Bash 布尔表达式及其赋值
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Bash boolean expression and its value assignment
提问by sakhunzai
Is there a way to to evaluate a boolean expression and assign its value to a variable?
有没有办法评估布尔表达式并将其值分配给变量？
In most of the scripting languages there is way to evaluates e.g
在大多数脚本语言中，都有评估的方法，例如
//PHS
$found= $count > 0 ; //evaluates to a boolean values
I want similar way to evaluate in bash:
我想要类似的方式在 bash 中进行评估：
BOOL=[ "$PROCEED" -ne "y" ] ;
This is not working and tried other way but could not get a boolean value. IS there a way to do this WITHOUT using IF ?
这不起作用并尝试了其他方式，但无法获得布尔值。有没有办法在不使用 IF 的情况下做到这一点？
回答by Mat
You could do:
你可以这样做：
[ "$PROCEED" = "y" ] ; BOOL=$?
If you're working with set -e, you can use instead:
如果您正在使用set -e，则可以改用：
[ "$PROCEED" = "y" ] && BOOL=0 || BOOL=1
BOOLset to zero when there is a match, to act like typical Unix return codes. Looks a bit weird.
BOOL匹配时设置为零，以类似于典型的 Unix 返回码。看起来有点奇怪。
This will not throw errors, and you're sure $BOOLwill be either 0 or 1 afterwards, whatever it contained before.
这不会抛出错误，并且您确定之后$BOOL将是 0 或 1，无论它之前包含什么。
回答by Andrew
I would suggest:
我会建议：
[ "$PROCEED" = "y" ] || BOOL=1
This has the advantage over checking $?that it works even when set -eis on. (See writing robust shell scripts.)
这比检查$?它是否工作更有优势，即使在set -e打开时也是如此。（请参阅编写健壮的 shell 脚本。）
回答by user11023646
Rather than using ... && BOOL=0 || BOOL=1suggested in the currently-accepted answer, it's clearer to use trueand false.
与使用... && BOOL=0 || BOOL=1当前接受的答案中的建议相比，使用trueand更清晰false。
And since this question is about bash specifically (not POSIX shell), it's also better to use [[instead of [(see e.g. 1and 2), which allows using ==instead of =.
而且由于这个问题特别是关于 bash（而不是 POSIX shell），所以最好使用[[而不是[（参见例如1和2），它允许使用==代替=.
So if you had to use a one-liner for something like this in bash, the following would be better:
因此，如果您必须在 bash 中为这样的事情使用单行，以下内容会更好：
[[ "$PROCEED" == "y" ]] && should_proceed=true || should_proceed=false
[[ "$PROCEED" == "y" ]] && should_proceed=true || should_proceed=false
Then you can use the derived variable ergonomically in boolean contexts...
然后您可以在布尔上下文中以符合人体工程学的方式使用派生变量...
if $should_proceed; then
echo "Proceeding..."
fi
...including with the !operator:
...包括!运营商：
if ! $should_proceed; then
echo "Bye for now."
exit 0
fi
回答by user unknown
Assignment:
任务：
found=$((count > 0))
