Sometimes people make mistakes and things don't go as expected. Eventually, they find out and say oops.
The next blog post for the a bashism a week series was scheduled for publishing on next Wednesday due to a mistake from my side. Sorry about that! given that it's a bit late today, let's better take a break this week.
August 28, 2013
August 21, 2013
A bashism a week: function names
The bashism of this week is easy to hit when overriding the execution of a command with a shell function. Think of the following example scenario:
Replacing the yes(1) command with a shell function:
Now every time yes is called the above-defined shell function will be called instead of /usr/bin/yes.
Apply the same principle to replace the run-parts(8) command with the following overly-simplified shell function:
It also works as expected. However, when running it under a shell that only supports the function names required by the POSIX:2001 specification it will fail. One such shell is dash, which aborts with a "Syntax error: Bad function name", another is posh which aborts with a "run-parts: invalid function name".
If you ever want to have function names with dashes, equal signs, commas, and other unusual characters make sure you use bash and ksh-like shells (and keep that code to yourself). Yes, you can even have batch-like silencing of stdout with
Update: there were missing quotation marks in the example @ function.
Replacing the yes(1) command with a shell function:
$ exec /bin/bash
$ type -t yes
file
$ yes() { while :; do echo ${1:-y}; done; }
$ type -t yes
function
Now every time yes is called the above-defined shell function will be called instead of /usr/bin/yes.
Apply the same principle to replace the run-parts(8) command with the following overly-simplified shell function:
$ run-parts() {
if [ "$1" = "--test" ]; then
shift;
simulate=true;
else
simulate=false;
fi;
for f in "$1"/*; do
[ -x "$f" ] && [ -f "$f" ] || continue;
case "$(basename "$f")" in
*[!a-zA-Z0-9_/-]*)
:
;;
*)
if $simulate; then
echo $f;
else
$f;
fi
;;
esac;
done
}
$ type -t run-parts
function
(note the use of negative matching)It also works as expected. However, when running it under a shell that only supports the function names required by the POSIX:2001 specification it will fail. One such shell is dash, which aborts with a "Syntax error: Bad function name", another is posh which aborts with a "run-parts: invalid function name".
If you ever want to have function names with dashes, equal signs, commas, and other unusual characters make sure you use bash and ksh-like shells (and keep that code to yourself). Yes, you can even have batch-like silencing of stdout with
function @ { "$@" > /dev/null ; }Update: there were missing quotation marks in the example @ function.
August 14, 2013
A bashism a week is back
After a while without posts on the a bashism a week series, it is coming back!
Next week, at the usual time and day of the week, the series of blog posts about bashisms will be back for at least one more month. Subscribe via Atom and don't miss any post and check all the previous posts.
The a bashism a week series cover some of the differences between bash and the behavior of other shells, and the requirements by the POSIX standard regarding shell scripting. Or put simply: they are a guide to common bashisms, allowing you to identify them and avoid their use for a more compatible and portable code.
Happy reading!
Next week, at the usual time and day of the week, the series of blog posts about bashisms will be back for at least one more month. Subscribe via Atom and don't miss any post and check all the previous posts.
The a bashism a week series cover some of the differences between bash and the behavior of other shells, and the requirements by the POSIX standard regarding shell scripting. Or put simply: they are a guide to common bashisms, allowing you to identify them and avoid their use for a more compatible and portable code.
Happy reading!
July 31, 2013
Ten years-old ebook reader
I own a ten years-old ebook reader.
Is it a smartphone?
It has features that make it pretty much like one, except that it can't make phone calls. So no. (However, I dare to say that making phone calls is one of the least used features of smartphones nowadays. Perhaps in the feature smartphones won't even be phones anymore, due to atrophy.)
What is it then?
It is a Sony CLIÉ, a PEG-SJ22 to be more precise. A PDA running Palm OS 4.1 that I'm now using as an ebook reader. To my surprise, the Plucker and iSilo readers still exist and at least the latter seems somewhat alive - there is even a version for Android.
Nowadays there are ebook readers with electronic paper displays, wifi or 3G connectivity, and other features but they all come down to the same thing: an ebook reader. Truth be told, the technology is quite old. In 2004, a year after the release of the PEG-SJ22, Sony also released the LIBRIé EBR-1000EP in Japan, an ebook reader with a 6" electronic paper display. A few years later is was released in the US as the Sony Reader.
Certainly, there have been advances since those devices were first released, but I have yet to see something that is really innovating. This year we are back to wrist watches, which were first released more than ten years ago - and one of them even ran linux.
Netbooks also had devices like the PEG-UX50 as their ancestor.
And if you were thinking about shoes, you've arrived late: the Puma RS already had some chips in them, and the Adidas 1 were also sported a few years ago. World, it's time to innovate.
Is it a smartphone?
It has features that make it pretty much like one, except that it can't make phone calls. So no. (However, I dare to say that making phone calls is one of the least used features of smartphones nowadays. Perhaps in the feature smartphones won't even be phones anymore, due to atrophy.)
What is it then?
It is a Sony CLIÉ, a PEG-SJ22 to be more precise. A PDA running Palm OS 4.1 that I'm now using as an ebook reader. To my surprise, the Plucker and iSilo readers still exist and at least the latter seems somewhat alive - there is even a version for Android.
Nowadays there are ebook readers with electronic paper displays, wifi or 3G connectivity, and other features but they all come down to the same thing: an ebook reader. Truth be told, the technology is quite old. In 2004, a year after the release of the PEG-SJ22, Sony also released the LIBRIé EBR-1000EP in Japan, an ebook reader with a 6" electronic paper display. A few years later is was released in the US as the Sony Reader.
Certainly, there have been advances since those devices were first released, but I have yet to see something that is really innovating. This year we are back to wrist watches, which were first released more than ten years ago - and one of them even ran linux.
Netbooks also had devices like the PEG-UX50 as their ancestor.
And if you were thinking about shoes, you've arrived late: the Puma RS already had some chips in them, and the Adidas 1 were also sported a few years ago. World, it's time to innovate.
July 09, 2013
Explaining segmentation fault errors
Want to fix that segfault you keep hitting or was reported to you? The first step is to understand the error message you get.
So you have a message like the following:
segfault at bfea3fec ip 080ee07e sp bfea3fa0 error 6
You might already know that ip means instruction pointer and sp means stack pointer and as such the addresses that follow them are the values in those registers. But what does the error number mean?
The error number, or code, actually gives you a better explanation of what the cause of the segfault is. The number's bits are flags describing the error and are architecture-dependent. For x86/x86_64 I just wrote an online converter/decoder that you can use to explain the segfault error code.
As an example, the above error code is explained as:
And the common error 4:
Enjoy.
So you have a message like the following:
segfault at bfea3fec ip 080ee07e sp bfea3fa0 error 6
You might already know that ip means instruction pointer and sp means stack pointer and as such the addresses that follow them are the values in those registers. But what does the error number mean?
The error number, or code, actually gives you a better explanation of what the cause of the segfault is. The number's bits are flags describing the error and are architecture-dependent. For x86/x86_64 I just wrote an online converter/decoder that you can use to explain the segfault error code.
As an example, the above error code is explained as:
The cause was a user-mode write resulting in no page being found.
And the common error 4:
The cause was a user-mode read resulting in no page being found.(also known as a null pointer dereference).
Enjoy.
June 12, 2013
Service update: 5 million a day
After the release of Debian wheezy traffic jumped to about 1 million requests per day, but as the weeks have passed by traffic has continued to increase to 5 million requests every day.
Even though it is a new record for the redirector it can not yet be compared to openSUSE.org's 20-40 million on their mirrorbrain instance. Let's see how long it takes to get there.
User adoption has increased but it has yet to become the default mirror in several places.
Even though it is a new record for the redirector it can not yet be compared to openSUSE.org's 20-40 million on their mirrorbrain instance. Let's see how long it takes to get there.
User adoption has increased but it has yet to become the default mirror in several places.
June 05, 2013
The "let the tool do the work" update
Over the last few weeks I've been making several changes to http.debian.net to detect mirrors that don't follow Debian's mirroring guidelines and end up causing problems to the end users. The changes will mean less hash mismatches and similar errors.
As I wrote back in December, the redirector is becoming nicer but also stricter. Some of the changes I recently made caused over 30 mirrors to be completely disabled from the redirector. This is not ideal and I don't like having to disable mirrors. They are contributions afterall.
The only thing I can do, and can't stress enough, is encourage people to use an up to date ftpsync script (available at project/ftpsync/ on every mirror) to mirror Debian.
It takes care for you of all the little but important things needed. Really. A mirror that uses ftpsync is easier for the administrator to properly configure, and provides a consistent mirror for the benefit of the users.
Speaking of ftpsync, there is a new version! If you use ftpsync please upgrade it as soon as possible.
Other improvements are on their way. Contributions are welcome (if you like refactoring, there's quite a bit of explicitly-redundant code in check.pl that should now give a better idea of the way it needs to be refactored.)
As I wrote back in December, the redirector is becoming nicer but also stricter. Some of the changes I recently made caused over 30 mirrors to be completely disabled from the redirector. This is not ideal and I don't like having to disable mirrors. They are contributions afterall.
The only thing I can do, and can't stress enough, is encourage people to use an up to date ftpsync script (available at project/ftpsync/ on every mirror) to mirror Debian.
It takes care for you of all the little but important things needed. Really. A mirror that uses ftpsync is easier for the administrator to properly configure, and provides a consistent mirror for the benefit of the users.
Speaking of ftpsync, there is a new version! If you use ftpsync please upgrade it as soon as possible.
Other improvements are on their way. Contributions are welcome (if you like refactoring, there's quite a bit of explicitly-redundant code in check.pl that should now give a better idea of the way it needs to be refactored.)
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