数字的绝对值 [英] Absolute value of a number

问题描述

我想通过bash中的以下代码获取数字的绝对值:

 #！/bin/bashecho输入第一个文件名:"首先阅读echo输入第二个文件名:"阅读第二s1 = $(stat --format =％s"$ first")s2 = $(stat -c'％s'"$ second")res = expr $ s2-$ s1如果["$ res" -lt 0]然后res = $ res \ * -1科幻回声$ res 

现在我面临的问题是if语句，无论我进行什么更改，它总是出现在if中，我尝试将 [[]] 放在语句周围，但是什么也没有.

这是错误:

  ./p6.sh:第13行:[::预期为整数表达式 

解决方案

  $ s2 = 5 s1 = 4$ echo $ s2 $ s15 4$ res = expr $ s2-$ s11个$ echo $ res 

第四行实际发生的事情是，将 res 设置为空并为 expr 命令导出.因此，当您运行 ["$ res" -lt 0] res 扩展为空时，您会看到错误.

您可以只使用算术表达式:

  $((res = s2-s1))$ echo $ res1个 

算术上下文保证结果将为整数，因此即使未定义所有术语，您也将获得整数结果(即零).

  $((res = whoknows-whocares));回声$ res0 

或者，您可以这样声明外壳，从而告诉外壳 res 是整数:

  $声明-i res$ res = s2-s1 

这里有趣的是，赋值的右侧在算术上下文中处理，因此您不需要 $ 进行扩展.

I want to take the absolute of a number by the following code in bash:

#!/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.

Here is the error:

./p6.sh: line 13: [: : integer expression expected

解决方案

$ 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 res is being set to nothing and exported for the expr command. Thus, when you run [ "$res" -lt 0 ] res is expanding to nothing and you see the error.

You could just use an arithmetic expression:

$ (( 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).

$ (( res = whoknows - whocares )); echo $res
0

Alternatively, you can tell the shell that res is an integer by declaring it as such:

$ 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.

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