Bash 中的三元运算符 (?:)
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Ternary operator (?:) in Bash
提问by En_t8
Is there a way to do something like this
有没有办法做这样的事情
int a = (b == 5) ? c : d;
using Bash?
使用 Bash？
回答by ghostdog74
ternary operator ? :is just short form of if/else
三元运算符? :只是if/else
case "$b" in
5) a=$c ;;
*) a=$d ;;
esac
Or
或者
[[ $b = 5 ]] && a="$c" || a="$d"
回答by Vladimir
Code:
代码：
a=$([ "$b" == 5 ] && echo "$c" || echo "$d")
回答by nemesisfixx
If the condition is merely checking if a variable is set, there's even a shorter form:
如果条件只是检查是否设置了变量，则还有更短的形式：
a=${VAR:-20}
will assign to athe value of VARif VARis set, otherwise it will assign it the default value 20-- this can also be a result of an expression.
将要分配给a的值VAR如果VAR被设置，否则将分配给它的默认值20-这也可以是一个表达的结果。
As alex notes in the comment, this approach is technically called "Parameter Expansion".
正如亚历克斯在评论中指出的那样，这种方法在技术上称为“参数扩展”。
回答by ivan_pozdeev
if [ "$b" -eq 5 ]; then a="$c"; else a="$d"; fi
The cond && op1 || op2expression suggested in other answers has an inherent bug: if op1has a nonzero exit status, op2silently becomes the result; the error will also not be caught in -emode. So, that expression is only safe to use if op1can never fail (e.g., :, trueif a builtin, or variable assignment without any operations that can fail (like division and OS calls)).
cond && op1 || op2其他答案中建议的表达式有一个固有的错误：如果op1退出状态非零，则op2默默地成为结果；错误也不会在-e模式中被捕获。所以，这表达仅仅是安全的使用，如果op1不能失败（例如:，true如果一个内置的，或者不能够失败（如部门和操作系统调用）的任何操作变量赋值）。
Note the ""quotes. The first pair will prevent a syntax error if $bis blank or has whitespace. Others will prevent translation of all whitespace into single spaces.
注意""引号。如果第一对$b为空白或有空格，则将防止语法错误。其他人会阻止将所有空格转换为单个空格。
回答by dutCh
(( a = b==5 ? c : d )) # string + numeric
回答by Sir Athos
[ $b == 5 ] && { a=$c; true; } || a=$d
This will avoid executing the part after || by accident when the code between && and || fails.
这将避免在 || 之后执行部分 && 和 || 之间的代码偶然出现 失败。
回答by emu
The letcommand supports most of the basic operators one would need:
该让命令支持基本算一个需要的最：
let a=b==5?c:d;
Naturally, this works only for assigning variables; it cannot execute other commands.
当然，这仅适用于分配变量；它不能执行其他命令。
回答by Jasonovich
Here is another option where you only have to specify the variable you're assigning once, and it doesn't matter whether what your assigning is a string or a number:
这是另一种选择，您只需指定一次要分配的变量，并且分配的内容是字符串还是数字都没有关系：
VARIABLE=`[ test ] && echo VALUE_A || echo VALUE_B`
Just a thought. :)
只是一个想法。:)
回答by Brad Parks
The following seems to work for my use cases:
以下似乎适用于我的用例：
Examples
例子
$ tern 1 YES NO
YES
$ tern 0 YES NO
NO
$ tern 52 YES NO
YES
$ tern 52 YES NO 52
NO
and can be used in a script like so:
并且可以在这样的脚本中使用：
RESULT=$(tern 1 YES NO)
echo "The result is $RESULT"
tern
燕鸥
function show_help()
{
echo ""
echo "usage: BOOLEAN VALUE_IF_TRUE VALUE_IF_FALSE {FALSE_VALUE}"
echo ""
echo "e.g. "
echo ""
echo "tern 1 YES NO => YES"
echo "tern 0 YES NO => NO"
echo "tern "" YES NO => NO"
echo "tern "ANY STRING THAT ISNT 1" YES NO => NO"
echo "ME=$(tern 0 YES NO) => ME contains NO"
echo ""
exit
}
if [ "" == "help" ]
then
show_help
fi
if [ -z "" ]
then
show_help
fi
# Set a default value for what is "false" -> 0
FALSE_VALUE=${4:-0}
function main
{
if [ "" == "$FALSE_VALUE" ]; then
echo
exit;
fi;
echo
}
main "" "" ""
回答by Sujay U N
We can use following three ways in Shell Scripting for ternary operator :
我们可以在 Shell Scripting 中使用以下三种方式进行三元运算符：
[ $numVar == numVal ] && resVar="Yop" || resVar="Nop"
Or
resVar=$([ $numVar == numVal ] && echo "Yop" || echo "Nop")
Or
(( numVar == numVal ? (resVar=1) : (resVar=0) ))
