ruby on Rails and a lot of Python (some Django, not much).

Consequently we do a lot of editing text, and that editing happens without an IDE because that’s easier to do remotely (or maybe we’re just stuck in our ways). For the Ruby guys, that means Textmate and a little bit of classic vi; for the Python folks gVim/CarbonEmacs (all on OS X). Strangely enough, most of the worst crashes all seem tied to editors (but maybe that’s just because we have them open all the time, just like how 90% of accidents occur within a mile of home). I’m about to downgrade from Snow Leopard because of other minor glitches with Spotlight but Berto’s been hitting kernel panics only when CarbonEmacs is open.

Funny, huh? Not for the second time in a day. And it turns out that Apple Engineering is investigating both of our issues, but nothing yet. At least they’re not duplicates of “existing issues.”

A Simpler Loopback Cable

Tue, 20 Oct 2009 21:08:56 -0700

Earlier today I posted some info on a double-loopback cable I made earlier this year. If you relax the requirement that the ends be different vendors, it's possible to make a much simpler one.

This cable has the TX/RX cris-crossed, and the grounds connected. That's technically all that's required for something like this, all without custom circuitry. Note that on this picture, black is at the top of both cables.

How to Make a Serial Loopback Cable

Tue, 20 Oct 2009 20:59:02 -0700

A regular loopback serial cable just ties the Tx and Rx lines, so any characters you send are echoed back on the receive side. For a special project, I needed one that tied together two whole bidirectional ports so I could connect a piece of software that wanted to use a serial port with another that emulated responses for testing. Here's the frankencable I came up with a couple of weeks ago.

Requirements

Be built with only parts on hand (that is, in my toolbox at the office, or things I regularly keep in my car, which has more of the same)
Consist of two different USB-Serial chip vendors, for easy differentiation with udev rules
Work out-of-the-box on most 2.6 Linux kernels (so the Keyspan 4-port adapter is automatically out, with the Ubuntu 8.x bug 149649)
End goal: write data into one usb-serial device and be able to read it out of the other, on the same machine.

This ended up being a pretty quick rummage. The only devices handy were two FTDI TTL232 which are TTL-level, and a bunch of Prolific PL2303-based RS232-level chips. The handy MAX232 does this conversion, but I didn#8217;t have time to prototype a board...

But the RepRap to the rescue. I had one of the old comms boards which had a MAX232 on it, but it would need a computer PSU to provide power, except...

It needs 5v which the FTDI cable can already provide.

A few jumpers, a spare servo cable, and some trial and error (I always forget whose perspective Tx and Rx are from) later, we have this wonderful gadget (where every connection is pretty much wired backwards from intended):