通过一条命令计算输出1+2+….+10的表达式，并计算出结果
方法有多种，如下：
-bash-4.1# echo seq -s "+" 10=seq -s "+" 10|bc
1+2+3+4+5+6+7+8+9+10=55
-bash-4.1# echo echo {1..10}|tr " " "+"=echo {1..10}|tr " " "+"|bc
1+2+3+4+5+6+7+8+9+10=55
-bash-4.1# echo seq -s "+" 10=$[seq -s "+" 10]
1+2+3+4+5+6+7+8+9+10=55
-bash-4.1# echo seq -s "+" 10=$(expr $[seq -s "+" 10])
1+2+3+4+5+6+7+8+9+10=55
-bash-4.1# echo seq -s "+" 10=$((seq -s "+" 10))
1+2+3+4+5+6+7+8+9+10=55