bash 数字的绝对值
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Absolute value of a number
提问by Ali Sajid
I want to take the absolute of a number by the following code in bash:
我想通过bash中的以下代码获取数字的绝对值：
#!/bin/bash
echo "Enter the first file name: "
read first
echo "Enter the second file name: "
read second
s1=$(stat --format=%s "$first")
s2=$(stat -c '%s' "$second")
res= expr $s2 - $s1
if [ "$res" -lt 0 ]
then
res=$res \* -1
fi
echo $res
Now the problem I am facing is in the if statement, no matter what I changes it always goes in the if, I tried to put [[ ]]around the statement but nothing.
现在我面临的问题是在 if 语句中，无论我如何更改它总是在 if 中，我试图[[ ]]绕过该语句但什么也没有。
Here is the error:
这是错误：
./p6.sh: line 13: [: : integer expression expected
采纳答案by kojiro
$ s2=5 s1=4
$ echo $s2 $s1
5 4
$ res= expr $s2 - $s1
1
$ echo $res
What's actually happening on the fourth line is that resis being set to nothing and exported for the exprcommand. Thus, when you run [ "$res" -lt 0 ]resis expanding to nothing and you see the error.
第四行实际发生的res是被设置为空并为expr命令导出。因此，当您运行时[ "$res" -lt 0 ]res扩展为空并且您会看到错误。
You could just use an arithmetic expression:
您可以只使用算术表达式：
$ (( res=s2-s1 ))
$ echo $res
1
Arithmetic context guarantees the result will be an integer, so even if all your terms are undefined to begin with, you will get an integer result (namely zero).
算术上下文保证结果将是一个整数，因此即使您的所有术语一开始都未定义，您也会得到一个整数结果（即零）。
$ (( res = whoknows - whocares )); echo $res
0
Alternatively, you can tell the shell that resis an integer by declaring it as such:
或者，您可以res通过如下声明来告诉外壳这是一个整数：
$ declare -i res
$ res=s2-s1
The interesting thing here is that the right hand side of an assignment is treated in arithmetic context, so you don't need the $for the expansions.
有趣的是，赋值的右侧是在算术上下文中处理的，因此您不需要$扩展。
回答by Suuuehgi
回答by bng44270
I know this thread is WAY old at this point, but I wanted to share a function I wrote that could help with this:
我知道这个线程在这一点上已经很老了，但我想分享一个我写的可以帮助解决这个问题的函数：
abs() {
[[ $[ $@ ] -lt 0 ]] && echo "$[ ($@) * -1 ]" || echo "$[ $@ ]"
}
This will take any mathematical/numeric expression as an argument and return the absolute value. For instance: abs -4 => 4 or abs 5-8 => 3
这将采用任何数学/数字表达式作为参数并返回绝对值。例如： abs -4 => 4 或 abs 5-8 => 3
