Archive for the ‘bash’ tag

How to pretend bash is a real programming language, tip #13


I wrote some throwaway shell code tonight that looked like this:

   for oo in $(cd .git/objects/ && ls ??/*); do
     # do something horrible with the Git object $o, which is in the file $oo

It doesn’t matter now exactly what the code was for. But a collaborator wrote back to me:

> > o=${oo%/*}${oo#*/}
> How does this line work/what is it supposed to accomplish?  In
> particular not sure what the %foo and #foo do.

Stop for a moment: do you know how that line works? I wouldn’t have in my first years writing shell scripts.

This line demonstrates one of a repertoire of tricks I’ve picked up to get some things done in bash that might otherwise require invoking a separate program. None of these will be news to shell-programming experts, but I sure didn’t know all of them when I started writing in shell. Here’s a little braindump on one of my favorite tricks, and where to read about more.

The best documentation for Bash is the info page—the specific pages I find myself referring to most often are under “info bash” -> “Basic Shell Features” -> “Shell Expansions”. (If you’ve never tried it, you’ve been missing out! Type “info bash” at your favorite prompt. But not on a Debian or Ubuntu machine, where the info page is missing due to a stupid licensing dispute. Info is the home of the best documentation available for Bash, GCC, GDB, Emacs, miscellaneous GNU utilities, and Info itself.)

This feature is under “Shell Parameter Expansion” there.

     The WORD is expanded to produce a pattern just as in filename
     expansion (*note Filename Expansion::).  If the pattern matches
     the beginning of the expanded value of PARAMETER, then the result
     of the expansion is the expanded value of PARAMETER with the
     shortest matching pattern (the `#' case) or the longest matching
     pattern (the `##' case) deleted.

The % and %% features work similarly, with “beginning” substituted with “end”.

My mnemonic for # versus % is that $ is for the variable; # is to the left of $, so it strips from the left, and % is to the right, so it strips from the right. I suspect this is the actual motivation for the choice of # and %, though I’m curious to see evidence to confirm or refute that thought.

So after my line o=${oo%/*}${oo#*/}, o consists of the part of oo to the left of the last slash, and then the part of oo to the right of the first slash. Since there should be just one slash in oo, it has the effect of making o be everything but the slash.

That makes one trick I use all the time. There’s plenty more, and those Info pages explain many of them. I’m not sure all these tricks are a good thing on balance—they serve as a crutch to make the shell go further, when maybe I should just be quicker to switch to a real programming language. But they sure come in handy.

Written by Greg Price

February 27th, 2010 at 4:56 am

Posted in Uncategorized

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