Shallow Thoughts

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

Tue, 02 Sep 2014

Using strace to find configuration file locations

I was using strace to figure out how to set up a program, lftp, and a friend commented that he didn't know how to use it and would like to learn. I don't use strace often, but when I do, it's indispensible -- and it's easy to use. So here's a little tutorial.

My problem, in this case, was that I needed to find out what configuration file I needed to modify in order to set up an alias in lftp. The lftp man page tells you how to define an alias, but doesn't tell you how to save it for future sessions; apparently you have to edit the configuration file yourself.

But where? The man page suggested a couple of possible config file locations -- ~/.lftprc and ~/.config/lftp/rc -- but neither of those existed. I wanted to use the one that already existed. I had already set up bookmarks in lftp and it remembered them, so it must have a config file already, somewhere. I wanted to find that file and use it.

So the question was, what files does lftp read when it starts up? strace lets you snoop on a program and see what it's doing.

strace shows you all system calls being used by a program. What's a system call? Well, it's anything in section 2 of the Unix manual. You can get a complete list by typing: man 2 syscalls (you may have to install developer man pages first -- on Debian that's the manpages-dev package). But the important thing is that most file access calls -- open, read, chmod, rename, unlink (that's how you remove a file), and so on -- are system calls.

You can run a program under strace directly:

$ strace lftp sitename
Interrupt it with Ctrl-C when you've seen what you need to see.

Pruning the output

And of course, you'll see tons of crap you're not interested in, like rt_sigaction(SIGTTOU) and fcntl64(0, F_GETFL). So let's get rid of that first. The easiest way is to use grep. Let's say I want to know every file that lftp opens. I can do it like this:

$ strace lftp sitename |& grep open

I have to use |& instead of just | because strace prints its output on stderr instead of stdout.

That's pretty useful, but it's still too much. I really don't care to know about strace opening a bazillion files in /usr/share/locale/en_US/LC_MESSAGES, or libraries like /usr/lib/i386-linux-gnu/libp11-kit.so.0.

In this case, I'm looking for config files, so I really only want to know which files it opens in my home directory. Like this:

$ strace lftp sitename |& grep 'open.*/home/akkana'

In other words, show me just the lines that have either the word "open" or "read" followed later by the string "/home/akkana".

Digression: grep pipelines

Now, you might think that you could use a simpler pipeline with two greps:

$ strace lftp sitename |& grep open | grep /home/akkana

But that doesn't work -- nothing prints out. Why? Because grep, under certain circumstances that aren't clear to me, buffers its output, so in some cases when you pipe grep | grep, the second grep will wait until it has collected quite a lot of output before it prints anything. (This comes up a lot with tail -f as well.) You can avoid that with

$ strace lftp sitename |& grep --line-buffered open | grep /home/akkana
but that's too much to type, if you ask me.

Back to that strace | grep

Okay, whichever way you grep for open and your home directory, it gives:

open("/home/akkana/.local/share/lftp/bookmarks", O_RDONLY|O_LARGEFILE) = 5
open("/home/akkana/.netrc", O_RDONLY|O_LARGEFILE) = -1 ENOENT (No such file or directory)
open("/home/akkana/.local/share/lftp/rl_history", O_RDONLY|O_LARGEFILE) = 5
open("/home/akkana/.inputrc", O_RDONLY|O_LARGEFILE) = 5
Now we're getting somewhere! The file where it's getting its bookmarks is ~/.local/share/lftp/bookmarks -- and I probably can't use that to set my alias.

But wait, why doesn't it show lftp trying to open those other config files?

Using script to save the output

At this point, you might be sick of running those grep pipelines over and over. Most of the time, when I run strace, instead of piping it through grep I run it under script to save the whole output.

script is one of those poorly named, ungoogleable commands, but it's incredibly useful. It runs a subshell and saves everything that appears in that subshell, both what you type and all the output, in a file.

Start script, then run lftp inside it:

$ script /tmp/lftp.strace
Script started on Tue 26 Aug 2014 12:58:30 PM MDT
$ strace lftp sitename

After the flood of output stops, I type Ctrl-D or Ctrl-C to exit lftp, then another Ctrl-D to exit the subshell script is using. Now all the strace output was in /tmp/lftp.strace and I can grep in it, view it in an editor or anything I want.

So, what files is it looking for in my home directory and why don't they show up as open attemps?

$ grep /home/akkana /tmp/lftp.strace

Ah, there it is! A bunch of lines like this:

access("/home/akkana/.lftprc", R_OK)    = -1 ENOENT (No such file or directory)
stat64("/home/akkana/.lftp", 0xbff821a0) = -1 ENOENT (No such file or directory)
mkdir("/home/akkana/.config", 0755)     = -1 EEXIST (File exists)
mkdir("/home/akkana/.config/lftp", 0755) = -1 EEXIST (File exists)
access("/home/akkana/.config/lftp/rc", R_OK) = 0

So I should have looked for access and stat as well as open. Now I have the list of files it's looking for. And, curiously, it creates ~/.config/lftp if it doesn't exist already, even though it's not going to write anything there.

So I created ~/.config/lftp/rc and put my alias there. Worked fine. And I was able to edit my bookmark in ~/.local/share/lftp/bookmarks later when I had a need for that. All thanks to strace.

Tags: , ,
[ 13:06 Sep 02, 2014    More linux/cmdline | permalink to this entry | comments ]

Thu, 28 Aug 2014

Debugging a mysterious terminal setting

For the last several months, I repeatedly find myself in a mode where my terminal isn't working quite right. In particular, Ctrl-C doesn't work to interrupt a running program. It's always in a terminal where I've been doing web work. The site I'm working on sadly has only ftp access, so I've been using ncftp to upload files to the site, and git and meld to do local version control on the copy of the site I keep on my local machine. I was pretty sure the problem was coming from either git, meld, or ncftp, but I couldn't reproduce it.

Running reset fixed the problem. But since I didn't know what program was causing the problem, I didn't know when I needed to type reset.

The first step was to find out which of the three programs was at fault. Most of the time when this happened, I wouldn't notice until hours later, the next time I needed to stop a program with Ctrl-C. I speculated that there was probably some way to make zsh run a check after every command ... if I could just figure out what to check.

Terminal modes and stty -a

It seemed like my terminal was getting put into raw mode. In programming lingo, a terminal is in raw mode when characters from it are processed one at a time, and special characters like Ctrl-C, which would normally interrupt whatever program is running, are just passed like any other character.

You can list your terminal modes with stty -a:

$ stty -a
speed 38400 baud; rows 32; columns 80; line = 0;
intr = ^C; quit = ^\; erase = ^?; kill = ^U; eof = ^D; eol = ;
eol2 = ; swtch = ; start = ^Q; stop = ^S; susp = ^Z; rprnt = ^R;
werase = ^W; lnext = ^V; flush = ^O; min = 1; time = 0;
-parenb -parodd cs8 -hupcl -cstopb cread -clocal -crtscts
ignbrk -brkint ignpar -parmrk -inpck -istrip -inlcr -igncr icrnl -ixon -ixoff
-iuclc -ixany -imaxbel iutf8
opost -olcuc -ocrnl onlcr -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0 ff0
-isig icanon -iexten echo echoe echok -echonl -noflsh -xcase -tostop -echoprt
echoctl echoke

But that's a lot of information. Unfortunately there's no single flag for raw mode; it's a collection of a lot of flags. I checked the interrupt character: yep, intr = ^C, just like it should be. So what was the problem?

I saved the output with stty -a >/tmp/stty.bad, then I started up a new xterm and made a copy of what it should look like with stty -a >/tmp/stty.good. Then I looked for differences: meld /tmp/stty.good /tmp/stty.bad. I saw these flags differing in the bad one: ignbrk ignpar -iexten -ixon, while the good one had -ignbrk -ignpar iexten ixon. So I should be able to run:

$ stty -ignbrk -ignpar iexten ixon
and that would fix the problem. But it didn't. Ctrl-C still didn't work.

Setting a trap, with precmd

However, knowing some things that differed did give me something to test for in the shell, so I could test after every command and find out exactly when this happened. In zsh, you do that by defining a precmd function, so here's what I did:

precmd()
{
    stty -a | fgrep -- -ignbrk > /dev/null
    if [ $? -ne 0 ]; then
        echo
        echo "STTY SETTINGS HAVE CHANGED \!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!"
        echo
    fi
}
Pardon all the exclams. I wanted to make sure I saw the notice when it happened.

And this fairly quickly found the problem: it happened when I suspended ncftp with Ctrl-Z.

stty sane and isig

Okay, now I knew the culprit, and that if I switched to a different ftp client the problem would probably go away. But I still wanted to know why my stty command didn't work, and what the actual terminal difference was.

Somewhere in my web searching I'd stumbled upon some pages suggesting stty sane as an alternative to reset. I tried it, and it worked.

According to man stty, stty sane is equivalent to

$ stty cread -ignbrk brkint -inlcr -igncr icrnl -iutf8 -ixoff -iuclc -ixany  imaxbel opost -olcuc -ocrnl onlcr -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0 ff0 isig icanon iexten echo echoe echok -echonl -noflsh -xcase -tostop -echoprt echoctl echoke

Eek! But actually that's helpful. All I had to do was get a bad terminal (easy now that I knew ncftp was the culprit), then try:

$ stty cread 
$ stty -ignbrk 
$ stty brkint
... and so on, trying Ctrl-C each time to see if things were back to normal. Or I could speed up the process by grouping them:
$ stty cread -ignbrk brkint
$ stty -inlcr -igncr icrnl -iutf8 -ixoff
... and so forth. Which is what I did. And that quickly narrowed it down to isig. I ran reset, then ncftp again to get the terminal in "bad" mode, and tried:
$ stty isig
and sure enough, that was the difference.

I'm still not sure why meld didn't show me the isig difference. But if nothing else, I learned a bit about debugging stty settings, and about stty sane, which is a much nicer way of resetting the terminal than reset since it doesn't clear the screen.

Tags: , ,
[ 15:41 Aug 28, 2014    More linux | permalink to this entry | comments ]

Sun, 24 Aug 2014

One of them Los Alamos liberals

[Adopt-a-Highway: One of them Los Alamos liberals] I love this Adopt-a-Highway sign on Highway 4 on the way back down from the Jemez.

I have no idea who it is (I hope to find out, some day), but it gives me a laugh every time I see it.

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[ 10:50 Aug 24, 2014    More humor | permalink to this entry | comments ]

Wed, 20 Aug 2014

Mouse Release Movie

[Mouse peeking out of the trap] We caught another mouse! I shot a movie of its release.

Like the previous mouse we'd caught, it was nervous about coming out of the trap: it poked its nose out, but didn't want to come the rest of the way.

[Mouse about to fall out of the trap] Dave finally got impatient, picked up the trap and turned it opening down, so the mouse would slide out.

It turned out to be the world's scruffiest mouse, which immediately darted toward me. I had to step back and stand up to follow it on camera. (Yes, I know my camera technique needs work. Sorry.)

[scruffy mouse, just released from trap] [Mouse bounding away] Then it headed up the hill a ways before finally lapsing into the high-bounding behavior we've seen from other mice and rats we've released. I know it's hard to tell in the last picture -- the photo is so small -- but look at the distance between the mouse and its shadow on the ground.

Very entertaining! I don't understand why anyone uses killing traps -- even if you aren't bothered by killing things unnecessarily, the entertainment we get from watching the releases is worth any slight extra hassle of using the live traps.

Here's the movie: Mouse released from trap. [Mouse released from trap]

Tags: , ,
[ 17:10 Aug 20, 2014    More nature | permalink to this entry | comments ]

Fri, 15 Aug 2014

Time-lapse photography: stitching movies together on Linux

[Time-lapse clouds movie on youtube] A few weeks ago I wrote about building a simple Arduino-driven camera intervalometer to take repeat photos with my DSLR. I'd been entertained by watching the clouds build and gather and dissipate again while I stepped through all the false positives in my crittercam, and I wanted to try capturing them intentionally so I could make cloud movies.

Of course, you don't have to build an Arduino device. A search for timer remote control or intervalometer will find lots of good options around $20-30. I bought one so I'll have a nice LCD interface rather than having to program an Arduino every time I want to make movies.

Setting the image size

Okay, so you've set up your camera on a tripod with the intervalometer hooked to it. (Depending on how long your movie is, you may also want an external power supply for your camera.)

Now think about what size images you want. If you're targeting YouTube, you probably want to use one of YouTube's preferred settings, bitrates and resolutions, perhaps 1280x720 or 1920x1080. But you may have some other reason to shoot at higher resolution: perhaps you want to use some of the still images as well as making video.

For my first test, I shot at the full resolution of the camera. So I had a directory full of big ten-megapixel photos with filenames ranging from img_6624.jpg to img_6715.jpg. I copied these into a new directory, so I didn't overwrite the originals. You can use ImageMagick's mogrify to scale them all:

mogrify -scale 1280x720 *.jpg

I had an additional issue, though: rain was threatening and I didn't want to leave my camera at risk of getting wet while I went dinner shopping, so I moved the camera back under the patio roof. But with my fisheye lens, that meant I had a lot of extra house showing and I wanted to crop that off. I used GIMP on one image to determine the x, y, width and height for the crop rectangle I wanted. You can even crop to a different aspect ratio from your target, and then fill the extra space with black:

mogrify img_6624.jpg -crop 2720x1450+135+315 -scale 1280 -gravity center -background black -extent 1280x720 *.jpg

If you decide to rescale your images to an unusual size, make sure both dimensions are even, otherwise avconv will complain that they're not divisible by two.

Finally: Making your movie

I found lots of pages explaining how to stitch together time-lapse movies using mencoder, and a few using ffmpeg. Unfortunately, in Debian, both are deprecated. Mplayer has been removed entirely. The ffmpeg-vs-avconv issue is apparently a big political war, and I have no position on the matter, except that Debian has come down strongly on the side of avconv and I get tired of getting nagged at every time I run a program. So I needed to figure out how to use avconv.

I found some pages on avconv, but most of them didn't actually work. Here's what worked for me:

avconv -f image2 -r 15 -start_number 6624 -i 'img_%04d.jpg' -vcodec libx264 time-lapse.mp4

Adjust the start_number and filename appropriately for the files you have.

Avconv produces an mp4 file suitable for uploading to youtube. So here is my little test movie: Time Lapse Clouds.

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[ 12:05 Aug 15, 2014    More photo | permalink to this entry | comments ]

Sat, 09 Aug 2014

Sphinx Moths

[White-lined sphinx moth on pale trumpets] We're having a huge bloom of a lovely flower called pale trumpets (Ipomopsis longiflora), and it turns out that sphinx moths just love them.

The white-lined sphinx moth (Hyles lineata) is a moth the size of a hummingbird, and it behaves like a hummingbird, too. It flies during the day, hovering from flower to flower to suck nectar, being far too heavy to land on flowers like butterflies do.

[Sphinx moth eye] I've seen them before, on hikes, but only gotten blurry shots with my pocket camera. But with the pale trumpets blooming, the sphinx moths come right at sunset and feed until near dark. That gives a good excuse to play with the DSLR, telephoto lens and flash ... and I still haven't gotten a really sharp photo, but I'm making progress.

Check out that huge eye! I guess you need good vision in order to make your living poking a long wiggly proboscis into long skinny flowers while laboriously hovering in midair.

Photos here: White-lined sphinx moths on pale trumpets.

Tags: , ,
[ 21:23 Aug 09, 2014    More nature | permalink to this entry | comments ]

Tue, 05 Aug 2014

Privacy Policy

I got an envelope from my bank in the mail. The envelope was open and looked like the flap had never been sealed.

Inside was a copy of their privacy policy. Nothing else.

The policy didn't say whether their privacy policy included sealing the envelope when they send me things.

Tags: , ,
[ 13:22 Aug 05, 2014    More humor | permalink to this entry | comments ]

Thu, 31 Jul 2014

Predicting planetary visibility with PyEphem

Part II: Predicting Conjunctions

After I'd written a basic script to calculate when planets will be visible, the next step was predicting conjunctions, times when two or more planets are close together in the sky.

Finding separation between two objects is easy in PyEphem: it's just one line once you've set up your objects, observer and date.

p1 = ephem.Mars()
p2 = ephem.Jupiter()
observer = ephem.Observer()  # and then set it to your city, etc.
observer.date = ephem.date('2014/8/1')
p1.compute(observer)
p2.compute(observer)

ephem.separation(p1, p2)

So all I have to do is loop over all the visible planets and see when the separation is less than some set minimum, like 4 degrees, right?

Well, not really. That tells me if there's a conjunction between a particular pair of planets, like Mars and Jupiter. But the really interesting events are when you have three or more objects close together in the sky. And events like that often span several days. If there's a conjunction of Mars, Venus, and the moon, I don't want to print something awful like

Friday:
  Conjunction between Mars and Venus, separation 2.7 degrees.
  Conjunction between the moon and Mars, separation 3.8 degrees.
Saturday:
  Conjunction between Mars and Venus, separation 2.2 degrees.
  Conjunction between Venus and the moon, separation 3.9 degrees.
  Conjunction between the moon and Mars, separation 3.2 degrees.
Sunday:
  Conjunction between Venus and the moon, separation 4.0 degrees.
  Conjunction between the moon and Mars, separation 2.5 degrees.

... and so on, for each day. I'd prefer something like:

Conjunction between Mars, Venus and the moon lasts from Friday through Sunday.
  Mars and Venus are closest on Saturday (2.2 degrees).
  The moon and Mars are closest on Sunday (2.5 degrees).

At first I tried just keeping a list of planets involved in the conjunction. So if I see Mars and Jupiter close together, I'd make a list [mars, jupiter], and then if I see Venus and Mars on the same date, I search through all the current conjunction lists and see if either Venus or Mars is already in a list, and if so, add the other one. But that got out of hand quickly. What if my conjunction list looks like [ [mars, venus], [jupiter, saturn] ] and then I see there's also a conjunction between Mars and Jupiter? Oops -- how do you merge those two lists together?

The solution to taking all these pairs and turning them into a list of groups that are all connected actually lies in graph theory: each conjunction pair, like [mars, venus], is an edge, and the trick is to find all the connected edges. But turning my list of conjunction pairs into a graph so I could use a pre-made graph theory algorithm looked like it was going to be more code -- and a lot harder to read and less maintainable -- than making a bunch of custom Python classes.

I eventually ended up with three classes: ConjunctionPair, for a single conjunction observed between two bodies on a single date; Conjunction, a collection of ConjunctionPairs covering as many bodies and dates as needed; and ConjunctionList, the list of all Conjunctions currently active. That let me write methods to handle merging multiple conjunction events together if they turned out to be connected, as well as a method to summarize the event in a nice, readable way.

So predicting conjunctions ended up being a lot more code than I expected -- but only because of the problem of presenting it neatly to the user. As always, user interface represents the hardest part of coding.

The working script is on github at conjunctions.py.

Tags: , , ,
[ 19:57 Jul 31, 2014    More science/astro | permalink to this entry | comments ]

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