Bash：在命令行上指定echo的环境变量？ [英] Bash: specifying environment variables for echo on command line?

问题描述

考虑这个片段：

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

这里我设置了 $ SOMEVAR 到 AAA 在第一行 - 当我回应它在第二行，我得到 AAA 内容为预期。

但是，如果我尝试在与 echo 相同的命令行上指定变量：

  $ SOMEVAR = BBB echo zzz $ SOMEVAR zzz 
 zzz AAA zzz 
  pre> 
 
 
 ...我没有按预期得到 BBB   - 我得到旧的值（ AAA ）。 
 
 
 
这是怎么回事？如果是这样，那么你可以指定诸如 LD_PRELOAD = / ...程序args ... 等变量的变量我缺少什么？
解决方案
你看到的是预期的行为。麻烦的是，在调用具有修改的环境的命令之前，父shell在命令行上评估 $ SOMEVAR 。您需要获得 $ SOMEVAR 的评估，直到环境设置为止。
 
 
 
您的即时选项包括： 
 
 
  SOMEVAR = BBB eval echo zzz'$ SOMEVAR'zzz 。 > 
 
  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 ;它仅在环境中设置（在单个命令的持续时间内暂时被评估）才被评估。
 
 
 
另一个选项是使用子shell符号（as也由 Marcus Kuhn 在他的 answer ）：
 （SOMEVAR = BBB; echo zzz $ SOMEVAR zzz）
  
变量仅在子shell中设置
 
Consider this snippet: 
$ SOMEVAR=AAA
$ echo zzz $SOMEVAR zzz
zzz AAA zzz
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. 

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

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?
解决方案
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.

Your immediate options include:

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

Another option is to use the sub-shell notation (as also suggested by Marcus Kuhn in his answer):
(SOMEVAR=BBB; echo zzz $SOMEVAR zzz)
The variable is set only in the sub-shell

                        
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