bash 将“测试”的结果分配给变量

Assigning the result of 'test' to a variable

提问by Barton Chittenden

I'm writing a script where I need to use the output of a file test in several places, including inside a shell function. I would like to assign the file existence to a shell variable, like this: file_exists=[ -f $myfile ].

我正在编写一个脚本，我需要在几个地方使用文件测试的输出，包括在 shell 函数内部。我想该文件存在分配给一个shell变量，就像这样：file_exists=[ -f $myfile ]。

Just to make sure that I've got my bases covered, I start by touching a file, and testing its existance:

只是为了确保我已经涵盖了我的基础，我从触摸一个文件开始，并测试它的存在：

file='a'
touch $file
if [ -f $file ]
then
    echo "1 -- '$file' exists"
fi

Output:

输出：

1 -- 'a' exists

The file was created successfully -- no surprises, but at least I know that I'm not dealing with any permissions issues or anything.

该文件已成功创建 - 毫不奇怪，但至少我知道我没有处理任何权限问题或任何事情。

Next I test to make sure that I can store a boolean expression in a variable:

接下来我测试以确保我可以在变量中存储布尔表达式：

mytest=/bin/true

if $mytest
then
    echo "2 -- $mytest is true"
fi

Output:

输出：

2 -- $mytest is true

So I've got the basics covered -- conditional expressions should emit the same output as /bin/trueor /bin/false... but that's not what I'm seeing:

所以我已经涵盖了基础知识——条件表达式应该发出与/bin/trueor相同的输出/bin/false……但这不是我所看到的：

mytest=[ -f $file ]
if $mytest
then
    echo "3 -- $mytest is true [expect true]"
else
    echo "3 -- $mytest is false [expect true]"
fi

This fails with the following error:

这失败并出现以下错误：

-f: command not found

I get the same error message if i use test -f $filerather than [ -f $file ].

如果我使用test -f $file而不是[ -f $file ].

If I put a space in front of the [, the error goes away...

如果我在 前面放一个空格[，错误就会消失......

mytest= [ -f $file ]
if $mytest
then
    echo "4 -- $mytest is true [expect true]"
else
    echo "4 -- $mytest is false [expect true]"
fi

The output appears to be correct:

输出似乎是正确的：

4 -- $mytest is true [expect true]

... but if I remove the file, I should get the opposite result:

...但如果我删除文件，我应该得到相反的结果：

rm $file
mytest= [ -f $file ]
if $mytest
then
    echo "5 -- $mytest is true [expect false]"
else
    echo "5 -- $mytest is false [expect false]"
fi

... and I don't:

......我没有：

5 -- $mytest is true [expect false]

To be fair, I expected the space to mess with the truth value:

公平地说，我预计这个空间会混淆真值：

mytest= /bin/false
if $mytest
then
    echo "6 -- $mytest is true [expect false]"
else
    echo "6 -- $mytest is false [expect false]"
fi

Outputs:

输出：

6 -- $mytest is true [expect false]

So, how do I store the output from the testbuiltin in a shell variable?

那么，如何将test内置函数的输出存储在 shell 变量中？

回答by Charles Duffy

As others have documented here, using the string "true" is a red herring; this is notan appropriate way to store boolean values in shell scripts, as evaluating it means dynamically invoking a command rather than simply inspecting the stored value using shell builtins hardcoded in your script.

正如其他人在此处记录的那样，使用字符串“true”是一个红鲱鱼；这不是在 shell 脚本中存储布尔值的合适方法，因为评估它意味着动态调用命令，而不是简单地使用脚本中硬编码的 shell 内置程序检查存储的值。

Instead, if you really must store an exit status, do so as a numeric value:

相反，如果您确实必须存储退出状态，请将其存储为数值：

[ -f "$file" ]               # run the test
result=$?                    # store the result

if (( result == 0 )); then   # 0 is success
  echo "success"
else                         # nonzero is failure
  echo "failure"
fi

回答by tripleee

You need to quote whitespace:

您需要引用空格：

mytest='[ -f $file ]'
if $mytest; then echo yes; fi

However, this is extremely brittle and potentially insecure. See http://mywiki.wooledge.org/BashFAQ/050for a detailed discussion and some better ways to accomplish something similar.

然而，这是非常脆弱的并且可能不安全。请参阅http://mywiki.wooledge.org/BashFAQ/050以获得详细的讨论和一些更好的方法来完成类似的事情。

If you want to encapsulate a complex piece of code, a function is usually the way to go:

如果要封装一段复杂的代码，通常可以使用函数：

mytest () { [ -f "$file" ]; }
if mytest; then echo yes; fi

If you want to run the code once and store its result so you can examine it later, I would rephrase it like this:

如果你想运行一次代码并存储它的结果以便你以后检查它，我会像这样改写它：

if [ -f "$file" ]; then
    mytest=true
else
    mytest=false
fi
if $mytest; then echo yes; fi

回答by Etan Reisner

mytest=/bin/trueis storing the string/bin/truein the $mytestvariable.

mytest=/bin/true将字符串存储/bin/true在$mytest变量中。

mytest=[ -f $file ]is setting the $mytestvariable to the value [for the duration of the command -f $file ](which as your output indicates fails as there is no -fcommand available).

mytest=[ -f $file ]正在将$mytest变量设置[为命令持续时间的值-f $file ]（如您的输出所示，由于没有-f可用命令而失败）。

mytest= [ -f $file ](like the above) sets the value of the $mytestvariable to blank for the duration of the [ -f $file ]command (and returns whatever [returns).

mytest= [ -f $file ]（像上面一样）$mytest在[ -f $file ]命令的持续时间内将变量的值设置为空白（并返回任何[返回值）。

mytest= /bin/falsethis is the same as the above case only the command being run is /bin/false.

mytest= /bin/false这与上述情况相同，只是正在运行的命令是/bin/false.

If you want to store the return code from a command in a variable you can do

如果要将命令的返回代码存储在变量中，则可以执行

/bin/true
ret=$?

if you want to store the output from a command in a variable you can do

如果要将命令的输出存储在变量中，则可以执行

out=$(/bin/true)

(though with /bin/truethat variable will be empty as it outputs no text.

（尽管/bin/true该变量将为空，因为它不输出文本。

For your case you want the former $?model.

对于您的情况，您需要以前的$?模型。

Also, using set -x(and/or set -v) in your scripts might have helped you diagnose this.

此外，在脚本中使用set -x(和/或set -v) 可能有助于您对此进行诊断。

回答by Jacques

A old one but left this here for reference for people that might need it. Not the most beautiful solution but it works in bash:

一个旧的，但留在这里供可能需要它的人参考。不是最漂亮的解决方案，但它适用于 bash：

mytest=$( [ -f $file ] ; echo $? )

More portable, using the test command, and the backticks:

更便携，使用 test 命令和反引号：

set mytest=`test -f $file ; echo $?`

In a subprocess (<!> system load), the condition is evaluated, and then the result echoed to the output that is captured by the variable $mytest.

在子流程（<!> 系统负载）中，评估条件，然后将结果回显到变量 $mytest 捕获的输出中。

回答by Václav Ku?el

Different version with test command.

带有测试命令的不同版本。

fileExists=$(test -f /path/to/file && echo true || echo false)
if [[ ${fileExists} == "true" ]]; then
    # Your code here
fi

Or even more simplier version.

或者更简单的版本。

fileExists=$(test -f /path/to/file && echo 1)
if [[ -z ${fileExists} ]]; then
    # Your code here
fi