bash 如何打印/回显环境变量？

How to print / echo environment variables?

提问by ThomasReggi

How do I print the environment variable just being set?

如何打印刚刚设置的环境变量？

NAME=sam echo "$NAME" # empty

You can see here using evalit works. Is this the way?

你可以在这里看到使用eval它的工作原理。这是方法吗？

NAME=sam eval 'echo $NAME' # => sam

回答by heemayl

These need to go as different commands e.g.:

这些需要作为不同的命令，例如：

NAME=sam; echo "$NAME"
NAME=sam && echo "$NAME"

The expansion $NAMEto empty string is done by the shell earlier, before running echo, so at the time the NAMEvariable is passed to the echocommand's environment, the expansion is already done (to null string).

扩展$NAME为空字符串是由 shell 在运行之前完成的echo，因此在将NAME变量传递给echo命令的环境时，扩展已经完成（到空字符串）。

To get the same result in one command:

要在一个命令中获得相同的结果：

NAME=sam printenv NAME

回答by mklement0

To bring the existing answers together with an important clarification:

将现有答案与重要说明结合在一起：

As stated, the problem with NAME=sam echo "$NAME"is that $NAMEgets expanded by the current shell beforeassignment NAME=samtakes effect.

如前所述，问题NAME=sam echo "$NAME"在于在分配生效之前$NAME被当前 shell 扩展。NAME=sam

Solutions that preserve the original semantics(of the (ineffective) solution attempt NAME=sam echo "$NAME"):

保留原始语义的解决方案（（无效的）解决方案尝试的NAME=sam echo "$NAME"）：

Use either eval^[1](as in the question itself), or printenv(as added by Aaron McDaid to heemayl's answer), or bash -c(from Ljm Dullaart's answer), in descending order of efficiency:

使用eval^[1]（如问题本身），或printenv（由 Aaron McDaid 添加到heemayl 的回答），或bash -c（来自Ljm Dullaart 的回答），按效率降序排列：

NAME=sam eval 'echo "$NAME"'  # use `eval` only if you fully control the command string
NAME=sam printenv NAME
NAME=sam bash -c 'echo "$NAME"'

^{printenvis not a POSIX utility, but it is available on both Linux and macOS/BSD.}

^{printenv不是 POSIX 实用程序，但它在 Linux 和 macOS/BSD 上都可用。}

What this style of invocation (<var>=<name> cmd ...) does is to define NAME:

这种调用方式 ( <var>=<name> cmd ...) 的作用是定义NAME：

• as an environmentvariable
• that is only defined for the command being invoked.

• 作为环境变量
• 即只对被调用的命令定义。

In other words: NAMEonly exists for the command being invoked, and has no effect on the current shell(if no variable named NAMEexisted before, there will be none after; a preexisting NAMEvariable remains unchanged).

换句话说：NAME只存在于被调用的命令中，对当前shell没有影响（如果NAME之前不存在named变量，则之后也不存在；先前存在的NAME变量保持不变）。

POSIX defines the rules for this kind of invocation in its Command Search and Executionchapter.

POSIX 在其命令搜索和执行章节中定义了这种调用的规则。

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The following solutions work very differently(from heemayl's answer):

以下解决方案的工作方式非常不同（来自heemayl 的回答）：

NAME=sam; echo "$NAME"
NAME=sam && echo "$NAME"

While they produce the same output, they instead define:

虽然它们产生相同的输出，但它们定义了：

• a shellvariable NAME(only) rather than an environmentvariable
  • if echowere a command that relied on environmentvariable NAME, it wouldn't be defined (or potentially defined differently from earlier).
• that lives onafter the command.

• 一个壳变量NAME（只），而不是一个环境变量
  • 如果echo是依赖于环境变量的命令，NAME它将不会被定义（或可能与之前的定义不同）。
• 是住在该命令后。

Note that every environment variable is also exposed as a shell variable, but the inverse is not true: shell variables are only visible to the current shell and its subshells, but notto child processes, such as external utilities and (non-sourced) scripts (unless they're marked as environment variables with exportor declare -x).

请注意，每个环境变量也作为 shell 变量公开，但反之则不然：shell 变量仅对当前 shell 及其子 shell 可见，而对子进程（例如外部实用程序和（非源）脚本）不可见（除非它们用export或标记为环境变量declare -x）。

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^{[1] Technically, bashis in violation of POSIX here (as is zsh): Since evalis a specialshell built-in, the preceding NAME=samassignment should cause the the variable $NAMEto remain in scope after the command finishes, but that's not what happens.
However, when you run bashin POSIX compatibility mode, it iscompliant.
dashand kshare always compliant.
The exact rules are complicated, and some aspects are left up to the implementations to decide; again, see Command Search and Execution.
Also, the usual disclaimer applies: Use evalonly on input you fully control or implicitly trust.}

^{[1] 从技术上讲，bash这里违反了 POSIX（原样zsh）：由于eval是一个特殊的内置 shell，前面的NAME=sam赋值应该导致$NAME命令完成后变量保持在作用域内，但事实并非如此。
但是，当您bash在 POSIX 兼容模式下运行时，它是兼容的。
dash并且ksh始终合规。
确切的规则很复杂，有些方面由实现来决定；再次，请参阅命令搜索和执行。
此外，通常的免责声明适用：仅在您完全控制或隐含信任的输入上使用eval。}

回答by ThomasReggi

This works too, with the semi-colon.

这也适用于分号。

NAME=sam; echo $NAME

NAME=sam; echo $NAME

回答by Ljm Dullaart

The syntax

语法

variable=value command

is often used to set an environment variables for a specific process. However, you must understand which process gets what variable and who interprets it. As an example, using two shells:

通常用于为特定进程设置环境变量。但是，您必须了解哪个过程获得了哪些变量以及由谁来解释它。例如，使用两个外壳：

a=5
# variable expansion by the current shell:
a=3 bash -c "echo $a"
# variable expansion by the second shell:
a=3 bash -c 'echo $a'

The result will be 5 for the first echo and 3 for the second.

第一个回声的结果为 5，第二个为 3。