什么命令在 Bash 的条件中意味着“什么都不做”？
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What command means "do nothing" in a conditional in Bash?
提问by Village
Sometimes when making conditionals, I need the code to do nothing, e.g., here, I want Bash to do nothing when $ais greater than "10", print "1" if $ais less than "5", otherwise, print "2":
有时在制作条件时，我需要代码什么都不做，例如，在这里，我希望 Bash 在$a大于“10”时什么都不做，如果$a小于“5”则打印“1 ”，否则打印“2”：
if [ "$a" -ge 10 ]
then
elif [ "$a" -le 5 ]
then
echo "1"
else
echo "2"
fi
This makes an error though. Is there a command which will do nothing and also not slow down my script?
但这会导致错误。是否有一个什么都不做也不会减慢我的脚本的命令？
回答by Barmar
The no-op command in shell is :(colon).
shell 中的 no-op 命令是:(colon)。
if [ "$a" -ge 10 ]
then
:
elif [ "$a" -le 5 ]
then
echo "1"
else
echo "2"
fi
From the bash manual:
从bash 手册：
:(a colon)
Do nothing beyond expanding arguments and performing redirections. The return status is zero.
:（冒号）
除了扩展参数和执行重定向之外什么都不做。返回状态为零。
回答by Flimzy
You can probably just use the truecommand:
您可能只需使用以下true命令：
if [ "$a" -ge 10 ]; then
true
elif [ "$a" -le 5 ]; then
echo "1"
else
echo "2"
fi
An alternative, in your example case (but not necessarily everywhere) is to re-order your if/else:
另一种选择，在您的示例情况下（但不一定在所有地方）是重新订购您的 if/else：
if [ "$a" -le 5 ]; then
echo "1"
elif [ "$a" -lt 10 ]; then
echo "2"
fi
回答by Basti M
Although I'm not answering the original question concering the no-op command, many (if not most) problems when one may think "in this branch I have to do nothing" can be bypassed by simply restructuring the logicso that this branch won't occur.
虽然我没有回答关于 no-op 命令的原始问题，但当人们可能认为“在这个分支我什么都不做”时，许多（如果不是大多数）问题可以通过简单地重构逻辑来绕过，以便这个分支获胜不发生。
I try to give a general rule by using the OPs example
我尝试通过使用 OP 示例给出一般规则
do nothing when $a is greater than "10", print "1" if $a is less than "5", otherwise, print "2"
当 $a 大于“10”时什么也不做，如果 $a 小于“5”则打印“1”，否则打印“2”
we have to avoid a branch where $agets more than 10, so $a < 10as a general condition can be applied to every other, following condition.
我们必须避免一个分支$a超过 10 个，因此$a < 10一般条件可以应用于每个其他条件，以下条件。
In general terms, when you say do nothing when X, then rephrase it as avoid a branch where X. Usually you can make the avoidance happen by simply negating X and applying it to all other conditions.
一般而言，当您说X 什么都不做时，请将其改写为避免 X 所在的分支。通常，您可以通过简单地否定 X 并将其应用于所有其他条件来避免发生。
So the OPs example with the rule applied may be restructured as:
因此，应用规则的 OP 示例可以重构为：
if [ "$a" -lt 10 ] && [ "$a" -le 5 ]
then
echo "1"
elif [ "$a" -lt 10 ]
then
echo "2"
fi
Just a variation of the above, enclosing everything in the $a < 10condition:
只是上述的变体，将所有内容都包含在$a < 10条件中：
if [ "$a" -lt 10 ]
then
if [ "$a" -le 5 ]
then
echo "1"
else
echo "2"
fi
fi
(For this specific example @Flimzys restructuringis certainly better, but I wanted to give a general rule for all the people searching how to do nothing.)
（对于这个特定的例子，@Flimzys 重组当然更好，但我想为所有寻求如何无所作为的人提供一个一般规则。）
