检查 bash 变量是否等于 0
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Check if bash variable equals 0
提问by Perlnika
I have a bash variable depth and I would like to test if it equals 0. In case yes, I want to stop executing of script. So far I have:
我有一个 bash 变量深度，我想测试它是否等于 0。如果是，我想停止执行脚本。到目前为止，我有：
zero=0;
if [ $depth -eq $zero ]; then
echo "false";
exit;
fi
Unfortunately, this leads to:
不幸的是，这导致：
[: -eq: unary operator expected
(might be a bit inaccurate due to translation)
（由于翻译可能有点不准确）
Please, how can I modify my script to get it working?
请问，我如何修改我的脚本以使其正常工作？
回答by cyon
Looks like your depthvariable is unset. This means that the expression [ $depth -eq $zero ]becomes [ -eq 0 ]after bash substitutes the values of the variables into the expression. The problem here is that the -eqoperator is incorrectly used as an operator with only one argument (the zero), but it requires two arguments. That is why you get the unary operatorerror message.
看起来您的depth变量未设置。这意味着在 bash 将变量的值代入表达式后，表达式[ $depth -eq $zero ]就[ -eq 0 ]变成了。这里的问题是-eq运算符被错误地用作只有一个参数（零）的运算符，但它需要两个参数。这就是为什么您会收到一元运算符错误消息的原因。
EDIT:As Doktor Jmentioned in his comment to this answer, a safe way to avoid problems with unset variables in checks is to enclose the variables in "". See his comment for the explanation.
编辑：正如Doktor J在对此答案的评论中提到的，避免检查中未设置变量出现问题的一种安全方法是将变量包含在"". 有关解释，请参阅他的评论。
if [ "$depth" -eq "0" ]; then
echo "false";
exit;
fi
An unset variable used with the [command appears empty to bash. You can verify this using the below tests which all evaluate to truebecause xyzis either empty or unset:
与[命令一起使用的未设置变量 在 bash 中显示为空。您可以通过下面的测试中，所有评估来验证这true是因为xyz为空或取消设置：
if [ -z ] ; then echo "true"; else echo "false"; fixyz=""; if [ -z "$xyz" ] ; then echo "true"; else echo "false"; fiunset xyz; if [ -z "$xyz" ] ; then echo "true"; else echo "false"; fi
if [ -z ] ; then echo "true"; else echo "false"; fixyz=""; if [ -z "$xyz" ] ; then echo "true"; else echo "false"; fiunset xyz; if [ -z "$xyz" ] ; then echo "true"; else echo "false"; fi
回答by Raviteja Gubba
Double parenthesis (( ... ))is used for arithmetic operations.
双括号(( ... ))用于算术运算。
Double square brackets [[ ... ]]can be used to compare and examine numbers (only integers are supported), with the following operators:
[[ ... ]]双方括号可用于比较和检查数字（仅支持整数），具有以下运算符：
· NUM1 -eq NUM2 returns true if NUM1 and NUM2 are numerically equal.
· NUM1 -ne NUM2 returns true if NUM1 and NUM2 are not numerically equal.
· NUM1 -gt NUM2 returns true if NUM1 is greater than NUM2.
· NUM1 -ge NUM2 returns true if NUM1 is greater than or equal to NUM2.
· NUM1 -lt NUM2 returns true if NUM1 is less than NUM2.
· NUM1 -le NUM2 returns true if NUM1 is less than or equal to NUM2.
For example
例如
if [[ $age > 21 ]] # bad, > is a string comparison operator
if [ $age > 21 ] # bad, > is a redirection operator
if [[ $age -gt 21 ]] # okay, but fails if $age is not numeric
if (( $age > 21 )) # best, $ on age is optional
回答by Jacek Dominiak
Try:
尝试：
zero=0;
if [[ $depth -eq $zero ]]; then
echo "false";
exit;
fi
回答by Gurubaran
you can also use this format and use comparison operators like '==' '<='
您也可以使用这种格式并使用比较运算符，例如 '==' '<='
if (( $total == 0 )); then
echo "No results for "
return
fi
回答by Stephen Niedzielski
Specifically: ((depth)). By example, the following prints 1.
具体：((depth))。例如，以下打印1.
declare -i x=0
((x)) && echo $x
x=1
((x)) && echo $x
回答by Prabhat Kumar Singh
You can try this:
你可以试试这个：
: ${depth?"Error Message"} ## when your depth variable is not even declared or is unset.
NOTE: Here it's just ?after depth.
注意：这里就?在depth.
or
或者
: ${depth:?"Error Message"} ## when your depth variable is declared but is null like: "depth=".
NOTE: Here it's :?after depth.
注意：这是:?在depth.
Here if the variable depthis found nullit will print the error message and then exit.
在这里，如果depth找到该变量null，它将打印错误消息然后退出。
