Shallow Thoughts : : linux

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

Fri, 15 May 2015

Of file modes, umasks and fmasks, and mounting FAT devices

I have a bunch of devices that use VFAT filesystems. MP3 players, camera SD cards, SD cards in my Android tablet. I mount them through /etc/fstab, and the files always look executable, so when I ls -f them, they all have asterisks after their names. I don't generally execute files on these devices; I'd prefer the files to have a mode that doesn't make them look executable.

I'd like the files to be mode 644 (or 0644 in most programming languages, since it's an octal, or base 8, number). 644 in binary is 110 100 100, or as the Unix ls command puts it, rw-r--r--.

There's a directive, fmask, that you can put in fstab entries to control the mode of files when the device is mounted. (Here's Wikipedia's long umask article.) But how do you get from the mode you want the files to be, 644, to the mask?

The mask (which corresponds to the umask command) represent the bits you don't want to have set. So, for instance, if you don't want the world-execute bit (1) set, you'd put 1 in the mask. If you don't want the world-write bit (2) set, as you likely don't, put 2 in the mask. So that's already a clue that I'm going to want the rightmost byte to be 3: I don't want files mounted from my MP3 player to be either world writable or executable.

But I also don't want to have to puzzle out the details of all nine bits every time I set an fmask. Isn't there some way I can take the mode I want the files to be -- 644 -- and turn them into the mask I'd need to put in /etc/fstab or set as a umask?

Fortunately, there is. It seemed like it ought to be straightforward, but it took a little fiddling to get it into a one-line command I can type. I made it a shell function in my .zshrc:

# What's the complement of a number, e.g. the fmask in fstab to get
# a given file mode for vfat files? Sample usage: invertmask 755
invertmask() {
    python -c "print '0%o' % (~(0777 & 0$1) & 0777)"
}

This takes whatever argument I give to it -- $1 -- and takes only the three rightmost bytes from it, (0777 & 0$1). It takes the bitwise NOT of that, ~. But the result of that is a negative number, and we only want the three rightmost bytes of the result, (result) & 0777, expressed as an octal number -- which we can do in python by printing it as %o. Whew!

Here's a shorter, cleaner looking alias that does the same thing, though it's not as clear about what it's doing:

invertmask1() {
    python -c "print '0%o' % (0777 - 0$1)"
}

So now, for my MP3 player I can put this in /etc/fstab:

UUID=0000-009E /mp3 vfat user,noauto,exec,fmask=133,shortname=lower 0 0

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[ 10:27 May 15, 2015    More linux/cmdline | permalink to this entry | comments ]

Mon, 23 Feb 2015

Tips for developing on a web host that offers only FTP

Generally, when I work on a website, I maintain a local copy of all the files. Ideally, I use version control (git, svn or whatever), but failing that, I use rsync over ssh to keep my files in sync with the web server's files.

But I'm helping with a local nonprofit's website, and the cheap web hosting plan they chose doesn't offer ssh, just ftp.

While I have to question the wisdom of an ISP that insists that its customers use insecure ftp rather than a secure encrypted protocol, that's their problem. My problem is how to keep my files in sync with theirs. And the other folks working on the website aren't developers and are very resistant to the idea of using any version control system, so I have to be careful to check for changed files before modifying anything.

In web searches, I haven't found much written about reasonable workflows on an ftp-only web host. I struggled a lot with scripts calling ncftp or lftp. But then I discovered curftpfs, which makes things much easier.

I put a line in /etc/fstab like this:

curlftpfs#user:password@example.com/ /servername fuse rw,allow_other,noauto,user 0 0

Then all I have to do is type mount /servername and the ftp connection is made automagically. From then on, I can treat it like a (very slow and somewhat limited) filesystem.

For instance, if I want to rsync, I can

rsync -avn --size-only /servername/subdir/ ~/servername/subdir/
for any particular subdirectory I want to check. A few things to know about this:
  1. I have to use --size-only because timestamps aren't reliable. I'm not sure whether this is a problem with the ftp protocol, or whether this particular ISP's server has problems with its dates. I suspect it's a problem inherent in ftp, because if I ls -l, I see things like this:
    -rw-rw---- 1 root root 7651 Feb 23  2015 guide-geo.php
    -rw-rw---- 1 root root 1801 Feb 14 17:16 guide-header.php
    -rw-rw---- 1 root root 8738 Feb 23  2015 guide-table.php
    
    Note that a file modified a week ago shows a modification time, but files modified today show only a day and year, not a time. I'm not sure what to make of this.
  2. Note the -n flag. I don't automatically rsync from the server to my local directory, because if I have any local changes newer than what's on the server they'd be overwritten. So I check the diffs by hand with tkdiff or meld before copying.
  3. It's important to rsync only the specific directories you're working on. You really don't want to see how long it takes to get the full file tree of a web server recursively over ftp.

How do you change and update files? It is possible to edit the files on the curlftpfs filesystem directly. But at least with emacs, it's incredibly slow: emacs likes to check file modification dates whenever you change anything, and that requires an ftp round-trip so it could be ten or twenty seconds before anything you type actually makes it into the file, with even longer delays any time you save.

So instead, I edit my local copy, and when I'm ready to push to the server, I cp filename /servername/path/to/filename.

Of course, I have aliases and shell functions to make all of this easier to type, especially the long pathnames: I can't rely on autocompletion like I usually would, because autocompleting a file or directory name on /servername requires an ftp round-trip to ls the remote directory.

Oh, and version control? I use a local git repository. Just because the other people working on the website don't want version control is no reason I can't have a record of my own changes.

None of this is as satisfactory as a nice git or svn repository and a good ssh connection. But it's a lot better than struggling with ftp clients every time you need to test a file.

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[ 19:46 Feb 23, 2015    More linux | permalink to this entry | comments ]

Thu, 19 Feb 2015

Finding core dump files

Someone on the SVLUG list posted about a shell script he'd written to find core dumps.

It sounded like a simple task -- just locate core | grep -w core, right? I mean, any sensible packager avoids naming files or directories "core" for just that reason, don't they?

But not so: turns out in the modern world, insane numbers of software projects include directories called "core", including projects that are developed primarily on Linux so you'd think they would avoid it ... even the kernel. On my system, locate core | grep -w core | wc -l returned 13641 filenames.

Okay, so clearly that isn't working. I had to agree with the SVLUG poster that using "file" to find out which files were actual core dumps is now the only reliable way to do it. The output looks like this:

$ file core
core: ELF 32-bit LSB core file Intel 80386, version 1 (SYSV), too many program headers (375)

The poster was using a shell script, but I was fairly sure it could be done in a single shell pipeline. Let's see: you need to run locate to find any files with 'core" in the name.

Then you pipe it through grep to make sure the filename is actually core: since locate gives you a full pathname, like /lib/modules/3.14-2-686-pae/kernel/drivers/edac/edac_core.ko or /lib/modules/3.14-2-686-pae/kernel/drivers/memstick/core, you want lines where only the final component is core -- so core has a slash before it and an end-of-line (in grep that's denoted by a dollar sign, $) after it. So grep '/core$' should do it.

Then take the output of that locate | grep and run file on it, and pipe the output of that file command through grep to find the lines that include the phrase 'core file'.

That gives you lines like

/home/akkana/geology/NorCal/pinnaclesGIS/core: ELF 32-bit LSB core file Intel 80386, version 1 (SYSV), too many program headers (523)

But those lines are long and all you really need are the filenames; so pass it through sed to get rid of anything to the right of "core" followed by a colon.

Here's the final command:

file `locate core | grep '/core$'` | grep 'core file' | sed 's/core:.*//'

On my system that gave me 11 files, and they were all really core dumps. I deleted them all.

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[ 12:54 Feb 19, 2015    More linux | permalink to this entry | comments ]

Mon, 22 Dec 2014

Passwordless ssh with a key: the part most tutorials skip

I'm working on my Raspberry Pi crittercam again. I got a battery, so it can be a standalone box -- it was such a hassle to set it up with two power cords dangling from it at all times -- and set it up to run automatically at boot time.

But there was one aspect of the camera that wasn't automated: if close enough to the house to see the wi-fi router, I want it to mount a filesystem from our server and store its image files there. That makes it a lot easier to check on its progress, and also saves wear on the Pi's SD card.

Only one problem: I was using sshfs to mount the disk remotely, and ssh always prompts me for a password.

Now, there are a gazillion tutorials on how to set up an ssh key. Just do a web search for ssh key or passwordless ssh key. They vary a bit in their details, but they're all the same in the important aspects. They're all the same in one other detail: none of them work for me. I generate a new key (various types) with no pass phrase, I copy it to the server's authorized keys file (several different ways, two possible filenames), I try to ssh -- and I'm prompted for a password.

After much flailing I finally found out what was missing. In addition to those two steps, you need to modify your .ssh/config file to tell it which key to use. This is especially critical if you have multiple keys on the client machine, or if you've named the file anything but the default id_dsa or id_rsa.

So here are the real steps for making an ssh key. Assume the server, the machine to which you want to ssh, is named "myserver". But these steps are all run on the client machine, the one from which you want to run ssh.

ssh-keygen -t rsa -C "Comment"
When it prompts you for a filename, give it a full pathname, e.g. ~/.ssh/id_rsa_myserver. Type in a pass phrase, or hit return twice if you want to be able to ssh without a password.
ssh-copy-id -i .ssh/id_rsa_myserver user@myserver
You can omit the user@ if you're using the same username on both machines. You'll have to type in your password on myserver.

Then edit ~/.ssh/config, and add an entry like this:

Host myserver
  User my_username
  IdentityFile ~/.ssh/id_rsa_myserver
The User line is optional, and refers to your username on myserver if it's different from the one on the client. For instance, on the Raspberry Pi, everything has to run as root because most of the hardware and camera libraries can't work any other way. But I want it using my user ID on the server side, not root.

Eliminating strict host key checking

Of course, you can use this to go the other way too, and ssh to your Pi without needing to type a password every time. If you do that, and if you have several Pis, Beaglebones, plug computers or other little Linux gizmos which sometimes share the same IP address, you may run into the annoying whine ssh is prone to:

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@    WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!     @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
The only way to get around this once it happens is by editing ~/.ssh/known_hosts, finding the line corresponding to the pi, and removing it (or just removing the whole file).

You're supposed to be able to turn off this check with StrictHostKeyChecking no, but it doesn't work. Fortunately, there's a trick I discovered several years ago and discussed in Three SSH tips. Here's how the Pi entry ends up looking in my desktop's ~/.ssh/config:

Host pipi
  HostName pi
  User pi
  StrictHostKeyChecking no
  UserKnownHostsFile /dev/null
  IdentityFile ~/.ssh/id_pi

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[ 16:25 Dec 22, 2014    More linux | permalink to this entry | comments ]

Thu, 18 Dec 2014

Firefox deprecates flash. How to get it back (on Debian).

Recently Firefox started refusing to run flash, including youtube videos (about the only flash I run). A bar would appear at the top of the page saying "This plug-in is vulnerable and should be upgraded". Apparently Adobe had another security bug. There's an "Update now" button in the Firefox bar, but it's a chimera: Firefox has never known how to install plug-ins for Linux (there are longstanding bugs filed on why it claims to be able to but can't), and it certainly doesn't know how to update a Debian package.

I use a Firefox downloaded from Mozilla.org, but flash from Debian's flashplugin-nonfree package. So I figured updating Debian -- apt-get update; apt-get dist-upgrade -- would fix it. Nope. I still got the same message.

A little googling found several pages recommending update-flashplugin-nonfree --install; I tried that but it didn't help either. It seemed to download a tarball, but as far as I could tell it never unpacked or installed the tarball it downloaded.

What finally did the trick was

apt-get install --reinstall flashplugin-nonfree
That downloaded a new tarball, AND unpacked and installed it. After restarting Firefox, I was able to view the video I'd been trying to watch.

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[ 15:21 Dec 18, 2014    More linux | permalink to this entry | comments ]

Tue, 02 Dec 2014

Ripping a whole CD on Linux

I recently discovered that my ancient stereo turntable didn't survive our move. So all those LPs I brought along, intending to rip to mp3 when I had more time, will never see bits.

So I need to buy new versions of some of that old music. In particular, I'd lately been wanting to listen to my old Flanders and Swann albums. Flanders and Swann were a terrific comedy music duo (think Tom Lehrer only less scientifically oriented) from the 1960s.

So I ordered a CD of The Complete Flanders & Swann, which contains all three of the albums I inherited from my parents. Woohoo! I ran a little script I have that rips a whole CD to a directory of separate MP3 songs, and I was all set.

Until I listened to it. It turns out that when the LP album was turned into a CD, they put the track breaks in the wrong place. These albums are recordings of live performances. Each song has a spoken intro, giving a little context for the song that follows. On the CD, each track starts with a song, and ends with the spoken intro for the next song. That's no problem if you always listen to whole albums in order. But I like to play individual tracks, or listen to music on random play. So this wasn't going to work at all.

I tried using audacity to copy the intro from the end of one track and paste it onto the beginning of another. That worked, but it was tedious and fiddly. A little research showed me a much better way.

First: Rip the whole CD

First I needed to rip the whole CD as one gigantic track. My script had been running cdparanoia tracknumber filename.wav. But it took some study of the cdparanoia manual before I finally found the way to rip a whole CD to one track: you can specify a range of tracks, starting at 0 and omitting the end track.

cdparanoia 0- outfile.wav

Use Audacity to split and save the tracks

Now what's the best way to split a recording into separate tracks? Fortunately the Audacity manual has a nice page on that very subject: Splitting a recording into separate tracks.

Mostly, the issue is setting labels -- with Tracks->Add Label at Selection or Tracks->Add Label at Playback Position. Use Ctrl-1 to zoom as much as you need to see where the short pauses are. Then listen to the audio, pausing or clicking and setting labels appropriately.

It's a bit fiddly. For instance, if you pause your listening to set a label, you might want to save the audacity project so you don't lose the label positions you've set so far. But you can't save unless you Stop the playback; and that loses the current playback position which you may not yet have set a label for. Even if you have set a label for it, you'll need to click to set the selection to the label you just made if you want to continue playing from where you left off. It all seems a little silly and unintuitive ... but after a few tries you'll find a routine that works for you.

When all your labels are set, then File->Export Multiple.... You will have to go through a bunch of dialogs involving metadata for each track; just hit return, since audacity ignores any metadata you type in and won't actually write it to the MP3 file. I have no idea why it always prompts for metadata then doesn't use it, but you can use a program like id3tool later to add proper metadata to the tracks.

So, no, the tools aren't perfect. On the other hand, I now have a nice set of Flanders and Swann tracks, and can listen to Misalliance, Ill Wind and The GNU Song complete with their proper introductions.

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[ 13:35 Dec 02, 2014    More linux | permalink to this entry | comments ]

Tue, 18 Nov 2014

Unix "remind" file for US holidays

Am I the only one who's always confused about when holidays happen?

Partly it's software, I guess. In these days of everybody keeping their schedules on Google's or Apple's servers, maybe most people keep up on these things.

But being the dinosaur I am, I'm still resistant to keeping my schedule in the cloud on a public server. What if I need to check for upcoming events while I'm on a trip out in the remote desert somewhere? (Not to mention the obvious privacy considerations.) For years I used PalmOS PDAs, but when I switched to Android and discovered how poor the offline calendar options are, I decided that I should learn how to use the old Unix standby.

It's been pretty handy. I run remind ~/[remind-file-name] when I log in in the morning, and it gives me a nice summary of upcoming events:

DPU Solar surcharge meeting, 5:30-8:30 tomorrow
NMGLUG meeting in 2 days' time

Of course, I can also have it email me with reminders, or pop up a window, but so far I haven't felt the need.

I can also display a nice calendar showing upcoming events for this month or the next several months. I made a couple of aliases:

mycal () {
        months=$1 
        if [[ x$months = x ]]
        then
                months=1 
        fi
        remind -c$months ~/Docs/Lists/remind
}

mycalp () {
        months=$1 
        if [[ x$months = x ]]
        then
                months=2 
        fi
        remind -p$months ~/Docs/Lists/remind | rem2ps -e -l > /tmp/mycal.ps
        gv /tmp/mycal.ps &
}

The first prints an ascii calendar; the second displays a nice postscript calendar complete with little icons for phases of the moon.

But what about those holidays?

Okay, that gives me a good way of storing reminders about appointments. But I still don't know when holidays are. (I had that problem with the PalmOS scheduling program, too -- it never knew about holidays either.)

Web searching didn't help much. Unfortunately, "remind" is a terrible name in this age of search engines. If someone has already solved this problem, I sure wasn't able to find any evidence of it. So instead, I went to Wikipedia's list of US holidays, with the remind man page in another tab, and wrote remind stanzas for each one -- except Easter, which is much more complicated.

But wait -- it turns out that remind already has code to calculate Easter! It just needs a slightly more complicated stanza: instead of the standard form of

REM  1 Apr +1 MSG April Fool's Day %b
I need to use this form:
REM  [trigger(easterdate(today()))] +1 MSG Easter %b

The %b in each case is what gives you the notice of when the event is in your reminders, e.g. "Easter tomorrow" or "Easter in two days' time". The +1 is how far beforehand you want to be reminded of each event.

So here's my remind file for US holidays. I make no guarantees that every one is right, though I did check them for the next 12 months and they all seem to be working.

#
# US Holidays
#
REM      1 Jan    +3 MSG New Year's Day %b
REM Mon 15 Jan    +2 MSG MLK Day %b
REM      2 Feb       MSG Groundhog Day %b
REM     14 Feb    +2 MSG Valentine's Day %b
REM Mon 15 Feb    +2 MSG President's Day %b
REM     17 Mar    +2 MSG St Patrick's Day %b
REM      1 Apr    +9 MSG April Fool's Day %b
REM  [trigger(easterdate(today()))] +1 MSG Easter %b
REM     22 Apr    +2 MSG Earth Day %b
REM Fri  1 May -7 +2 MSG Arbor Day %b
REM Sun  8 May    +2 MSG Mother's Day %b
REM Mon  1 Jun -7 +2 MSG Memorial Day %b
REM Sun 15 Jun       MSG Father's Day
REM      4 Jul    +2 MSG 4th of July %b
REM Mon  1 Sep    +2 MSG Labor Day %b
REM Mon  8 Oct    +2 MSG Columbus Day %b
REM     31 Oct    +2 MSG Halloween %b
REM Tue  2 Nov    +4 MSG Election Day %b
REM     11 Nov    +2 MSG Veteran's Day %b
REM Thu 22 Nov    +3 MSG Thanksgiving %b
REM     25 Dec    +3 MSG Christmas %b

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[ 14:07 Nov 18, 2014    More linux | permalink to this entry | comments ]

Sun, 14 Sep 2014

Global key bindings in Emacs

Global key bindings in emacs. What's hard about that, right? Just something simple like

(global-set-key "\C-m" 'newline-and-indent)
and you're all set.

Well, no. global-set-key gives you a nice key binding that works ... until the next time you load a mode that wants to redefine that key binding out from under you.

For many years I've had a huge collection of mode hooks that run when specific modes load. For instance, python-mode defines \C-c\C-r, my binding that normally runs revert-buffer, to do something called run-python. I never need to run python inside emacs -- I do that in a shell window. But I fairly frequently want to revert a python file back to the last version I saved. So I had a hook that ran whenever python-mode loaded to override that key binding and set it back to what I'd already set it to:

(defun reset-revert-buffer ()
  (define-key python-mode-map "\C-c\C-r" 'revert-buffer) )
(setq python-mode-hook 'reset-revert-buffer)

That worked fine -- but you have to do it for every mode that overrides key bindings and every binding that gets overridden. It's a constant chase, where you keep needing to stop editing whatever you wanted to edit and go add yet another mode-hook to .emacs after chasing down which mode is causing the problem. There must be a better solution.

A web search quickly led me to the StackOverflow discussion Globally override key bindings. I tried the techniques there; but they didn't work.

It took a lot of help from the kind folks on #emacs, but after an hour or so they finally found the key: emulation-mode-map-alists. It's only barely documented -- the key there is "The “active” keymaps in each alist are used before minor-mode-map-alist and minor-mode-overriding-map-alist" -- and there seem to be no examples anywhere on the web for how to use it. It's a list of alists mapping names to keymaps. Oh, clears it right up! Right?

Okay, here's what it means. First you define a new keymap and add your bindings to it:

(defvar global-keys-minor-mode-map (make-sparse-keymap)
  "global-keys-minor-mode keymap.")

(define-key global-keys-minor-mode-map "\C-c\C-r" 'revert-buffer)
(define-key global-keys-minor-mode-map (kbd "C-;") 'insert-date)

Now define a minor mode that will use that keymap. You'll use that minor mode for basically everything.

(define-minor-mode global-keys-minor-mode
  "A minor mode so that global key settings override annoying major modes."
  t "global-keys" 'global-keys-minor-mode-map)

(global-keys-minor-mode 1)

Now build an alist consisting of a list containing a single dotted pair: the name of the minor mode and the keymap.

;; A keymap that's supposed to be consulted before the first
;; minor-mode-map-alist.
(defconst global-minor-mode-alist (list (cons 'global-keys-minor-mode
                                              global-keys-minor-mode-map)))

Finally, set emulation-mode-map-alists to a list containing only the global-minor-mode-alist.

(setf emulation-mode-map-alists '(global-minor-mode-alist))

There's one final step. Even though you want these bindings to be global and work everywhere, there is one place where you might not want them: the minibuffer. To be honest, I'm not sure if this part is necessary, but it sounds like a good idea so I've kept it.

(defun my-minibuffer-setup-hook ()
  (global-keys-minor-mode 0))
(add-hook 'minibuffer-setup-hook 'my-minibuffer-setup-hook)

Whew! It's a lot of work, but it'll let me clean up my .emacs file and save me from endlessly adding new mode-hooks.

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[ 16:46 Sep 14, 2014    More linux/editors | permalink to this entry | comments ]