How to print Specific keyword, by using awk?

Shell Programming and Scripting
1

How to print Specific keyword, by using awk.?

prime:root:[/usr/home/myserver/$ cat input.log 
1620002_M_20120309033204_78.txt
1620002_M_20120309033232_1457.txt
1620002_M_20120309033240_10000.txt
1620002_M_20120309033241_10000.txt
1620002_M_20120309033242_5985.txt
1620002_M_20120309033243_307.txt
1620002_M_20120309033246_10000.txt
1620002_M_20120309033247_10000.txt
1620002_M_20120309033248_10000.txt
1620002_M_20120309033249_10000.txt
1620002_M_20120309033250_3760.txt
1620002_M_20120309033251_692.txt
1620002_M_20120309033254_6656.txt
1620002_M_20120309033255_157.txt
1620002_M_20120309033302_696.txt
1620002_M_20120309033303_127.txt
1620002_M_20120309033304_7408.txt

I want output.

78
1457
10000
10000
5985
307
10000
10000
10000
10000
3760
692
6656
157
696
127
7408

I try with command

cat input.log |awk -F"_" '{print$4}'

Please Guide Line.

Thank

2 
 awk -F'[_.]' '{print $4}' inputfilename > newfilename
3

I Try but not work.

4

In what way does it "not work"?

What's your operating system?

Have you tried nawk instead of awk?

5

Why awk?

while IFS='.' read a b; do echo ${a##*_}; done < input.log