grtail.groovy

final COUNT_TO_WRITE = 5

def lineNo = 1
def writableCount = 0

System.in.eachLine {
    if (it =~ /hoge/) {
        writableCount = COUNT_TO_WRITE
    }
    if (writableCount > 0) {
        println "$lineNo: $it"
        writableCount--
    }
    lineNo++
}

Alternative implementation of "grep -A".

$ cat test.txt | groovy grtail.groovy
15: xhoge
16: x1
17: x2
18: x3
19: x4
30: yhoge
31: y1
32: y2
33: y3
34: y4
45: zhoge
46: z1
47: z2
48: z3
49: z4

I recommend to use the groovyclient of GroovyServ instead of groovy!