这个bash语句(命令|| true)的想法是什么? [英] What is the idea in this bash statement ( command || true )?

问题描述

我在互联网上看到了以下bash语句:

I saw the following bash statement being used on the internet:

PYTHON_BIN_PATH=$(which python || which python3 || true)

我知道，如果哪个python 失败，则将执行哪个python3 ，但是我不理解 true 的目的在条件结束时.有什么主意吗?

I understand that if which python fails, then which python3 will be executed, but I don't understand the purpose of the true at the end of the condition. Any idea?

推荐答案

尝试运行:(请注意 bla )

which python_bla || which python3_bla_bla || true
echo $?
0

您将获得 RC = 0 .这意味着它是成功执行下一条命令的一种构造.在这里我们知道 python_bla 或 python3_bla_bla 不存在，但是命令仍然给出了 rc = 0 示例:检查以下三个命令的RC，我已将 date 命令的拼写更改为不正确，但 true 导致 RC 保留为 0 .

You will get RC=0. It means it a construct to successfully proceed to next command. Here we know python_bla or python3_bla_bla does not exist,but still command gave rc=0 Example: Check the RC of following three commands, I have changed the spelling of date command to incorrect but true is causing RC to remain 0.

date;echo $?
Thu Nov  9 01:40:44 CST 2017
0
datea;echo $?
If 'datea' is not a typo you can use command-not-found to lookup the package that contains it, like this:
    cnf datea
127
datea||true;echo $?
If 'datea' is not a typo you can use command-not-found to lookup the package that contains it, like this:
    cnf datea
0

注意:您也可以使用:运算符代替true来获得相同的结果.示例:

Note: You can also use : operator instead of true to get the same results.Example:

command || :

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