Linux “$”是什么意思？给我们一个shell脚本？
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What does "$?" give us exactly in a shell script?
提问by Khushman Patel
I saw the code written somewhere online, and I wanted to know what exactly does "$?" do/give us. Googling did not help.
我看到网上某处写的代码，我想知道“$”到底是什么？做/给我们。谷歌搜索没有帮助。
Here's the code I saw it in:
这是我在其中看到的代码：
#!/bin/sh
ping -c 2 localhost
if [ $? != 0 ] ; then
echo "Couldn't ping localhost, weird"
fi
ping -c 2 veryweirdhostname.noend
if [ $? != 0 ] ; then
echo "Surprise, Couldn't ping a very weird hostname.."
fi
echo "The pid of this process is $$"
Taken from: http://efod.se/writings/linuxbook/html/shell-scripts.html
取自：http: //efod.se/writings/linuxbook/html/shell-scripts.html
采纳答案by Brendan Long
$?is a variable holding the return value of the last command you ran.
$?是一个变量，保存您运行的最后一个命令的返回值。
Example C program (example.c):
示例 C 程序 ( example.c)：
int main() { return 1; }
Example Bash:
示例 Bash：
gcc -o example example.c
./example
echo $? # prints 1
回答by evil otto
It's the return code from the most recently executed command.
它是最近执行的命令的返回码。
By convention 0 is a successful exit and non-zero indicates some kind of error.
按照惯例，0 是成功退出，非零表示某种错误。
回答by Brendan Long
This special variable shows the exit status of the last command that was run in a script or command-line. For example, in a command-line, the user could type
此特殊变量显示在脚本或命令行中运行的最后一个命令的退出状态。例如，在命令行中，用户可以键入
who; echo $?
The output would then be
然后输出将是
user tty7 2014-07-13 19:47
0
This shows the output of whoand the exit status of the command. A script would be the same.
这显示了who的输出和命令的退出状态。脚本将是相同的。
#!/bin/bash
who
echo $?
Output: 0
输出：0
回答by Jens
Most of the answers are missing a bit of detail. A definitive answer is found in the POSIX standard for the shell, in the section on special parameters:
大多数答案都缺少一些细节。在 shell的POSIX 标准中，在有关特殊参数的部分中找到了明确的答案：
$? Expands to the decimal exit status of the most recent pipeline (see Pipelines ).
$? 扩展到最近管道的十进制退出状态（请参阅管道）。
Don't be surprised by the word pipeline, because even a simple command such as lsis grammatically a pipeline consisting of a single command. But then, what is $?for a multi-command pipeline? It's the exit status of the lastcommand in the pipeline.
不要对管道这个词感到惊讶，因为即使是一个简单的命令，例如在ls语法上也是一个由单个命令组成的管道。但是，$?多命令管道是什么？它是管道中最后一个命令的退出状态。
And what about pipelines executing in the background, like grep foo bigfile|head -n 10 > result &?
那么在后台执行的管道grep foo bigfile|head -n 10 > result &呢，比如？
Their exit status can be retrieved through waitonce the pipeline's last command has finished.
The background process pid is available as $!, and $?only reports whether the background command was correctly started.
wait一旦管道的最后一个命令完成，就可以检索它们的退出状态。后台进程 pid 可用$!，$?仅报告后台命令是否正确启动。
Another detail worth mentioning is that the exit status is usually in the range 0 through 255, with 128 to 255 indicating the process exited due to a signal. Returning other values from a C program is likely to not be reflected accurately in $?.
另一个值得一提的细节是退出状态通常在 0 到 255 的范围内，128 到 255 表示进程因信号退出。从 C 程序返回其他值可能不会在$?.
回答by craq
the other answers cover bash pretty well, but you don't specify a shell in your question. In csh (and tcsh) $?can be used to query the existence of variables, e.g.
其他答案很好地涵盖了 bash，但您没有在问题中指定 shell。在csh（和tcsh）$?中可以用来查询变量是否存在，例如
if $?my_var then
echo my_var exists
endif
