At the risk of turning this into a Groovy blog, here's an extended comment on a neat post by Jesse Wilson.
With its Java-friendly syntax and command-line options, Groovy opens up some serious possibilities on the command line.
Check out the doc for all the details and examples.
Here are some of the basics:
The -e option
-e option accepts a one-liner as the input program. Many languages allow this but few offer such tight integration with Java.
For example, we can format a date. (Note that I'll use Unix-style line continuations here, but these are a single line.)
Note how easy it is to experiment with the format string. I work in an open "war-room", and often use little snippets like this to answer questions on the behaviour of various Java libraries.
$ groovy -e " println \
> new java.text.SimpleDateFormat('yyyy.MM.dd G') \
> .format( new Date() ) "
$ 2008.08.25 AD
Here we use
java.util.Random. (Note: Groovy imports
$ groovy -e " println new Random().nextInt(25) "
The -n option
-noption instructs Groovy to use standard input. The doc asks if there is a bug, but it seems to works fine for these examples.
In this mode, Groovy will provide the
countvariables like so (note the content of the file is shown first):
When combined with Bash, this opens up a whole new world.
$ cat abc.txt
$ cat abc.txt | groovy -n -e " println count + ' : ' + line "
1 : a
2 : b
3 : c
It is difficult to duplicate Jesse's program with a single line. However, it is easy to use Java's regular expressions inline.
First, build an input file:
$ echo "http://codetojoy.blogspot.com CodeToJoy" > foo.txt
Here is the Groovy, in multi-line form for clarity:
// The =~ operator applies line against
// a regex within the /'s, and returns a Java matcher.
// The regex is a plain old Java regex.
// If new to Groovy, 'def' is like 'Object' (for now)
def m = ( line =~ /http:\/\/([\w.]+)\S*/ )
// Using Groovy truth: if m matched, then the matching
// groups can be accessed via an array.
if( m ) println m
The one-liner looks like this (again Unix-style line continuations here):
$ cat foo.txt | groovy -n -e " \
> def m = ( line =~ /http:\/\/([\w.]+)\S*/ ) ; \
> if( m ) println m "
Grails gets a lot of press, but Groovy offers a lot of utility, even on the modest command line. This is especially true when combined with Unix shells like Bash.
Sometimes, I want to use other tools but this damn language just won't go away as a useful option.