如何在 bash 中构建条件赋值？

Question

提问by Neuquino

I'm looking a way to build conditional assignments in bash:

我正在寻找一种在 bash 中构建条件赋值的方法：

In Java it looks like this:

在 Java 中，它看起来像这样：

int variable= (condition) ? 1 : 0;

Answer 1

回答by Demosthenex

If you want a way to define defaults in a shell script, use code like this:

如果您想要一种在 shell 脚本中定义默认值的方法，请使用如下代码：

: ${VAR:="default"}

Yes, the line begins with ':'. I use this in shell scripts so I can override variables in ENV, or use the default.

是的，该行以“:”开头。我在 shell 脚本中使用它，所以我可以覆盖 ENV 中的变量，或者使用默认值。

This is related because this is my most common use case for that kind of logic. ;]

这是相关的，因为这是我对这种逻辑最常见的用例。;]

Answer 2

回答by Anthony Forloney

As per Jonathan's comment:

根据乔纳森的评论：

variable=$(( 1 == 1 ? 1 : 0 ))

EDIT:

编辑：

I revised the original answer which just echo'd the value of the condition operator, it didn't actually show any assignment.

我修改了原始答案，它只是echo'd'条件运算符的值，它实际上没有显示任何赋值。

Answer 3

回答by Kevin Little

myvar="default" && [[ <some_condition_is_true> ]]  && myvar="non-default"

real examples:

真实例子：

DELIM="" && [[ "$APP_ENV_RESOLVED" != "" ]] && DELIM=$INNER_DELIM

The condition can be "(( ... ))" as well:

条件也可以是“(( ... ))”：

filepath=/proc/drbd && (( $# > 0 )) && filepath=

Answer 4

回答by Martin Rüegg

In addition to the other more general answers (particularly as per Jonathan's commentand Kevin's more general answer[which also supports strings]) I'd like to add the following two solutions:

除了其他更一般的答案（特别是根据乔纳森的评论和凯文更一般的答案[也支持字符串]），我想添加以下两个解决方案：

setting the variable to either `0`or `1`based on the condition:

将变量设置为`0`或`1`基于条件：

(as the question's example suggests.)

（正如问题的例子所暗示的那样。）

The general form would read

一般形式将阅读

(condition); variable=$?;

where $variableresults in being either 0or 1and conditioncan be any valid conditional expression.

where$variable结果是0or1和condition可以是任何有效的条件表达式。

E.g. checking a variable ...

例如检查一个变量...

[[ $variableToCheck == "$othervariable, string or number to match" ]]
variable=$?

... or checking a file's existence ...

... 或检查文件是否存在 ...

[ -f "$filepath" ]
fileExists=$?

... or checking the nummerical value of $myNumber:

...或检查的数值$myNumber：

(( myNumber >= 1000000000 ))
is_huge_number=$?

The advantages of this solution is that

这种解决方案的优点是

it supports arbitrary conditional expressions, including strings
(which are not supported in arithmetic expressionsof Jonathan's solution)
that variablegets declaredin any case, unlike in griffon's answer:
[ -z "$variable" ] && variable="defaultValue"
Which would matter in case you want to namerefit later on (e.g. from within a function).

它支持任意条件表达式，包括字符串
（乔纳森解决方案的算术表达式不支持）
这在任何情况下variable都会被声明，与 griffon 的回答不同：
[ -z "$variable" ] && variable="defaultValue"
如果您稍后想对其进行命名（例如从函数内），这将很重要。

Please note:In Bash, the special variable $?always contains the exit code of the previously executed statement (or statement block; see the man bashfor more details). As such, a positive result is generally represented by the value 0, not 1(See my comment below, thanks Assimilater for pointing it out). Thus, if the condition is true (e.g [[2 eq 2]]) then $?=0.

请注意：在 Bash 中，特殊变量$?始终包含先前执行的语句（或语句块；有关详细信息，请参阅man bash）的退出代码。因此，积极的结果通常由 value 表示0，而不是1（请参阅下面我的评论，感谢 Assimilater指出）。因此，如果条件为真（例如[[2 eq 2]]），则$?=0。

If instead you need 0 or 1 in your variable (e.g. to print or do mathematical calculations) then you need to employ boolean negation using a leading exclamation mark (as pointed out by GypsySpellweaver in the commentsbelow): ( ! condition ); variable=$?or ! ( condition ); variable=$?. (However, readability in terms of what happens might be a bit less obvious.)

相反，如果您需要在变量中使用 0 或 1（例如打印或进行数学计算），那么您需要使用带有前导感叹号的布尔否定（正如 GypsySpellweaver 在下面的评论中指出的那样）：( ! condition ); variable=$?或! ( condition ); variable=$?. （但是，就发生的事情而言，可读性可能不太明显。）

Another possible solutionby Jonathan would be variable=$(( 1 == 1 ? 1 : 0 ))- which, however, is creating a subshell.

乔纳森的另一个可能的解决方案是variable=$(( 1 == 1 ? 1 : 0 ))- 然而，它正在创建一个子外壳。

If you want to avoid the creation of a subshel, keep good readability or have arbitrary conditions, use one of the following solutions.

如果您想避免创建 subshel，保持良好的可读性或具有任意条件，请使用以下解决方案之一。

setting the variable to arbitrary values:

将变量设置为任意值：

as it is done in most other answers, it could adapted as follows:

正如在大多数其他答案中所做的那样，它可以调整如下：

(condition) \
    && variable=true \
    || variable=false

e.g as in

例如在

[[ $variableToCheck == "$othervariable, string or number to match" ]] \
    && variable="$valueIfTrue" \
    || variable="$valueIfFalse"

or to get 1 in a positive check, and 0 upon failure (like in the question's example):

或在正面检查中获得 1，在失败时获得 0（如问题示例中所示）：

[[ $variableToCheck == "$othervariable, string or number to match" ]] \
    && variable=1 \
    || variable=0

(for the last example, - as already mentioned in the notes above - the same can be achieved with boolean negation using a leading exclamation mark:

（对于最后一个示例， - 正如上面的注释中已经提到的 - 可以通过使用前导感叹号的布尔否定来实现相同的效果：

[[ ! $variableToCheck == "$othervariable, string or number to match" ]]
variable=$?

The advantages of this solution is that

这种解决方案的优点是

it might be considered a bit better readablethan Kevin's answer
myvar="default" && [[ <some_condition_is_true> ]] && myvar="non-default", and
the $valueIfTrueis conditionally evaluated onlyif needed,
which would matter in case you'd do something
- with side-effect, like
  - variable=$((i++)), or
  - { variable=$1; shift; }
- high computation, like
  - variable=$(find / -type f -name ListOfFilesWithThisNameOnMySystem)
is a bit shorterthan ghostdog74's answer
(which, however is great if you have multiple conditions!)
does not open a subshell as in Pierre's answer
and as above:
- it supports arbitrary conditional expressions, including strings
  (which are not supported in arithmetic expressionsof Jonathan's solution)
- that variablegets declaredin any case, unlike in griffon's answer:
  [ -z "$variable" ] && variable="defaultValue"
  Which would matter in case you want to namerefit later on (e.g. from within a function).

它可能被认为比凯文的答案更具可读性，并且
myvar="default" && [[ <some_condition_is_true> ]] && myvar="non-default"
在$valueIfTrue有条件评估仅在需要时，
它会在你做一些无关紧要的情况下
- 有副作用，比如
  - variable=$((i++))，或者
  - { variable=$1; shift; }
- 高计算，如
  - variable=$(find / -type f -name ListOfFilesWithThisNameOnMySystem)
有点短比ghostdog74的答案
（这不过是伟大的，如果你有多个条件！）
不像皮埃尔的回答那样打开子shell
和上面一样：
- 它支持任意条件表达式，包括字符串
  （乔纳森解决方案的算术表达式不支持）
- 这在任何情况下variable都会被声明，与 griffon 的回答不同：
  [ -z "$variable" ] && variable="defaultValue"
  如果您稍后想对其进行命名（例如从函数内），这将很重要。

Answer 5

回答by Pierre Maoui

I wanted to do a conditional assignment with strings and I ended up with :

我想用字符串进行条件赋值，结果是：

SOWHAT=$([ "$MYVALUE" = "value" ] && echo "YES" || echo "NO")

Answer 6

回答by Alex Gray

Big upsto @Demosthenex and especially @Dennis Williamson for the shortest and easiestsolution I've seen. Leave it to bashto require a bunch of parenthesesfor a simple ternary assignment. Ahh, the 60s! And to put it all together in an example...

大起大落对@Demosthenex，尤其是@Dennis威廉姆森的最短和最简单的我见过的解决方案。将它留给 bash以需要一堆括号来进行简单的三元赋值。啊，60年代！并把它们放在一个例子中......

echo $BASHRULES;             # not defined
                             # no output
: ${BASHRULES:="SCHOOL"}     # assign the variable
echo $BASHRULES              # check it
SCHOOL                       # correct answer
: ${BASHRULES="FOREVER?"}    # notice the slightly different syntax for the conditional assignment
echo $BASHRULES              # let's see what happened!
SCHOOL                       # unchanged! (it was already defined)

I wrote that a long time ago.. these days I'd probably get more excited over a solution like...

我很久以前写过……这些天我可能会对像……这样的解决方案感到更加兴奋。

PLATFORM=iphonesimulator
OTHERSDK=iphone && [[ $PLATFORM=~os ]]      \
                &&     OTHERSDK+=simulator  \
                ||     OTHERSDK+=os

$OTHERSDK? iphoneos

Answer 7

回答by griffon vulture

If you want to assign a value unless variable is empty use this:

如果您想分配一个值，除非变量为空，请使用以下命令：

[ -z "$variable" ] && variable="defaultValue"

You can put as well, each other condition on the []

你也可以把对方的条件放在 []

Answer 8

回答by ghostdog74

another way

其它的办法

case "$variable" in
  condition ) result=1;;
  *) result=0;;
esac

如何在 bash 中构建条件赋值？

提问by Neuquino

回答by Demosthenex

回答by Anthony Forloney

EDIT:

编辑：

回答by Kevin Little

回答by Martin Rüegg

setting the variable to either `0`or `1`based on the condition:

将变量设置为`0`或`1`基于条件：

The advantages of this solution is that

这种解决方案的优点是

setting the variable to arbitrary values:

将变量设置为任意值：

The advantages of this solution is that

这种解决方案的优点是

回答by Pierre Maoui

回答by Alex Gray

回答by griffon vulture

回答by ghostdog74

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最近更新

标签

如何在 bash 中构建条件赋值？

提问by Neuquino

回答by Demosthenex

回答by Anthony Forloney

EDIT:

编辑：

回答by Kevin Little

回答by Martin Rüegg

setting the variable to either 0or 1based on the condition:

将变量设置为0或1基于条件：

The advantages of this solution is that

这种解决方案的优点是

setting the variable to arbitrary values:

将变量设置为任意值：

The advantages of this solution is that

这种解决方案的优点是

回答by Pierre Maoui

回答by Alex Gray

回答by griffon vulture

回答by ghostdog74

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最近更新

标签

setting the variable to either `0`or `1`based on the condition:

将变量设置为`0`或`1`基于条件：