blocks|key|4448174|text|我认为，您错过了/bin/true+Returs0和/bin/false返回1|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4448175|回显$+r='res:'；{/+|4448176|+/true；r%2B=$?；}+2>/dev/null；回显$r；|blockquote|4448177|4448178|分辨率:0|4448179|和|4448180|回显$+r='res:'；{/bin/+|4448181|；r%2B=$?；}+2>/dev/null；回显$r；|4448182|4448183|分辨率:1|4448184|entityMap^0|0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|V|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|W|8|@]|9|@]|A|$]]|$1|G|3|-4|5|6|7|X|8|@]|9|@]|A|$]]|$1|H|3|I|5|6|7|Y|8|@]|9|@]|A|$]]|$1|J|3|K|5|6|7|Z|8|@]|9|@]|A|$]]|$1|L|3|M|5|6|7|10|8|@]|9|@]|A|$]]|$1|N|3|O|5|F|7|11|8|@]|9|@]|A|$]]|$1|P|3|-4|5|6|7|12|8|@]|9|@]|A|$]]|$1|Q|3|R|5|6|7|13|8|@]|9|@]|A|$]]|$1|S|3|-4|5|6|7|14|8|@]|9|@]|A|$]]]|T|$]]

I think, you miss that /bin/true returs 0 and /bin/false returns 1

<blockquote>
 <blockquote>
 $ r='res:'; { /bin/true; r+=$?; } 2>/dev/null; echo $r;
 
 res:0
 </blockquote>
</blockquote>

And

<blockquote>
 <blockquote>
 $ r='res:'; { /bin/false; r+=$?; } 2>/dev/null; echo $r;
 
 res:1
 </blockquote>
</blockquote>

blocks|key|2980627|text|我尝试了一个测试程序：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2980628|x=0
{+x=$$+;+echo+"$$+$BASHPID+$x"+;+}
echo+$x

x=0
{+x=$$+;+echo+"$$+$BASHPID+$x"+;+}+%7C+cat
echo+$x|code-block|syntax|javascript|2980629|事实上-看起来管道将先前的代码强制到另一个进程中，但没有重新初始化bash+-所以$BASHPID改变了，但$$改变了。|offset|length|style|CODE|2980630|有关$$和$BASHPID之间区别的更多详细信息，请参阅Difference+between+bash+pid+and+$$。|2980631|输出$BASH_SUBSHELL还会显示第二个比特正在子外壳中运行(级别1)，第一个比特在级别0。|2980632|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/questions/7858191/difference-between-bash-pid-and^0|0|0|15|8|1I|2|0|2|2|5|8|S|Y|0|0|2|E|0^^$0|@$1|2|3|4|5|6|7|Y|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|Z|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|10|8|@$I|11|J|12|K|L]|$I|13|J|14|K|L]]|9|@]|A|$]]|$1|M|3|N|5|6|7|15|8|@$I|16|J|17|K|L]|$I|18|J|19|K|L]]|9|@$I|1A|J|1B|1|1C]]|A|$]]|$1|O|3|P|5|6|7|1D|8|@$I|1E|J|1F|K|L]]|9|@]|A|$]]|$1|Q|3|-4|5|6|7|1G|8|@]|9|@]|A|$]]]|R|$S|$5|T|U|V|A|$W|X]]]]

I tried a test program:

<pre><code>x=0
{ x=$$ ; echo "$$ $BASHPID $x" ; }
echo $x

x=0
{ x=$$ ; echo "$$ $BASHPID $x" ; } | cat
echo $x
</code></pre>

And indeed - it looks like the pipe forces the prior code into another process, but without reinitialising bash - so <code>$BASHPID</code> changes but <code>$$</code> does.

See <a href="https://stackoverflow.com/questions/7858191/difference-between-bash-pid-and">Difference between bash pid and $$</a> for more details of the different between <code>$$</code> and <code>$BASHPID</code>.

Also outputting <code>$BASH_SUBSHELL</code> shows that the second bit is running in a subshell (level 1), and the first is at level 0.

blocks|key|933669|text|bash将管道的所有元素作为子进程执行；如果它们是shell内置或命令组，这意味着它们在subshell中执行，因此它们设置的任何变量都不会传播到父shell。这通常很难解决，但如果您只需要命令组的退出状态，则可以使用$PIPESTATUS数组来获取它：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|933670|$+{+false;+}+%7C+cat;+echo+"${PIPESTATUS[@]}"
1+0
$+{+false;+}+%7C+cat;+r=${PIPESTATUS[0]};+echo+$r
1
$+{+true;+}+%7C+cat;+r=${PIPESTATUS[0]};+echo+$r
0|code-block|syntax|javascript|933671|请注意，这仅适用于获取组中最后一个命令的退出状态：|933672|$+{+false;+true;+false;+uselessvar=$?;+}+%7C+cat;+r=${PIPESTATUS[0]};+echo+$r
0|933673|..。因为uselessvar=$?成功了。|offset|length|style|CODE|933674|entityMap^0|0|0|0|0|5|D|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|U|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|W|8|@$M|X|N|Y|O|P]]|9|@]|A|$]]|$1|Q|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]]|R|$]]

bash executes all elements of a pipeline as subprocesses; if they're shell builtins or command groups, that means they execute in subshells and so any variables they set don't propagate to the parent shell. This can be tricky to work around in general, but if all you need is the exit status of the command group, you can use the $PIPESTATUS array to get it:

<pre><code>$ { false; } | cat; echo "${PIPESTATUS[@]}"
1 0
$ { false; } | cat; r=${PIPESTATUS[0]}; echo $r
1
$ { true; } | cat; r=${PIPESTATUS[0]}; echo $r
0
</code></pre>

Note that this only works for getting the exit status of the last command in the group:

<pre><code>$ { false; true; false; uselessvar=$?; } | cat; r=${PIPESTATUS[0]}; echo $r
0
</code></pre>

... because <code>uselessvar=$?</code> succeeded.

The group command <code>{ list; }</code> should execute list in the current shell environment.

This allows things like variable assignments to be visible outside of the command group (<a href="http://mywiki.wooledge.org/BashGuide/CompoundCommands" rel="nofollow noreferrer">http://mywiki.wooledge.org/BashGuide/CompoundCommands</a>).

I use it to send output to a logfile as well as terminal:

<pre><code>{ { echo "Result is 13"; echo "ERROR: division by 0" 1&gt;&amp;2; } | tee -a stdout.txt; } 3&gt;&amp;1 1&gt;&amp;2 2&gt;&amp;3 | tee -a stderr.txt;
</code></pre>

On the topic "pipe stdout and stderr to two different processes in shell script?" read here: <a href="https://stackoverflow.com/questions/9112979/pipe-stdout-and-stderr-to-two-different-processes-in-shell-script">pipe stdout and stderr to two different processes in shell script?</a>.

<pre><code>{ echo "Result is 13"; echo "ERROR: division by 0" 1&gt;&amp;2; }
</code></pre>

simulates a command with output to stdout and stderr.

I want to evaluate the exit status also. <code>/bin/true</code> and <code>/bin/false</code> simulate a command that may succeed or fail. So I try to save <code>$?</code> to a variable <code>r</code>:

<pre><code>~$ r=init; { /bin/true; r=$?; } | cat; echo $r;
init
~$ r=init; { /bin/true; r=$?; } 2&gt;/dev/null; echo $r;
0
</code></pre>

As you can see the above pipeline construct does not set variable <code>r</code> while the second command line leads to the expected result. Is it a bug or is it my fault? Thanks.

I tested Ubuntu 12.04.2 LTS (<code>~$</code>) and Debian GNU/Linux 7.0 (wheezy) (<code>~#</code>) with the following versions of bash:

<pre><code>~$ echo $BASH_VERSION
4.2.25(1)-release

~# echo $BASH_VERSION
4.2.37(1)-release
</code></pre>

setting variables inside a compound command in bash fails (command group)

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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组命令{ list; }应该在当前shell环境中执行list。这允许诸如变量赋值之类的事情在命令组()之外可见。我使用它将输出发送到日志文件和终端：{ { echo "Result is 13"; echo "ERROR: division by 0" 1>&2; } | tee -a stdout.txt; } 3...

问在bash中设置复合命令内的变量失败(命令组)
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问在bash中设置复合命令内的变量失败(命令组)EN