Thursday, June 07, 2007

Kickin' it Old School: Inspecting $CLASSPATH with sed and grep

Here's a fun sed one-liner that I used today to break up the entries in my $CLASSPATH:

echo $CLASSPATH | sed 's/:/\<return>

Note that the <return> above means to actually hit the return key following the backslash. This bit of awkwardness is sed's way of specifying a literal newline as part of the substitution string (how literal can you get?). The net effect is to replace the colon characters with newlines, resulting in a display of my classpath with one entry per line.

I needed this in the context of figuring out a broken Ant build while testing some changes I'm making to JRuby. Unfortunately, even the pretty-printed version of my Ant classpath was too long to sift through with the naked eye, so I turned to grep to look for exactly what I needed:

echo $CLASSPATH | sed 's/:/\<return>
/g | grep jruby.jar'

That is, break up the classpath into one line per entry, and show me only the entries for jruby.jar. With this, I was able to determine that I had an older version of jruby.jar on my classpath that is incompatible with the current trunk. Problem solved!

I love a happy ending.


Lee said...

tr ':' '\n'

David Rupp said...

Nice. I vaguely remember reading somewhere that tr has some weirdness about it, but I can't think where it was. I'll have to go look it up again.

Anyway, thanks for pointing that out. I will add it to my bag of tricks.