UNIX for Dummies Questions & Answers

newfile=$( echo 04302007_xxx_yyy_aaa_bbb.txt  |awk 'BEGIN { FS = "."}
						 {	
							n = split($1,s1,"_")
							for(i=2;i<=n;i++) printf "%s_",s1[i]
							printf "%s.%s\n",s1[1] , $2
						 }' )
echo $newfile

# ./test.sh
xxx_yyy_aaa_bbb_04302007.txt

$ nm=04302007_xxx_yyy_aaa_bbb.txt
$ flnm=${nm#*_}
$ dt=${nm%%_*}
$ ext=${nm##*.}
$ echo ${flnm%.*}"_"${dt%?}"."${ext}
xxx_yyy_aaa_bbb_0430200.txt

       ${name#pattern}, ${name##pattern}
              If pattern matches the beginning of the value of parameter name,
              the matched text is deleted from the result of substitution.   A
              single  #  results in the shortest match, two #'s results in the
              longest match.

       ${name%pattern}, ${name%%pattern}
              Like ${..#..} substitution, but it deletes from the end  of  the
              value.