为什么我不能指定环境变量并在同一命令行中回显它? [英] Why can't I specify an environment variable and echo it in the same command line?

问题描述

考虑这个片段:

$ SOMEVAR=AAA
$ echo zzz $SOMEVAR zzz
zzz AAA zzz

在这里，我在第一行将 $SOMEVAR 设置为 AAA - 当我在第二行回显它时，我得到 AAA内容符合预期.

Here I've set $SOMEVAR to AAA on the first line - and when I echo it on the second line, I get the AAA contents as expected.

但是，如果我尝试在与 echo 相同的命令行上指定变量:

But then, if I try to specify the variable on the same command line as the echo:

$ SOMEVAR=BBB echo zzz $SOMEVAR zzz
zzz AAA zzz

... 我没有得到预期的 BBB - 我得到了旧值 (AAA).

... I do not get BBB as I expected - I get the old value (AAA).

事情应该是这样的吗?如果是这样，那你怎么能指定像 LD_PRELOAD=/... program args ... 这样的变量并让它工作?我错过了什么?

Is this how things are supposed to be? If so, how come then you can specify variables like LD_PRELOAD=/... program args ... and have it work? What am I missing?

推荐答案

你看到的是预期的行为.问题在于，父 shell 在使用修改后的环境调用命令之前，会在命令行上评估 $SOMEVAR.您需要将 $SOMEVAR 的评估推迟到设置环境之后.

What you see is the expected behaviour. The trouble is that the parent shell evaluates $SOMEVAR on the command line before it invokes the command with the modified environment. You need to get the evaluation of $SOMEVAR deferred until after the environment is set.

您的直接选择包括:

1. SOMEVAR=BBB eval echo zzz '$SOMEVAR' zzz.
2. SOMEVAR=BBB sh -c 'echo zzz $SOMEVAR zzz'.

这两个都使用单引号来防止父 shell 评估 $SOMEVAR;只有在环境中设置后才会评估它(暂时，在单个命令的持续时间内).

Both these use single quotes to prevent the parent shell from evaluating $SOMEVAR; it is only evaluated after it is set in the environment (temporarily, for the duration of the single command).

另一种选择是使用 sub-shell 表示法(正如 Marcus Kuhn 在他的 回答):

Another option is to use the sub-shell notation (as also suggested by Marcus Kuhn in his answer):

(SOMEVAR=BBB; echo zzz $SOMEVAR zzz)

变量只在子shell中设置

The variable is set only in the sub-shell

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