Posts Tagged ‘linux’
Ack
June 18th, 2010 at 11:03 pm
At the end of last week’s post about trusses, I mentioned that I had found an old photo of a roof truss by greping for the word “truss” in a folder of project reports on my work computer. I should not have grep’d; I should have ack’d.
Ack is a Perl script that’s meant to be a replacement of and improvement on the venerable Unix search tool grep. I’d read about ack quite a while ago—there’s a TextMate bundle called Ack in Project that’s well thought of—but kept forgetting to install it. Now that I have it installed, I see some obvious improvements.
- Speed. I haven’t done any formal timing tests, but
ackis clearly faster. - Presentation.
Ackshows its results in a much clearer format. I must say, though, that I’m not thrilled with some of the colors it uses in the output—maybe those can be changed in the.ackrcconfiguration file. - Better regular expressions. And by that I mean Perl’s regular expressions.
- Subdirectory search by default. I usually want my searches to walk down into the subdirectories but am always forgetting to turn on
grep’s recursive search option (-r), so having recursive search as the default is a big help. - No searching in revision control subdirectories. For me, that means the default is to avoid my
.gitfolders, butackalso knows to avoid folders named.svn,.hg,.bzr,CVS,RCS, and so on.
A couple of ack’s defaults are not to my liking.
- It doesn’t search in files that it considers “text” files. Since a lot of my searching is in just these kinds of files—mostly Markdown—this default works against me.
- It’s searches are case-sensitive. This is no different than
grep’s default, but that doesn’t mean I like having to always use the-iswitch.
These defaults betray ack’s origins as a programmer’s tool. Programmers usually want to search through their source code and avoid extraneous hits in, say, the documentation files. And most of today’s languages are case-sensitive, so it only makes sense for the default searches to be so as well.
Luckily for me, ack’s defaults can be changed through an .ackrc file in my home directory. Right now my .ackrc has just two lines:
--type=text
--smart-case
The first line tells ack to go ahead and search the text files. The second tells it to do case-insensitive searches unless the search term has a capital letter. The format of the .ackrc is simple: each line represents a command line switch and is written just as if you were adding it to an ack command—there’s no special dotfile syntax.
I’ll probably add more lines to my .ackrc as I learn more about it. For now, I’m just wondering why I didn’t switch from grep earlier.
Feet down below his knee
May 27th, 2009 at 9:44 am
Following up this post from yesterday, I downloaded and compiled GNU units to give it a test run. Although it’s feature-packed, it won’t let me use units like \sqrt{\textrm{in}}. Also, the inclusion of addition/subtraction and the change to equal precedence for multiplication and division make it, I think, a little less usable than the traditional units. For example, in traditional units, btu/hr ft degF is interpreted as
because multiplication has higher precedence than division. In GNU units, you’d have to do that as btu/hr/ft/degF. Since you’d say the unit as “BTU per hour foot degree Fahrenheit,” I think the tradition method is better.
I made a couple of additions to my local units.lib file to handle, in a limited way, the \sqrt{\textrm{in}} and \sqrt{\textrm{m}} units that come up in fracture toughness values.
rootm !k!
rootin 0.1593737745 rootm
This defines rootm as a fundamental unit and rootin as a derived unit. The proportionality constant is the square root of 0.0254. Now I can do things like
You have: 25 MPa rootm
You want: ksi rootin
* 22.751193
/ 0.04395374
which is pretty much all I need the root units for.
Help me get my feet back on the ground
May 26th, 2009 at 8:00 am
units is a Unix command line program that just doesn’t translate well to a graphical user interface.
You have: 105 m/s^2
You want: g
* 10.70702
/ 0.093396667
You have: 240 cc
You want: floz
* 8.1153654
/ 0.12322304
You have: 16 m/s
You want: mph
* 35.790981
/ 0.02794
You have: 2.7 ft water
You want: lbf/ft^2
* 168.55549
/ 0.0059327642
You have: 2.7 ft water
You want: kpascal
* 8.0704807
/ 0.12390836
You have: .6 watt/m degC
You want: btu/hr ft degF
* 0.34667223
/ 2.8845691
This is an interactive session with units. The strengths of the program are:
- It thinks the way you do, using common abbreviations and common prefixes for powers of ten. It’s much easier to just type
kpascalthan to go through a menu or tabbed interface to find the “pascal” entry among the various pressure terms—and then divide the answer by 1000 to convert from pascals to kilopascals. (Of course, you can’t have everything.gis commonly used for both grams and the acceleration of gravity, butunitsisn’t smart enough to use context to distinguish between the two. It has to choose one or the other, and it chosegto mean gravitational acceleration. Grams aregmandgram.) - It has a huge library of units. The
waterunit is the weight density of water; multiplying it by a length gives you the pressure at the bottom of a column of water of that height. The designer of a GUI unit converter would think long and hard before adding a unit likewater, because every additional unit takes up space in a window or a menu and makes finding the other units a little bit harder. No such problem with a command line program. - It allows you to combine units through multiplication and division, which effectively expands the library to a nearly infinite variety.
unitsdoesn’t need to have a special set of entries for thermal conductivity; it constructs thewatt/m degCandbtu/hr ft degFunits on the fly because it knows all the individual components.
By the way, if you’re wondering why every conversion has a / line, it’s because the fundamental behavior of units—which I seldom use—is to provide the conversion factors to go back and forth between two sets of units. In this fundamental mode, you would have a session like this:
You have: m/s
You want: mph
* 2.2369363
/ 0.44704
You have: ft water
You want: kpascal
* 2.9890669
/ 0.33455256
So, for example, to go from m/s to mph, you multiply by 2.237 or divide by 0.447. It just so happens that if you put a pure number in front of the “have” units, the * line gives you the converted value in the “want” units. You can put numbers in front of the “want” units, too, but the interpretation of the result may involve more thinking than you want to do.
You have: 2.7 ft water
You want: 10 lbf/ft^2
* 16.855549
/ 0.059327642
2.7 in feet of water is equivalent to 16.86 in tens of pounds per square foot. The / line in conversions like this has an even weirder interpretation: if you have a pressure in tens of pounds per square foot, you divide it by 0.0593 to get it in “2.7s” of feet of water. It’s probably best not to think that way.
The units program itself is just a conversion engine—by itself, it doesn’t know any units at all. It learns the units at startup by reading a data file. By default, this data file is /usr/share/misc/units.lib on a Mac or /usr/share/lib/unittab or /usr/share/units.dat on other Unix platforms. You can tell units to read another data file by invoking it with the -f option:
units -f /path/to/data/file
The data file is just a text file, with each line defining a new unit (or another name for a previously defined unit). On my computer, I have a units.lib file in my home directory that is a copy of the standard file with these additions at the end:
/ Local additions
Pa pascal
W watt
ksi 1000 psi
pcf lbf/ft3
psf lbf/ft2
ksf kip/ft2
tsf ton/ft2
The Pa and W lines are just synonyms so I don’t have to type out pascal and watt. The others are pressure, stress, and density units common in civil and mechanical engineering.
I have an alias in my .bashrc file,
alias units='units -f ~/units.lib'
so that I can simply type units at the command line to get my augmented units instead of just the standard ones.
One deficiency in units is that it doesn’t allow fractional exponents1. I can think of only one use for fractional exponents, but it’s one that comes up commonly in the work I do: the fracture toughness of materials is expressed in units like \textrm{ksi} \sqrt{\textrm{in}} or \textrm{MPa} \sqrt{\textrm{m}}. Because I can’t express values like this in units, I still have to convert these by hand.
The GNU version of units differs from the standard BSD/Mac version in that it allows addition and subtraction in the conversions (standard units allows only multiplication and division), and it uses the Readline library in interactive mode. The manual says it can do fractional exponents, but all the examples involve roots of units that are already raised to a power—there’s nothing like \sqrt{\textrm{in}}. Maybe I’ll download it and do some experimenting.
Update (5/27/09)
There’s now a followup post on \sqrt{\textrm{in}} and GNU units.
-
As far as I know. If you know better, please tell me about it. ↩
Linux fonts and fontconfig
August 18th, 2005 at 12:15 am