bash shell 脚本中的负数
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Negative number in bash shell scripting
提问by kwasbob
I need some help with a bash mathematical expression script. I need the expression below to be a negative 2 to the power of 63 but I have tried all sorts of combinations from "", '', (()) and even keeping the positive value but multiplying it by -1 but I can't seem to get the syntax right.
我需要一些有关 bash 数学表达式脚本的帮助。我需要下面的表达式是 2 的 63 次方的负数，但我已经尝试了 "", '', (()) 的各种组合，甚至保持正值但乘以 -1 但我可以'似乎语法正确。
Original expression:
原文表达：
kw=`expr 2^63 | bc`
gives me 9223372036854775808 (which is correct) but I need to get a -9223372036854775808.
给了我 9223372036854775808（这是正确的），但我需要得到 -9223372036854775808。
I want to be able to do a kw=expr -2^63 | bcto get a negative results but it's not as straight-forward as I'd hoped. Hence my numerous attempts of different permutations to the expression.
我希望能够执行 kw=expr -2^63 | bc以获得负面结果，但这并不像我希望的那样直接。因此，我多次尝试对表达式进行不同的排列。
Any help will be very appreciated.
任何帮助将不胜感激。
回答by janos
Here you go:
干得好：
$ kw=$(echo -2^63 | bc)
$ echo $kw
-9223372036854775808
UPDATE
更新
@DigitalTraumais right, if you're in bash, then using a bash here stringis better (one less process, more efficient):
@DigitalTrauma是对的，如果你在bash，那么在这里使用bash 字符串更好（少一个过程，更高效）：
kw=$(bc <<< -2^63)
回答by Digital Trauma
Since this is bash, you don't even need the echo; you can use a bash here stringinstead:
由于这是bash，您甚至不需要echo; 您可以在此处使用bash 字符串：
$ kw=$(bc <<< -2^63)
$ echo $kw
-9223372036854775808
