"env" bash中"echo"的奇怪行为 [英] strange behavior of 'echo' in 'env' bash

问题描述

当我在'env'命令中尝试echo变量时，我什么都没得到，但是我可以使用'printenv'命令看到它:

When I am trying echo variable inside 'env' command I got nothing, but I can see it using 'printenv' command:

root@devel:~# env xxx=23 echo $xxx

root@devel:~# env xxx=23 printenv | grep xxx
xxx=23

这是怎么了?

推荐答案

env xxx=23 echo $xxx

在上面，shell在执行env之前先评估$xxx.因此，没有任何回声.

In the above, the shell evaluates $xxx before env is executed. Thus, nothing is echoed.

更详细地，外壳程序看到四个词env，xxx=23，echo和$xxx.它将env解释为命令名称，并将xxx=23，echo和$xxx解释为将传递给命令env的三个参数.在将$xxx 传递给命令env之前，它会进行评估.

In more detail, the shell sees the four words env, xxx=23, echo and $xxx. It interprets env as a command name and xxx=23, echo, and $xxx as three arguments which will be passed to the command env. It evaluates $xxx before passing it to the command env.

相比之下，在下文中，没有要评估的Shell变量.而是使用两个自变量xxx=23和printenv执行env. env设置环境变量xxx，然后执行printenv:

By contrast, in the following, there are no shell variables for the shell to evaluate. Instead env is executed with two arguments, xxx=23 and printenv. env sets the environment variable xxx and then executes printenv:

$ env xxx=23 printenv | grep xxx
xxx=23

类似地，观察:

$ env xxx=23 sh -c 'echo $xxx'
23

由于$xxx在单引号内，因此外壳程序不对其进行求值.而是运行带有四个参数的env:xxx=23，sh，-c和echo $xxx. env设置环境变量xxx后，它将使用参数-c和echo $xxx执行sh.在执行sh时对$xxx进行求值，因此它会看到变量xxx.

Since $xxx is inside single-quotes, the shell does not evaluate it. Instead is runs env with four arguments: xxx=23, sh, -c, and echo $xxx. After env sets environment variable xxx, it executes sh with arguments -c and echo $xxx. The $xxx is evaluated when sh is executed and hence it sees the variable xxx.

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