shell 加法

i=$i+1

是在i的变量值后加上字符串'+1'

总结：其他语言中的$i++操作在shell中表示如下：
#!/bin/bash

n=1;echo -n "$n "

let "n = $n + 1"
echo -n "$n "

: $((n = $n + 1))
echo -n "$n "

(( n = n +1 ))
echo -n "$n "

: $[ n = $n +1 ]
echo -n "$n "

n=$[ $n + 1 ]
echo -n "$n "

let "n++"
echo -n "$n "

(( n++ ))
echo -n "$n "

: $[ n++ ]
echo -n "$n "

echo

运行结果：1 2 3 4 5 6 7 8 9