“$？”是什么意思？给我们确切的shell脚本吗？
我在网上看到了代码，我想知道“$？”到底是什么意思？做/给我们。 谷歌搜索没有帮助。
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 $$"
I saw the code written somewhere online, and I wanted to know what exactly does "$?" do/give us.
Googling did not help.
Here's the code I saw it in:
#!/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
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评论（5）
$?是一个变量，保存您运行的最后一个命令的返回值。C 程序示例 (
example.c)：Bash 示例：
$?is a variable holding the return value of the last command you ran.Example C program (
example.c):Example Bash:
大多数答案都缺少一些细节。明确的答案可以在 shell 的 POSIX 标准中有关特殊参数的部分中找到：
不要对管道这个词感到惊讶，因为即使是像 ls 这样的简单命令在语法上也是由单个命令组成的管道。但是，对于多命令管道来说，
$?是什么？它是管道中最后命令的退出状态。那么在后台执行的管道又如何呢，比如 grep foo bigfile|head -n 10 >结果&？
一旦管道的最后一个命令完成，就可以通过
wait检索它们的退出状态。后台进程 pid 以
$!形式提供，而$?仅报告后台命令是否正确启动。另一个值得一提的细节是退出状态通常在0到255范围内，其中128到255表示进程由于信号而退出。从 C 程序返回其他值可能无法在
$?中准确反映。Most of the answers are missing a bit of detail. A definitive answer is found in the POSIX standard for the shell, in the section on special parameters:
Don't be surprised by the word pipeline, because even a simple command such as
lsis grammatically a pipeline consisting of a single command. But then, what is$?for a multi-command pipeline? It's the exit status of the last command in the pipeline.And what about pipelines executing in the background, like
grep foo bigfile|head -n 10 > result &?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.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
$?.它是最近执行的命令的返回代码。
按照约定，0 表示成功退出，非零表示出现某种错误。
It's the return code from the most recently executed command.
By convention 0 is a successful exit and non-zero indicates some kind of error.
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
输出将为
这显示 who 的输出以及命令的退出状态。脚本也是一样的。
输出：0
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
The output would then be
This shows the output of who and the exit status of the command. A script would be the same.
Output: 0
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
$?可用于 查询变量是否存在，egthe 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.