为什么我不能指定一个环境变量并在同一个命令行中回显它? [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
- 当我在第二行回显时,我得到 AAA
代码> 内容如预期.
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 $SOMEVAR zzz
zzz AAA zzz
...我没有得到我预期的 BBB
- 我得到了旧值 (AAA
).
... I do not get BBB
as I expected - I get the old value (AAA
).
事情应该是这样的吗?如果是这样,那么您为什么可以指定像 LD_PRELOAD=/... program args ...
这样的变量并让它工作?我错过了什么?
Is this how things are supposed to be? If so, how come then you can specify variables like LD_PRELOAD=/... program args ...
and have it work? What am I missing?
推荐答案
您看到的是预期的行为.问题在于,父shell 在使用修改后的环境调用命令之前,会在命令行上评估$SOMEVAR
.您需要将 $SOMEVAR
的评估推迟到环境设置之后.
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 $SOMEVAR
deferred until after the environment is set.
您的直接选择包括:
SOMEVAR=BBB eval echo zzz '$SOMEVAR' zzz
.SOMEVAR=BBB sh -c 'echo zzz $SOMEVAR zzz'
.
两者都使用单引号来防止父 shell 评估 $SOMEVAR
;它仅在环境中设置后(临时,在单个命令的持续时间内)才被评估.
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 符号(正如 Marcus Kuhn 在他的
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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