shell使用eval进行赋值bc计算，bad substitution

开始我认为是这样的：

[root@jiangyi02.sqa.zmf /home/ahao.mah/ALIOS_TEST]
#cat bbb.sh
#!/bin/sh
eval $1_new=123
eval $1_old=100
eval echo ${$1_new}
eval echo "${$1_new} - ${$1_old}" | bc
aaa=`eval echo "${$1_new} - ${$1_old}" | bc`
echo $aaa

[root@jiangyi02.sqa.zmf /home/ahao.mah/ALIOS_TEST]
#sh  bbb.sh  aa
123
23
bbb.sh: line 6: ${$1_new} - ${$1_old}: bad substitution

其实，在使用eval的时候，如果使用了 ``需要 使用两个

#cat  bbb.sh
#!/bin/sh
eval $1_new=123
eval $1_old=100
eval echo ${$1_new}
eval echo "${$1_new} - ${$1_old}" | bc
aaa=`eval echo "\${$1_new} - \${$1_old}" | bc`
echo $aaa

[root@jiangyi02.sqa.zmf /home/ahao.mah/ALIOS_TEST]
#sh  bbb.sh aa
123
23
23

http://www.linuxquestions.org/questions/linux-general-1/why-i-get-bad-substitution-when-using-eval-898198/