Shallow Thoughts : : linux
Akkana's Musings on Open Source, Science, and Nature.
Tue, 22 Jun 2010
Another in the continuing series of
"This isn't documented anywhere and Google searches aren't helpful":
Dave's printer was failing to print on Ubuntu Lucid. The only
failure mode: pages came out with the single printed line,
Unable to open the initial device, quitting.
What a great, helpful error message! Thanks, CUPS!
Web searching found lots of people using HP printers, using various
versions of the HPLIP software to installer newer drivers and
otherwise reconfigure their HP setups.
Only problem: this printer isn't an HP. It's a Brother HL2070N,
which has given very good service and, until now, worked flawlessly
with every OS we've tried including Linux.
The solution? It turns out the problem really is HPLIP -- even if
the printer isn't an HP. What this message really meant was:
HPLIP isn't installed, and Ubuntu's CUPS refuses to work without it.
apt-get install hplip hplip-data got the printer
working again.
Wouldn't it be nice if CUPS bothered to print error messages that
gave some hint of the real problem? Ideally, visible on the computer
to the user,
rather than on the printed page, so you don't have to waste 25 pages
of dead tree while you try to narrow down the problem?
Update: After further testing, we have established that a standard
Gnome-based Ubuntu Lucid machine needs hplip and hplip-data installed,
while a Lucid machine without Gnome needs those two plus hpijs. Or you
can get around needing any of them by ignoring CUPS' recommendation
for which driver to use, and choosing the Gutenprint driver in the
CUPS configuration screens.
A reader asked me if we had checked the CUPS error log. On one
machine, the log file was virtually empty; on another, there
actually were some lines about hpijs (nothing about hplip),
intermixed with a lot of debug chatter and a large number of errors
that were fairly clearly unrelated to anything we were doing.
Tags: linux, printing, cups
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21:46 Jun 22, 2010
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Fri, 18 Jun 2010
While I was in Europe, Dave stumbled on a handy alias on his Mac to
check the time where I was:
date -v +10 (+10 is the offset
from the current time). But when he tried to translate this to Linux,
he found that the -v flag from FreeBSD's
date program
wasn't available on the GNU
date on Linux.
But I suggested he could do the same thing with the TZ environment variable.
It's not documented well anywhere I could find, but if you set TZ to
the name of a time zone, date will print out the time for
that zone rather than your current one.
So, for bash:
$ TZ=Europe/Paris date # time in Paris
$ TZ=GB date # time in Great Britain
$ TZ=GMT-02 date # time two timezones east of GMT
or for csh:
% ( setenv TZ Europe/Paris; date)
% ( setenv TZ GB; date)
% ( setenv TZ GMT-02; date)
That's all very well. But when I tried
% ( setenv TZ UK; date)
% ( setenv TZ FR; date)
they gave the wrong time, even though Wikipedia's
list
of time zones seemed to indicate that those abbreviations were okay.
The trick seems to be that setting TZ only works for abbreviations
in /usr/share/zoneinfo/, or maybe in /usr/share/zoneinfo/posix/.
If you give an abbreviation, like UK or FR or America/San_Francisco,
it won't give you an error, it'll just print GMT as if that was what
you had asked for.
So this trick is useful for printing times abroad -- but if you want
to be safe, either stick to syntaxes like GMT-2, or make a script that
checks whether your abbreviation exists in the directory before
calling date, and warns you rather than just printing the wrong time.
Tags: linux, tips, travel, cmdline
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13:04 Jun 18, 2010
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Sun, 13 Jun 2010
Update: though the rest of this article is still useful in
explaining how to un-blacklist the pcspkr module, unfortunately
that module works very erratically. Sometimes you'll get a beep,
sometimes not. So this article may be a good start but it still
doesn't explain why Ubuntu's kernels have such a flaky pcspkr module.
For years I've used Ubuntu with my own kernels rather than the kernels
Ubuntu provides. I have several reasons: home-built kernels boot a lot
faster (when I say a lot, I mean like saving 30 seconds off a one-minute
boot) and offer more control over options. But a minor reason is that
Ubuntu kernels generally don't support the system beep, so for example
there's no way to tell in vim when you get out of insert mode. (In the
past I've sometimes used the excellent
fancy beeper
module to play sounds, but I don't always want that.)
On Ubuntu's latest "Lucid Lynx", I'm using their kernel (so far).
The Ubuntu kernel team has made huge improvements in boot time and number
of modules loaded, so it's much more efficient than past kernels.
But it did leave me without a beeper.
modprobe pcspkr failed to do anything except print the enigmatic:
WARNING: All config files need .conf: /etc/modprobe.d/00local, it will be ignored in a future release.
modprobe -v pcspkr (verbose) was no help -- it printed
install /bin/true which didn't make anything clearer.
To get my beep back, I had to do two things:
First, edit /etc/modprobe.d/blacklist.conf and comment out the line
blacklisting pcspeakr. It looks like this:
# ugly and loud noise, getting on everyone's nerves; this should be done by a
# nice pulseaudio bing (Ubuntu: #77010)
blacklist pcspkr
(They don't seem to be concerned about anyone who doesn't run Pulse,
or about the various other bugs involved -- there's quite a laundry
list in
bug
486154.)
Secomd. pcspkr was blacklisted a second time in a different way, in that
file so confusingly alluded to by the warning. /etc/modprobe.d/00local
was apparently left over from a previous version of Ubuntu, and never
removed by any upgrade script, and consisted of this:
install pcspkr /bin/true
Aha! So that's why modprobe -v pcspkr printed
install /bin/true -- because that's all it was doing instead
of loading the module like I'd asked.
So rm /etc/modprobe.d/00local was the second step, and
once I'd done that, modprobe pcspkr loaded the module and
gave me my system beep.
Tags: ubuntu, lucid, audio
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13:02 Jun 13, 2010
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Sun, 09 May 2010
Ubuntu's latest release, 10.04 "Lucid Lynx", really seems remarkably
solid. It boots much faster than any Ubuntu of the past three years,
and has some other nice improvements too.
But like every release, they made some pointless random undocumented
changes that broke stuff. The most frustrating has been getting my
front-panel flash card reader to work under Lucid's new udev,
so I could read SD cards from my camera and PDA.
The SD card slot shows up as /dev/sdb, but unless there's a card
plugged in at boot time, there's no /dev/sdb1 that you can
actually mount.
hal vs udisks
Prior to Lucid, the "approved" way of creating sdb1 was to
let hald-addons-storage poll every USB device every so
often, to see if anyone has plugged in a card and if so, check its
partition table and create appropriate devices.
That's a lot of polling -- and in any case, hald isn't standard on
Lucid, and even when it's installed, it sometimes runs and sometimes
doesn't. (I haven't figured out what controls whether it decides to run).
Hal isn't even supposed to be needed on Lucid -- it's supposed to use
devicekit (renamed to) udisks for that.
Except I guess they couldn't quite figure out how to get udisks working
in time, so they patched things together so that on Gnome systems, hald
does the same old polling stuff -- and on non Gnome systems, well,
maybe it does and maybe it doesn't. And maybe you can't read your
camera cards. Oh well!
udev rules
But on systems prior to Lucid there was another way:
make a udev rule to create sdb1 through sdb15 every time. I have an older
article
on setting up udev rules for multicard readers, but none of my old
udev rules worked on Lucid.
After many rounds of udevadm info -a -p /block/sdb
and udevadm test /block/sdb, service udev restart,
and many reboots, I finally found a rule that worked.
Create a /etc/udev/rules.d/71-multicard-reader.rules file
containing the following:
# Create all devices for multicard reader:
KERNEL=="sd[b-g]", SUBSYSTEMS=="usb", ATTRS{idVendor}=="1d6b", ATTRS{idProduct}=="0002", OPTIONS+="all_partitions,last_rule"
Replace the 1d6b and 0002 with the vendor and product of your own device,
as determined with udevadm info -a -p /block/sdb ... and
don't be tempted to use the vendor and device ID you get from lsusb,
because those are different.
What didn't work that used to? String matches. Some of them.
For example, this worked:
KERNEL=="sd[b-g]", SUBSYSTEMS=="scsi", ATTRS{model}=="*SD*", NAME{all_partitions}="sdcard"
but these didn't:
KERNEL=="sd[b-g]", SUBSYSTEMS=="scsi", ATTRS{model}=="*SD*Reader*", NAME{all_partitions}="sdcard"
KERNEL=="sd[a-g]", SUBSYSTEMS=="scsi", ATTRS{model}=="USB SD Reader ", NAME{all_partitions}="cardsd"
Update: The first of those two lines does indeed work now, whereas
it didn't when I was testing. It's possible that this has something
to do with saving hardware states and needing an extra
udevadm trigger, as suggested in Alex's
Changes in Ubuntu Lucid to udev.
According to udevadm info, the model is "USB SD Reader " (three
spaces at the end). But somehow "*SD*" matches this while "*SD*Reader*"
and the exact string do not. Go figure.
Numeric order
I'd like to have this rule run earlier, so it runs before
/lib/udev/rules.d/60-persistent-storage.rules and could use
OPTIONS+="last_rule" to keep the persistent storage rules from firing
(they run a lot of unnecessary external programs for each device).
But if I rename the rule from 71-multicard-reader.rules to 59-,
it doesn't run at all. Why? Shrug. It's not like udevadm test
will tell me.
Other things I love (not) about the new udev
- I love how if you give the
udevadm info arguments in the wrong
order, -p -a, it means something else and gives an error message.
- I love how
udevadm test doesn't actually test the same
rules udev will use, so it's completely unrelated to anything.
- I love the complete lack of documentation on things like string
matching and how the numeric order is handled.
- I love how you can't match both the device name (a string) and
the USB IDs in the same rule, because one is SUBSYSTEMS=="scsi" and the
other is SUBSYSTEMS=="usb".
- Finally, I love how there's no longer any way to test udev rules on
a running system -- if you want it to actually create new devices, you
have to reboot for each new test.
service udev restart and
udevadm control --reload-rules
don't touch existing devices.
Gives me that warm feeling like maybe I'm not missing out on the full
Windows experience by using Linux.
Tags: linux, ubuntu, kernel, udev, install
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20:51 May 09, 2010
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Wed, 05 May 2010
On a Linux list, someone was having trouble with wireless networking,
and someone else said he'd had a similar problem and solved it by
reinstalling Kubuntu from scratch. Another poster then criticised
him for that:
"if
the answer is reinstall, you might as well downgrade to Windows.",
and later added,
"if
"we should understand a problem, and *then* choose a remedy to match."
As someone who spends quite a lot of time trying to track down root
causes of problems so that I can come up with a fix that doesn't
involve reinstalling, I thought that was unfair.
Here is how I replied on the list (or you can go straight to
the
mailing list version):
I'm a big fan of understanding the root cause of a problem and solving it
on that basis. Because I am, I waste many days chasing down problems
that ought to "just work", and probably would "just work" if I
gave in and installed a bone stock Ubuntu Gnome desktop with no
customizations. Modern Linux distros (except maybe Gentoo) are
written with the assumption that you aren't going to change anything
-- so reverting to the original (reinstalling) will often fix a problem.
Understanding this stuff *shouldn't* take days of wasted time -- but
it does, because none of this crap has decent documentation. With a
lot of the underlying processes in Linux -- networking, fonts, sound,
external storage -- there are plenty of "Click on the System Settings
menu, then click on ... here's a screenshot" howtos, but not much
"Then the foo daemon runs the /etc/acpi/bar.sh script, which calls
ifconfig with these arguments". Mostly you have to reverse-engineer
it by running experiments, or read the source code.
Sometimes I wonder why I bother. It may be sort of obsessive-compulsive
disorder, but I guess it's better than washing my hands 'til they bleed,
or hoarding 100 cats. At least I end up with a nice customized system
and more knowledge about how Linux works. And no cat food expenses.
But don't get on someone's case because he doesn't have days to
waste chasing down deep understanding of a system problem. If
you're going to get on someone's case, go after the people who
write these systems and then don't document how they actually work,
so people could debug them.
Tags: linux, tech, documentation
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18:37 May 05, 2010
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Thu, 15 Apr 2010
Quick tip from Dave, passed along to someone else trying to use
an Apple keyboard on Linux:
On Linux, for some reason Apple keyboards' function keys don't work
by default.
Most of them try to run special functions instead, like
volume up/down or play/pause.
But you can get normal function keys by talking to the kernel module
that drives the keyboard:
echo 2 > /sys/module/hid_apple/parameters/fnmode
This will only last until shutdown, so put that line in /etc/rc.local
or a similar place so it runs every time you boot.
Here's an
Ubuntu help page on
Apple Keyboards with more information and other tricks.
Tags: linux, apple, kernel, keyboard
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15:03 Apr 15, 2010
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Thu, 01 Apr 2010
Second time I've run into this -- time to write it down.
Trying to run
debootstrap
to install the new Ubuntu beta, I kept hitting a snag right at the
beginning: debootstrap kept saying the filesystem was mounted with
the wrong permissions, and it couldn't create an executable file
or a device.
I have lines for each of my spare filesystems in /etc/fstab, like this:
/dev/sda4 /lucid ext3 relatime,user,noauto 0 0
That way, if I'm booted into one OS but I want to check a file from
another, I can mount it without needing sudo, just by typing
mount /lucid.
Not being able to create executable files means it's mounted with
the noexec flag. I checked another machine and saw
that it was using lines like
/dev/sda4 /lucid ext3 exec,user,noauto 0 0
I added the "exec," to fstab, unmounted and remounted ... and it was
still mounted with noexec.
Turns out on some Linux versions, making a filesystem user
mountable turns off exec even if you've specified it
explicitly. You have to add the exec after the
user.
But that still didn't make debootstrap happy, because it couldn't
create a device. That's a separate fstab option, dev,
and user implies nodev.
So here's the fstab entry that finally worked:
/dev/sda4 /lucid ext3 relatime,user,exec,dev,noauto 0 0
Tags: linux, filesystem, fstab, tips
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22:07 Apr 01, 2010
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Mon, 29 Mar 2010
I maintain the websites for several clubs. No surprise there -- anyone
with even a moderate knowledge of HTML, or just a lack of fear of
it, invariably gets drafted into that job in any non-computer club.
In one club, the person in charge of scheduling sends out an elaborate
document every three months in various formats -- Word, RTF, Excel, it's
different each time. The only regularity is that it's always full of
crap that makes it hard to turn it into a nice simple HTML table.
This quarter, the formats were Word and RTF. I used unrtf to turn
the RTF version into HTML -- and found a horrifying page full of
lines like this:
<body><font size=3></font><font size=3><br>
</font><font size=3></font><b><font size=4></font></b><b><font size=4><table border=2>
</font></b><b><font size=4><tr><td><b><font size=4><font face="Arial">Club Schedule</font></font></b><b><font size=4></font></b><b><font size=4></font></b></td>
<font size=3></font><font size=3><td><font size=3><b><font face="Arial">April 13</font></b></font><font size=3></font><font size=3><br>
</font><font size=3></font><font size=3><b></b></font></td>
I've put the actual page content in bold; the rest is just junk,
mostly doing nothing, mostly not even legal HTML,
that needs to be eliminated if I want
the page to load and display reasonably.
I didn't want to clean up that mess by hand! So I needed some regular
expressions to clean it up in an editor.
I tried emacs first, but emacs makes it hard to try an expression then
modify it a little when the first try doesn't work, so I switched to vim.
The key to this sort of cleanup is non-greedy regular expressions.
When you have a bad tag sandwiched in the middle of a line containing
other tags, you want to remove everything from the <font
through the next > -- but no farther, or else you'll delete
real content. If you have a line like
<td><font size=3>Hello<font> world</td>
you only want to delete through the <font>, not through the </td>.
In general, you make a regular expression non-greedy by adding a ?
after the wildcard -- e.g. <font.*?>. But that doesn't work
in vim. In vim, you have to use \{M,N} which matches
from M to N repetitions of whatever immediately precedes it.
You can also use the shortcut \{-} to mean the same thing
as *? (0 or more matches) in other programs.
Using that, I built up a series of regexp substitutes to clean up
that unrtf mess in vim:
:%s/<\/\{0,1}font.\{-}>//g
:%s/<b><\/b>//g
:%s/<\/b><b>//g
:%s/<\/i><i>//g
:%s/<td><\/td>/<td><br><\/td>/g
:%s/<\/\{0,1}span.\{-}>//g
:%s/<\/\{0,1}center>//g
That took care of 90% of the work, leaving me with hardly any cleanup
I needed to do by hand. I'll definitely keep that list around for
the next time I need to do this.
Tags: regexp, html, editors, vim
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