shell只提取数字的方法大集合

$ echo ${aa#aa*} | awk -F. '{print $1 $2 $3}'
008877

$ aa="aa00.88.77"
$ echo $aa | grep -o '[0-9]\{2\}'
00
88
77

$ aa=009999
$ [ "${aa##[0-9]*}" != "" ] && echo "a string" || echo "a digit"
a digit