Shallow Thoughts : : Mar

Akkana's Musings on Open Source Computing and Technology, Science, and Nature.

Fri, 31 Mar 2017

Show mounted filesystems

Used to be that you could see your mounted filesystems by typing mount or df. But with modern Linux kernels, all sorts are implemented as virtual filesystems -- proc, /run, /sys/kernel/security, /dev/shm, /run/lock, /sys/fs/cgroup -- I have no idea what most of these things are except that they make it much more difficult to answer questions like "Where did that ebook reader mount, and did I already unmount it so it's safe to unplug it?" Neither mount nor df has a simple option to get rid of all the extraneous virtual filesystems and only show real filesystems.

http://unix.stackexchange.com/questions/177014/showing-only-interesting-mount-p oints-filtering-non-interesting-types had some suggestions that got me started:

mount -t ext3,ext4,cifs,nfs,nfs4,zfs
mount | grep -E --color=never  '^(/|[[:alnum:]\.-]*:/)'
Another answer there says it's better to use findmnt --df, but that still shows all the tmpfs entries (findmnt --df | grep -v tmpfs might do the job).

And real mounts are always mounted on a filesystem path starting with /, so you can do mount | grep '^/'.

But it also turns out that mount will accept a blacklist of types as well as a whitelist: -t notype1,notype2... I prefer the idea of excluding a blacklist of filesystem types versus restricting it to a whitelist; that way if I mount something unusual like curlftpfs that I forgot to add to the whitelist, or I mount a USB stick with a filesystem type I don't use very often (ntfs?), I'll see it.

On my system, this was the list of types I had to disable (sheesh!):

mount -t nosysfs,nodevtmpfs,nocgroup,nomqueue,notmpfs,noproc,nopstore,nohugetlbfs,nodebugfs,nodevpts,noautofs,nosecurityfs,nofusectl

df is easier: like findmnt, it excludes most of those filesystem types to begin with, so there are only a few you need to exclude:

df -hTx tmpfs -x devtmpfs -x rootfs

Obviously I don't want to have to type either of those commands every time I want to check my mount list. SoI put this in my .zshrc. If you call mount or df with no args, it applies the filters, otherwise it passes your arguments through. Of course, you could make a similar alias for findmnt.

# Mount and df are no longer useful to show mounted filesystems,
# since they show so much irrelevant crap now.
# Here are ways to clean them up:
mount() {
    if [[ $# -ne 0 ]]; then
        /bin/mount $*
        return
    fi

    # Else called with no arguments: we want to list mounted filesystems.
    /bin/mount -t nosysfs,nodevtmpfs,nocgroup,nomqueue,notmpfs,noproc,nopstore,nohugetlbfs,nodebugfs,nodevpts,noautofs,nosecurityfs,nofusectl
}

df() {
    if [[ $# -ne 0 ]]; then
        /bin/df $*
        return
    fi

    # Else called with no arguments: we want to list mounted filesystems.
    /bin/df -hTx tmpfs -x devtmpfs -x rootfs
}

Update: Chris X Edwards suggests lsblk or lsblk -o 'NAME,MOUNTPOINT'. it wouldn't have solved my problem because it only shows /dev devices, not virtual filesystems like sshfs, but it's still a command worth knowing about.

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[ 12:25 Mar 31, 2017    More linux | permalink to this entry | comments ]

Sat, 25 Mar 2017

Reading keypresses in Python

As part of preparation for Everyone Does IT, I was working on a silly hack to my Python script that plays notes and chords: I wanted to use the computer keyboard like a music keyboard, and play different notes when I press different keys. Obviously, in a case like that I don't want line buffering -- I want the program to play notes as soon as I press a key, not wait until I hit Enter and then play the whole line at once. In Unix that's called "cbreak mode".

There are a few ways to do this in Python. The most straightforward way is to use the curses library, which is designed for console based user interfaces and games. But importing curses is overkill just to do key reading.

Years ago, I found a guide on the official Python Library and Extension FAQ: Python: How do I get a single keypress at a time?. I'd even used it once, for a one-off Raspberry Pi project that I didn't end up using much. I hadn't done much testing of it at the time, but trying it now, I found a big problem: it doesn't block.

Blocking is whether the read() waits for input or returns immediately. If I read a character with c = sys.stdin.read(1) but there's been no character typed yet, a non-blocking read will throw an IOError exception, while a blocking read will wait, not returning until the user types a character.

In the code on that Python FAQ page, blocking looks like it should be optional. This line:

fcntl.fcntl(fd, fcntl.F_SETFL, oldflags | os.O_NONBLOCK)
is the part that requests non-blocking reads. Skipping that should let me read characters one at a time, block until each character is typed. But in practice, it doesn't work. If I omit the O_NONBLOCK flag, reads never return, not even if I hit Enter; if I set O_NONBLOCK, the read immediately raises an IOError. So I have to call read() over and over, spinning the CPU at 100% while I wait for the user to type something.

The way this is supposed to work is documented in the termios man page. Part of what tcgetattr returns is something called the cc structure, which includes two members called Vmin and Vtime. man termios is very clear on how they're supposed to work: for blocking, single character reads, you set Vmin to 1 (that's the number of characters you want it to batch up before returning), and Vtime to 0 (return immediately after getting that one character). But setting them in Python with tcsetattr doesn't make any difference.

(Python also has a module called tty that's supposed to simplify this stuff, and you should be able to call tty.setcbreak(fd). But that didn't work any better than termios: I suspect it just calls termios under the hood.)

But after a few hours of fiddling and googling, I realized that even if Python's termios can't block, there are other ways of blocking on input. The select system call lets you wait on any file descriptor until has input. So I should be able to set stdin to be non-blocking, then do my own blocking by waiting for it with select.

And that worked. Here's a minimal example:

import sys, os
import termios, fcntl
import select

fd = sys.stdin.fileno()
newattr = termios.tcgetattr(fd)
newattr[3] = newattr[3] & ~termios.ICANON
newattr[3] = newattr[3] & ~termios.ECHO
termios.tcsetattr(fd, termios.TCSANOW, newattr)

oldterm = termios.tcgetattr(fd)
oldflags = fcntl.fcntl(fd, fcntl.F_GETFL)
fcntl.fcntl(fd, fcntl.F_SETFL, oldflags | os.O_NONBLOCK)

print "Type some stuff"
while True:
    inp, outp, err = select.select([sys.stdin], [], [])
    c = sys.stdin.read()
    if c == 'q':
        break
    print "-", c

# Reset the terminal:
termios.tcsetattr(fd, termios.TCSAFLUSH, oldterm)
fcntl.fcntl(fd, fcntl.F_SETFL, oldflags)

A less minimal example: keyreader.py, a class to read characters, with blocking and echo optional. It also cleans up after itself on exit, though most of the time that seems to happen automatically when I exit the Python script.

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[ 12:42 Mar 25, 2017    More programming | permalink to this entry | comments ]

Mon, 20 Mar 2017

Everyone Does IT (and some Raspberry Pi gotchas)

I've been quiet for a while, partly because I've been busy preparing for a booth at the upcoming Everyone Does IT event at PEEC, organized by LANL.

In addition to booths from quite a few LANL and community groups, they'll show the movie "CODE: Debugging the Gender Gap" in the planetarium, I checked out the movie last week (our library has it) and it's a good overview of the problem of diversity, and especially the problems women face in in programming jobs.

I'll be at the Los Alamos Makers/Coder Dojo booth, where we'll be showing an assortment of Raspberry Pi and Arduino based projects. We've asked the Coder Dojo kids to come by and show off some of their projects. I'll have my RPi crittercam there (such as it is) as well as another Pi running motioneyeos, for comparison. (Motioneyeos turned out to be remarkably difficult to install and configure, and doesn't seem to do any better than my lightweight scripts at detecting motion without false positives. But it does offer streaming video, which might be nice for a booth.) I'll also be demonstrating cellular automata and the Game of Life (especially since the CODE movie uses Life as a background in quite a few scenes), music playing in Python, a couple of Arduino-driven NeoPixel LED light strings, and possibly an arm-waving penguin I built a few years ago for GetSET, if I can get it working again: the servos aren't behaving reliably, but I'm not sure yet whether it's a problem with the servos and their wiring or a power supply problem.

The music playing script turned up an interesting Raspberry Pi problem. The Pi has a headphone output, and initially when I plugged a powered speaker into it, the program worked fine. But then later, it didn't. After much debugging, it turned out that the difference was that I'd made myself a user so I could have my normal shell environment. I'd added my user to the audio group and all the other groups the default "pi" user is in, but the Pi's pulseaudio is set up to allow audio only from users root and pi, and it ignores groups. Nobody seems to have found a way around that, but sudo apt-get purge pulseaudio solved the problem nicely.

I also hit a minor snag attempting to upgrade some of my older Raspbian installs: lightdm can't upgrade itself (Errors were encountered while processing: lightdm). Lots of people on the web have hit this, and nobody has found a way around it; the only solution seems to be to abandon the old installation and download a new Raspbian image.

But I think I have all my Raspbian cards installed and working now; pulseaudio is gone, music plays, the Arduino light shows run. Now to play around with servo power supplies and see if I can get my penguin's arms waving again when someone steps in front of him. Should be fun, and I can't wait to see the demos the other booths will have.

If you're in northern New Mexico, come by Everyone Does IT this Tuesday night! It's 5:30-7:30 at PEEC, the Los Alamos Nature Center, and everybody's welcome.

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[ 12:29 Mar 20, 2017    More education | permalink to this entry | comments ]

Fri, 10 Mar 2017

At last! A roadrunner!

We live in what seems like wonderful roadrunner territory. For the three years we've lived here, we've hoped to see a roadrunner, and have seen them a few times at neighbors' places, but never in our own yard.

Until this morning. Dave happened to be looking out the window at just the right time, and spotted it in the garden. I grabbed the camera, and we watched it as it came out from behind a bush and went into stalk mode.

[Roadrunner stalking]

And it caught something!

[close-up, Roadrunner with fence lizard] We could see something large in its bill as it triumphantly perched on the edge of the garden wall, before hopping off and making a beeline for a nearby juniper thicket.

It wasn't until I uploaded the photo that I discovered what it had caught: a fence lizard. Our lizards only started to come out of hibernation about a week ago, so the roadrunner picked the perfect time to show up.

I hope our roadrunner decides this is a good place to hang around.

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[ 14:33 Mar 10, 2017    More nature/birds | permalink to this entry | comments ]

Sun, 05 Mar 2017

The Curious Incident of the Junco in the Night-Time

Dave called from an upstairs bedroom. "You'll probably want to see this."

He had gone up after dinner to get something, turned the light on, and been surprised by an agitated junco, chirping and fluttering on the sill outside the window. It evidently was tring to fly through the window and into the room. Occasionally it would flutter backward to the balcony rail, but no further.

There's a piñon tree whose branches extend to within a few feet of the balcony, but the junco ignored the tree and seemed bent on getting inside the room.

As we watched, hoping the bird would calm down, instead it became increasingly more desperate and stressed. I remembered how, a few months earlier, I opened the door to a deck at night and surprised a large bird, maybe a dove, that had been roosting there under the eaves. The bird startled and flew off in a panic toward the nearest tree. I had wondered what happened to it -- whether it had managed to find a perch in the thick of a tree in the dark of night. (Unlike San Jose, White Rock gets very dark at night.)

And that thought solved the problem of our agitated junco. "Turn the porch light on", I suggested. Dave flipped a switch, and the porch light over the deck illuminated not only the deck where the junco was, but the nearest branches of the nearby piñon.

Sure enough, now that it could see the branches of the tree, the junco immediately turned around and flew to a safe perch. We turned the porch light back off, and we heard no more from our nocturnal junco.

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[ 11:27 Mar 05, 2017    More nature/birds | permalink to this entry | comments ]