blocks|key|1545617|text|$$是当前进程的id。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1545618|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

$$ is the id of the current process.

blocks|key|1760516|text|$$是当前外壳进程的pid。这不是生成唯一文件名的好方法。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1760517|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

$$ is the pid of the current shell process. It isn't a good way to generate unique filenames.

blocks|key|1760544|text|在Bash中，$$是进程ID，在注释中指出，由于各种原因，使用它作为临时文件名是不安全的。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1760545|对于临时文件名，请使用mktemp命令。|1760546|entityMap^0|7|2|0|B|6|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|M|8|@$9|N|A|O|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|P|8|@]|D|@]|E|$]]]|I|$]]

In Bash <code>$$</code> is the process ID, as noted in the comments it is not safe to use as a temp filename for a variety of reasons.

For temporary file names, use the <code>mktemp</code> command.

blocks|key|1760594|text|它是bash进程的进程ID。任何并发进程都不会有相同的PID。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1760595|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

It's the process ID of the bash process. No concurrent processes will ever have the same PID.

blocks|key|1545779|text|$$是运行脚本的shell的进程id。有关更多详细信息，请参阅sh或bash的手册页。可以使用命令行"man+sh“或在web上搜索"shell+manpage”来找到手册页。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1545780|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

The $$ is the process id of the shell in which your script is running. For more details, see the man page for sh or bash. The man pages can be found be either using a command line "man sh", or by searching the web for "shell manpage"

blocks|key|1760707|text|$$是您的PID。它不会真正生成一个唯一的文件名，除非你很小心，没有其他人用完全相同的方式来做。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1760708|通常，您会创建类似于/tmp/myprogramname$$的内容|1760709|有很多方法可以打破这一点，如果您正在写入其他人可以写入的位置，那么在许多OSes上预测您将拥有的PID并不是很困难--假设您是以根用户身份运行的，我创建了/tmp/yourprocname13395作为指向/etc/passwd的符号链接--然后写入其中。|1760710|这是在shell脚本中做的一件坏事。如果你打算使用临时文件来做什么，你应该使用一种更好的语言，它至少可以让你在打开(创建)文件时添加“独占”标志。然后你就可以确定你不是在打击其他东西。|1760711|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|J|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|K|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|L|8|@]|9|@]|A|$]]|$1|F|3|G|5|6|7|M|8|@]|9|@]|A|$]]|$1|H|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|I|$]]

$$ is your PID. It doesn't really generate a unique filename, unless you are careful and no one else does it exactly the same way. 

Typically you'd create something like /tmp/myprogramname$$

There're so many ways to break this, and if you're writing to locations other folks can write to it's not too difficult on many OSes to predict what PID you're going to have and screw around -- imagine you're running as root and I create /tmp/yourprogname13395 as a symlink pointing to /etc/passwd -- and you write into it.

This is a bad thing to be doing in a shell script. If you're going to use a temporary file for something, you ought to be using a better language which will at least let you add the "exclusive" flag for opening (creating) the file. Then you can be sure you're not clobbering something else.

blocks|key|1545890|text|$$是运行脚本的shell解释器的pid+(进程id)。目前在系统上运行的每个进程都是不同的，但随着时间的推移，pid会卷土重来，在您退出后，将会有另一个进程具有相同的pid+eventually.As，只要您正在运行，pid对您来说是唯一的。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1545891|从上面的定义可以明显看出，无论您在脚本中使用$$多少次，它都将返回相同的数字。|1545892|您可以使用，例如/tmp/myscript.scatch.$$作为临时文件，用于不需要非常可靠或安全的内容。在脚本末尾删除此类临时文件是一种很好的做法，例如，使用陷阱命令：|1545893|trap+"echo+'Cleanup+in+progress';+rm+-r+$TMP_DIR"+EXIT|code-block|syntax|javascript|1545894|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|N|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|O|8|@]|9|@]|A|$]]|$1|F|3|G|5|H|7|P|8|@]|9|@]|A|$I|J]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

$$ is the pid (process id) of the shell interpreter running your script. It's different for each process running on a system at the moment, but over time the pid wraps around, and after you exit there will be another process with same pid eventually.As long as you're running, the pid is unique to you.

From the definition above it should be obvious that no matter how many times you use $$ in a script, it will return the same number. 

You can use, e.g. /tmp/myscript.scratch.$$ as your temp file for things that need not be extremely reliable or secure. It's a good practice to delete such temp files at the end of your script, using, for example, trap command:

<pre><code>trap "echo 'Cleanup in progress'; rm -r $TMP_DIR" EXIT
</code></pre>

blocks|key|1545943|text|$$是bash中的进程ID+(PID)。使用$$不是一个好主意，因为它通常会造成竞争条件，并允许攻击者破坏您的shell脚本。例如，查看创建不安全临时文件并必须发布安全建议的所有these+people。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1545944|相反，请使用mktemp。Linux+man+page+for+mktemp非常棒。下面是其中的一些示例代码：|1545945|tempfoo=`basename+$0`
TMPFILE=`mktemp+-t+${tempfoo}`+%7C%7C+exit+1
echo+"program+output"+>>+$TMPFILE|code-block|syntax|javascript|1545946|entityMap|0|LINK|mutability|MUTABLE|url|http://www.google.com/search?q=tmp%2Brace|1|http://linux.die.net/man/1/mktemp^0|0|2|M|2|2H|C|0|0|6|6|D|P|1|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@$9|X|A|Y|B|C]|$9|Z|A|10|B|C]]|D|@$9|11|A|12|1|13]]|E|$]]|$1|F|3|G|5|6|7|14|8|@$9|15|A|16|B|C]]|D|@$9|17|A|18|1|19]]|E|$]]|$1|H|3|I|5|J|7|1A|8|@]|D|@]|E|$K|L]]|$1|M|3|-4|5|6|7|1B|8|@]|D|@]|E|$]]]|N|$O|$5|P|Q|R|E|$S|T]]|U|$5|P|Q|R|E|$S|V]]]]

<code>$$</code> is the process ID (PID) in bash. Using <code>$$</code> is a bad idea, because it will usually create a race condition, and allow your shell-script to be subverted by an attacker. See, for example, all <a href="http://www.google.com/search?q=tmp+race" rel="noreferrer">these people</a> who created insecure temporary files and had to issue security advisories.

Instead, use <code>mktemp</code>. The <a href="http://linux.die.net/man/1/mktemp" rel="noreferrer">Linux man page for mktemp</a> is excellent. Here's some example code from it:

<pre><code>tempfoo=`basename $0`
TMPFILE=`mktemp -t ${tempfoo}` || exit 1
echo "program output" &gt;&gt; $TMPFILE
</code></pre>

I once read that one way to obtain a unique filename in a shell for temp files was to use a double dollar sign (<code>$$</code>). This does produce a number that varies from time to time... but if you call it repeatedly, it returns the same number. (The solution is to just use the time.)

I am curious to know what <code>$$</code> actually is, and why it would be suggested as a way to generate unique filenames.

What does $$ mean in the shell?

我曾经读过，在shell中获取临时文件的唯一文件名的一种方法是使用双美元符号($$)。这确实会产生一个随时间变化的数字...但如果您重复调用它，它将返回相同的数字。(解决方案就是利用时间。)我很想知道$$到底是什么，以及为什么建议使用它来生成唯一的文件名。

问$$在外壳中是什么意思？
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