Shallow Thoughts

Useful shell pipeline: Who checks in how much?

I wanted to get a list of who'd been contributing the most in a particular open source project. Most projects of any size have a ChangeLog file, in which check-ins have entries like this:

2008-08-26  Jane Hacker  <hacker@domain.org>

        * src/app/print.c: make sure the Portrait and Landscape
        * buttons update according to the current setting.

I wanted to take each entry, save the name of the developer checking in, then eventually count the number of times each name occurs (the number of times that developer checked in) and print them in order from most check-ins to least.

Getting the names is easy: for check-ins in the last 9 years, I just want the lines that start with "200". (Of course, if I wanted earlier check-ins I could make the match more general.)

grep "^200" ChangeLog

But now I want to trim the line so it includes only the contributor's name. A bit of sed geekery can do that: the date is a fixed format (four characters, a dash, two, dash, two, then two spaces, so "^....-..-.. " matches that pattern.

But I want to remove the email address part too (sometimes people use different email addresses when they check in). So I want a sed pattern that will match something at the front (to discard), something in the middle (keep that part) and something at the end (discard).

Here's how to do that in sed:

grep "^200" ChangeLog | sed 's/^....-..-..  \(.*\)<.*$/\1/'

In English, that says: "For each line in the ChangeLog that starts with 200, find a pattern at the beginning consisting of any four characters, a dash, two characters, dash, two characters, dash, and two spaces; then immediately after that, save all characters up to a < symbol; then throw away the < and any characters that follow until the end of the line."

That works pretty well! But it's not quite right: it includes the two spaces after the name as part of the name. In sed, \s matches any space character (like space or tab). So you'd think this should work:

grep "^200" ChangeLog | sed 's/^....-..-..  \(.*\)\s+<.*$/\1/'

\s+ means it will require that at least one and maybe more space characters immediately before the < are also discarded. But it doesn't work. It turns out the reason is that the \(.*\) expression is "greedier" than the \s+: so the saved name expression grabs the first space, leaving only the second to the \s+.

The way around that is to make the name expression specify that it can't end with a space. \S is the term for "anything that's not a space character"; so the expression becomes

grep "^200" ChangeLog | sed 's/^....-..-..  \(.*\S\)\s\+<.*$/\1/'

(the + turned out to need a backslash before it).

We have the list of names! Add a | sort on the end to sort them alphabetically -- that will make sure you get all the "Jane Hacker" lines listed together. But how to count them? The Unix program most frequently invoked after sort is uniq, which gets rid of all the repeated lines. On a hunch, I checked out the man page, man uniq, and found the -c option: "prefix lines by the number of occurrences". Perfect! Then just sort them by the number, from largest to smallest:

grep "^200" ChangeLog | sed 's/^....-..-..  \(.*\S\)\s+<.*$/\1/' | sort | uniq -c | sort -rn

And we're done!

Now, this isn't perfect since it doesn't catch "Checking in patch contributed by susan@otherhost.com" attributions -- but those aren't in a standard format in most projects, so they have to be handled by hand.

Disclaimer: Of course, number of check-ins is not a good measure of how important or productive someone is. You can check in a lot of one-line fixes, or you can write an important new module and submit it for someone else to merge in. The point here wasn't to rank developers, but just to get an idea who was checking into the tree and how often.

Well, that ... and an excuse to play with nifty Linux shell pipelines.

Tags: shell, CLI, linux, pipelines, regexp
[ 11:12 Aug 31, 2008    More linux | permalink to this entry ]

Random Wallpaper

I was talking about desktop backgrounds -- wallpaper -- with some friends the other day, and it occurred to me that it might be fun to have my system choose a random backdrop for me each morning.

Finding backgrounds is no problem: I have plenty of images stored in ~/Backgrounds -- mostly photos I've taken over the years, with a smattering of downloads from sites like the APOD. So all I needed was a way to select one file at random from the directory.

This is Unix, so there's definitely a commandline way to do it, right? Well, surprisingly, I couldn't find an easy way that didn't involve any scripting. Some shells have a random number generator built in ($RANDOM in bash) but you still have to do some math on the result.

Of course, I could have googled, since I'm sure other people have written random-wallpaper scripts ... but what's the fun in that? If it has to be a script, I might as well write my own.

Rather than write a random wallpaper script, I wanted something that could be more generally useful: pick one random line from standard input and print it. Then I could pass it the output of ls -1 $HOME/Backgrounds, and at the same time I'd have a script that I could also use for other purposes, such as choosing a random quotation, or choosing a "flash card" question when studying for an exam.

The obvious approach is to read all of standard input into an array, count the lines, then pick a random number between one and $num_lines and print that array element. It took no time to whip that up in Python and it worked fine. But it's not very efficient -- what if you're choosing a line from a 10Mb file?

Then Sara Falamaki (thanks, Sara!) pointed me to a page with a neat Perl algorithm. It's Perl so it's not easy to read, but the algorithm is cute. You read through the input line by line, keeping track of the line number. For each line, the chance that this line should be the one printed at the end is the reciprocal of the line number: in other words, there's one chance out of $line_number that this line is the one to print.

So if there's only one line, of course you print that line; when you get to the second line, there's one chance out of two that you should switch; on the third, one chance out of three, and so on.

A neat idea, and it doesn't require storing the whole file in memory. In retrospect, I should have thought of it myself: this is basically the same algorithm I used for averaging images in GIMP for my silly Chix Stack Mars project, and I later described the method in the image stacking section of my GIMP book. To average images by stacking them, you give the bottom layer 100% opacity, the second layer 50% opacity, the third 33% opacity, and so on up the stack. Each layer makes an equal contribution to the final result, so what you see is the average of all layers.

The randomline script, which you can inspect here, worked fine, so I hooked it up to accomplish the original problem: setting a randomly chosen desktop background each day. Since I use a lightweight window manager (fvwm) rather than gnome or kde, and I start X manually rather than using gdm, I put this in my .xinitrc:

(xsetbg -fullscreen -border black `find $HOME/Backgrounds -name "*.*" | randomline`) &

Update: I've switched to using hsetroot, which is a little more robust than xsetbg. My new command is:

hsetroot -center `find -L $HOME/Backgrounds -name "*.*" | randomline`

So, an overlong article about a relatively trivial but nontheless nifty algorithm. And now I have a new desktop background each day. Today it's something prosaic: mud cracks from Death Valley. Who knows what I'll see tomorrow?

Tags: programming, pipelines, shell
[ 13:02 Feb 28, 2007    More programming | permalink to this entry ]

The Joys of Shell Pipelines ...

A friend called me for help with a sysadmin problem they were having at work. The problem: find all files bigger than one gigabyte, print all the filenames, add up all the sizes and print the total. And for some reason (not explained to me) they needed to do this all in one command line.

This is Unix, so of course it's possible somehow! The obvious place to start is with the find command, and man find showed how to find all the 1G+ files:

find / -size +1G

(Turns out that's a GNU find syntax, and BSD find, on OS X, doesn't support it. I left it to my friend to check man find for the OS X equivalent of -size _1G.)

But for a problem like this, it's pretty clear we'd need to get find to execute a program that prints both the filename and the size. Initially I used ls -ls, but Saz (who was helping on IRC) pointed out that du on a file also does that, and looks a bit cleaner. With find's unfortunate syntax, that becomes:

find / -size +1G -exec du "{}" \;

But now we needed awk, to collect and add up all the sizes while printing just the filenames. A little googling (since I don't use awk very often) and experimenting led to the final solution:

find / -size +1G -exec du "{}" \; | awk '{print $2; total += $1} END { print "Total is", total}'

Ah, the joys of Unix shell pipelines!

Update: Ed Davies suggested an easier way to do the same thing. turns out du will handle it all by itself: du -hc `find . -size +1G` Thanks, Ed!

Tags: linux, CLI, shell, backups, pipelines
[ 16:53 Dec 29, 2006    More linux | permalink to this entry ]

Linkifying with Regular Expressions

I had a page of plaintext which included some URLs in it, like this:

Tour of the Hayward Fault
http://www.mcs.csuhayward.edu/~shirschf/tour-1.html

Technical Reports on Hayward Fault
http://quake.usgs.gov/research/geology/docs/lienkaemper_docs06.htm

I wanted to add links around each of the urls, so that I could make it part of a web page, more like this:

Tour of the Hayward Fault
http://www.mcs.csu hayward.edu/~shirschf/tour-1.html

Technical Reports on Hayward Fault
htt p://quake.usgs.gov/research/geology/docs/lienkaemper_docs06.htm

Surely there must be a program to do this, I thought. But I couldn't find one that was part of a standard Linux distribution.

But you can do a fair job of linkifying just using a regular expression in an editor like vim or emacs, or by using sed or perl from the commandline. You just need to specify the input pattern you want to change, then how you want to change it.

Here's a recipe for linkifying with regular expressions.

Within vim:

:%s_\(https\=\|ftp\)://\S\+_<a href="&">&</a>_

If you're new to regular expressions, it might be helpful to see a detailed breakdown of why this works:

:

Tell vim you're about to type a command.

%

The following command should be applied everywhere in the file.

s_

Do a global substitute, and everything up to the next underscore will represent the pattern to match.

\(

This will be a list of several alternate patterns.

http

If you see an "http", that counts as a match.

s\=

Zero or one esses after the http will match: so http and https are okay, but httpsssss isn't.

\|

Here comes another alternate pattern that you might see instead of http or https.

ftp

URLs starting with ftp are okay too.

\)

We're done with the list of alternate patterns.

://

After the http, https or ftp there should always be a colon-slash-slash.

\S

After the ://, there must be a character which is not whitespace.

\+

There can be any number of these non-whitespace characters as long as there's at least one. Keep matching until you see a space.

_

Finally, the underscore that says this is the end of the pattern to match. Next (until the final underscore) will be the expression which will replace the pattern.

<a href="&">

An ampersand, &, in a substitute expression means "insert everything that was in the original pattern". So the whole url will be inserted between the quotation marks.

&</a>

Now, outside the <a href="..."> tag, insert the matched url again, and follow it with a </a> to close the tag.

_

The final underscore which says "this is the end of the replacement pattern". We're done!

Linkifying from the commandline using sed

Sed is a bit trickier: it doesn't understand \S for non-whitespace, nor = for "zero or one occurrence". But this expression does the trick:
sed -e 's_\(http\|https\|ftp\)://[^ \t]\+_<a href="&">&</a>_' <infile.txt >outfile.html

Addendum: George Riley tells me about VST for Vim 7, which looks like a nice package to linkify, htmlify, and various other useful things such as creating HTML presentations. I don't have Vim 7 yet, but once I do I'll definitely check out VST.

Tags: linux, editors, pipelines, regexp, shell, CLI
[ 12:40 May 14, 2006    More linux/editors | permalink to this entry ]

How to Search Your Mozilla Cache

Ever want to look for something in your browser cache, but when you go there, it's just a mass of oddly named files and you can't figure out how to find anything?

(Sure, for whole pages you can use the History window, but what if you just want to find an image you saw this morning that isn't there any more?)

Here's a handy trick.

First, change directory to your cache directory (e.g. $HOME/.mozilla/firefox/blahblah/Cache).

Next, list the files of the type you're looking for, in the order in which they were last modified, and save that list to a file. Like this:

% file `ls -1t` | grep JPEG | sed 's/: .*//' > /tmp/foo

In English: ls -t lists in order of modification date, and -1 ensures that the files will be listed one per line. Pass that through grep for the right pattern (do a file * to see what sorts of patterns get spit out), then pass that through sed to get rid of everything but the filename. Save the result to a temporary file.

The temp file now contains the list of cache files of the type you want, ordered with the most recent first. You can now search through them to find what you want. For example, I viewed them with Pho:

pho `cat /tmp/foo`

For images, use whatever image viewer you normally use; if you're looking for text, you can use grep or whatever search you lke. Alternately, you could ls -lt `cat foo` to see what was modified when and cut down your search a bit further, or any other additional paring you need.

Of course, you don't have to use the temp file at all. I could have said simply:

pho `ls -1t` | grep JPEG | sed 's/: .*//'`

Making the temp file is merely for your convenience if you think you might need to do several types of searches before you find what you're looking for.

Tags: tech, web, mozilla, firefox, pipelines, CLI, shell, regexp
[ 21:40 Oct 10, 2005    More tech/web | permalink to this entry ]

Desktop Search -- What you need if you don't have grep

I've been surprised by the recent explosion in Windows desktop search tools. Why does everyone think this is such a big deal that every internet company has to jump onto the bandwagon and produce one, or be left behind?

I finally realized the answer this morning. These people don't have grep! They don't have any other way of searching out patterns in files.

I use grep dozens of times every day: for quickly looking up a phone number in a text file, for looking in my Sent mailbox for that url I mailed to my mom last week, for checking whether I have any saved email regarding setting up CUPS, for figuring out where in mozilla urlbar clicks are being handled.

Every so often, some Windows or Mac person is opining about how difficult commandlines are and how glad they are not to have to use them, and I ask them something like, "What if you wanted to search back through your mail folders to find the link to the cassini probe images -- e.g. lines that have both http:// and cassini in them?" I always get a blank look, like it would never occur to them that such a search would ever be possible.

Of course, expert users have ways of doing such searches (probably using command-line add-ons such as cygwin); and Mac OS X has the full FreeBSD commandline built in. And more recent Windows versions (Win2k and XP) now include a way to search for content in files (so in the Cassini example, you could search for http:// or cassini, but probably not both at once.) But the vast majority of Windows and Mac users have no way to do such a search, the sort of thing that Linux commandline users do casually dozens of times per day. Until now.

Now I see why desktop search is such a big deal.

But rather than installing web-based advertising-drive apps with a host of potential privacy and security implications ...

wouldn't it be easier just to install grep?

Tags: tech, pipelines, CLI, shell
[ 11:45 Jan 19, 2005    More tech | permalink to this entry ]