bash 关键字“if”如何测试一个值是真还是假？
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/3924182/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
How does the keyword “if” test if a value is true or false?
提问by kit.yang
In bash script
在 bash 脚本中
if [ 1 ]
then
echo "Yes"
else
echo "No"
fi
Output: Yes
输出： Yes
It represents that '1' is treated as true value.
它表示“1”被视为真值。
But in code:
但在代码中：
word = Linux
letter = nuxi
if echo "$word" | grep -q "$letter"
then
echo "Yes"
else
echo "No"
fi
Output: No
输出： No
But echo "$word" | grep -q "$letter"will return 1, so why is the result is No.
但是echo "$word" | grep -q "$letter"会返回 1，那么为什么结果是No.
How does the keyword iftest the value returned by the command after if?
关键字如何if测试后命令返回的值if？
回答by Lekensteyn
The return value of a command is checked. [ 1 ]has a return value of 0(true). Any other return value (like 1) indicates an error.
检查命令的返回值。[ 1 ]返回值为0(true)。任何其他返回值（如1）表示错误。
You can display the return value of the last executed command using the $?variable:
您可以使用$?变量显示上次执行命令的返回值：
true
echo $?
# returned 0
false
echo $?
# returned 1
echo $?
# returned 0 as the last executed command is 'echo', and not 'false'
回答by Nick
In unix land, 0 is true and 1 is false.
在 unix 领域，0 为真，1 为假。
For your first example:
对于您的第一个示例：
if [ 1 ]
then
echo "Yes"
else
echo "No"
fi
"If" checks the exit code of the given command for true/false (i.e. zero/non-zero).
“If”检查给定命令的退出代码是否为真/假（即零/非零）。
The square brackets actually invoke the "test" command (see "man test" for more information) and give the exit code to if.
方括号实际上调用了“test”命令（有关更多信息，请参阅“man test”）并将退出代码提供给 if。
"test 1" (or indeed "test any_string") returns true (0) so "Yes" is output.
“test 1”（或者实际上是“test any_string”）返回真（0）所以输出“Yes”。
For your second example, this outputs "No" because "nuxi" isn't found in "Linux", if you change "nuxi" to "nux" (perhaps this was a typo?) and remove the spaces around the = then you will get the behaviour you expect. e.g.
对于您的第二个示例，这将输出“否”，因为在“Linux”中找不到“nuxi”，如果您将“nuxi”更改为“nux”（也许这是一个错字？）并删除 = 周围的空格，那么您将获得您期望的行为。例如
word=Linux
letter=nux
if echo "$word" | grep -q "$letter"
then
echo "Yes"
else
echo "No"
fi
回答by Ibrahim
This is because the grep failed to find the $letter in $word, hence the exit code is 1. Whenever a process in linux return a code other than 0 then it means it failed. 0 means exited successfully. You can verify this by echo "Linux" | grep -d "nuxi"; echo $?
这是因为 grep 未能在 $word 中找到 $letter，因此退出代码为 1。每当 linux 中的进程返回 0 以外的代码时，这意味着它失败了。0 表示成功退出。您可以通过以下方式验证echo "Linux" | grep -d "nuxi"; echo $?
On the other hand in scripting world 0 means false and 1 mean true. So the grep failed to find the word and send 1 as an exit code to if, which took it as a true value.
另一方面，在脚本世界中，0 表示假，1 表示真。因此 grep 未能找到该单词并将 1 作为退出代码发送给 if，后者将其作为真值。
