在 bash shell 脚本中使用复合条件
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using compound conditions in bash shell script
提问by Sanjan Grero
I want to do something like below in bash script. how do i implement in bash syntax.
我想在 bash 脚本中做类似下面的事情。我如何在 bash 语法中实现。
if !((a==b) && (a==c))
then
do something
end if
回答by Paused until further notice.
For numeric comparison, you can do:
对于数字比较，您可以执行以下操作：
if ! (( (a == b) && (a == c) ))
For string comparison:
对于字符串比较：
if ! [[ "$a" == "$b" && "$a" == "$c" ]]
In Bash, the double parentheses set up an arithmetic context (in which dollar signs are mostly optional, by the way) for a comparison (also used in for ((i=0; i<=10; i++))and $(())arithmetic expansion) and is used to distinguish the sequence from a set of single parentheses which creates a subshell.
在bash，双括号设置算术上下文（其中美元符号大多是可选的，通过的方式）用于比较（在也使用for ((i=0; i<=10; i++))和$(())算术膨胀）和用于将序列从一组单括号其产生分辨一个子外壳。
This, for example, executes the command trueand, since it's always true it does the action:
例如，这将执行命令，true并且由于它始终为真，因此执行以下操作：
if (true); then echo hi; fi
This is the same as
这与
if true; then echo hi; fi
except that a subshell is created. However, if ((true))tests the value of a variable named "true".
除了创建了一个子shell。但是，if ((true))测试名为“true”的变量的值。
If you were to include a dollar sign, then "$true" would unambiguously be a variable, but the ifbehavior with single parentheses (or without parentheses) would change.
如果您要包含一个美元符号，那么“$true”将毫无if疑问地成为一个变量，但带有单括号（或不带括号）的行为会发生变化。
if ($true)
or
或者
if $true
would execute the contents of the variable as a command and execute the conditional action based on the command's exit value (or give a "command not found" message if the contents aren't a valid command).
将变量的内容作为命令执行，并根据命令的退出值执行条件操作（如果内容不是有效命令，则给出“未找到命令”消息）。
if (($true))
does the same thing as if ((true))as described above.
做与上述相同的事情if ((true))。
回答by wilhelmtell
if [ "$a" != "$b" -o "$a" != "$c" ]; then
# ...
else
# ...
fi
回答by SiegeX
#!/bin/bash
a=2
b=3
c=4
if ! (( (a == b) && (a == c) )); then
# stuff here
fi
You could also use the following which I personally find more clear:
您还可以使用以下我个人觉得更清楚的内容：
#!/bin/bash
a=2
b=3
c=4
if (( (a != b) || (a != c) )); then
# stuff here
fi
Technically speaking you don't need the parens around the sub expressions since the equality operators == !=have higher precedence then both the compound comparison operators && ||but I think it's wise to keep them in there to show intent if nothing else.
从技术上讲，您不需要子表达式周围的括号，因为相等运算符的== !=优先级高于复合比较运算符，&& ||但我认为将它们保留在那里以显示意图是明智的。
