为什么以下脚本使用 Dash (sh) 时会出现错误，而使用 Bash 时却可以正常运行？
我看不到错误
感谢您的帮助。
#!/bin/bash
# Calculate the length of the hypotenuse of a Pythagorean triangle
# using hypotenuse^2 = adjacent^2 + opposite^2
echo -n "Enter the Adjacent length: "
read adjacent
echo -n "Enter the Opposite length: "
read opposite
asquared=$(($adjacent ** 2)) # get a^2
osquared=$(($opposite ** 2)) # get o^2
hsquared=$(($osquared + $asquared)) # h^2 = a^2 + o^2
hypotenuse=`echo "scale=3;sqrt ($hsquared)" | bc` # bc does sqrt
echo "The Hypotenuse is $hypotenuse"
结果：
myname@myhost:~$ sh ./hypotenusa.sh
Enter the Adjacent length: 12
Enter the Opposite length: 4
./hypotenusa.sh: 12: ./hypotenusa.sh: arithmetic expression: expecting primary: "12 ** 2"
myname@myhost:~$ bash ./hypotenusa.sh
Enter the Adjacent length: 12
Enter the Opposite length: 4
The Hypotenuse is 12.649
我的 Ubuntu 版本是 13.04。
答案1
答案很简单，它dash不支持通过**运算符进行幂运算（这不是 POSIX 兼容的 shell 解释器的要求）。
你可以使用该实用程序检查特定脚本是否使用了此类“bashisms” checkbashisms，例如
$ cat > myscript.sh
#!/bin/sh
echo $((12 ** 2))
Ctrl+d
$ checkbashisms myscript.sh
possible bashism in myscript.sh line 3 (exponentiation is not POSIX):
echo $((12 ** 2))
$
答案2
**运算符无法被 sh 识别。sh（Shell 命令语言）是一种由POSIX 标准，而 bash 会改变有效 POSIX shell 脚本的行为，所以 bash 本身并不是一个有效的 POSIX shell。**指数运算符在 bash 2.02 版本中引入过。
资料来源：
在 bash 和 sh 中，^是将数字转换为另一个数字的默认运算符（请参阅man bc）。因此，您的脚本应如下所示：
#!/bin/bash
# Calculate the length of the hypotenuse of a Pythagorean triangle
# using hypotenuse^2 = adjacent^2 + opposite^2
echo -n "Enter the Adjacent length: "
read adjacent
echo -n "Enter the Opposite length: "
read opposite
asquared="($adjacent ^ 2)" # get a^2
osquared="($opposite ^ 2)" # get o^2
hsquared="($osquared + $asquared)" # h^2 = a^2 + o^2
hypotenuse=`echo "scale=3;sqrt ($hsquared)" | bc` # bc does sqrt
echo "The Hypotenuse is $hypotenuse"