awk使用'['和']'做分隔符

类似于下面的日志，需要统计tm字段的值的和，请问我如何从tm[23]中提取出纯数字23？
谢谢

QUOTE:

NOTICE: 09-16 01:00:02:  server. * 114703 sign[1770396815:3988304213] rslt[0] tm[23] src[1]
NOTICE: 09-16 01:00:02:  server. * 319528 sign[1831438967:4109678927] rslt[0] tm[22] src[1]
NOTICE: 09-16 01:00:02:  server. * 172054 sign[1984265647:345815627] rslt[0] tm[20] src[1]
NOTICE: 09-16 01:00:02:  server. * 196633 sign[1758830922:260725986] rslt[0] tm[36] src[4]
NOTICE: 09-16 01:00:02:  server. * 57352 sign[1713835886:4224720248] rslt[0] tm[31] src[1]
NOTICE: 09-16 01:00:02:  server. * 262177 sign[1985386887:620124813] rslt[0] tm[20] src[1]
NOTICE: 09-16 01:00:02:  server. * 450616 sign[1804753829:733626451] rslt[0] tm[20] src[1]
NOTICE: 09-16 01:00:02:  server. * 163861 sign[2249492205:582976983] rslt[0] tm[7781] src[4]
NOTICE: 09-16 01:00:02:  server. * 524353 sign[1859063525:4103190671] rslt[0] tm[27] src[1]
NOTICE: 09-16 01:00:02:  server. * 213019 sign[1533101819:3801889919] rslt[2] tm[22] src[2]

CODE:

awk -F'[][]' '{sum+=$6}END{print "total of tm is",sum}'

CODE:

cat filename|awk '{print $9}'|tr 'tm[]' ' 'From: http://bbs.chinaunix.net/thread-992296-1-1.html