使用env为bash中的一个程序调用设置环境变量 [英] Setting environment variable for one program call in bash using env

问题描述

我尝试通过命令 env 调用具有修改环境的shell命令。

根据手册

  env HELLO ='Hello World'echo $ HELLO 
  

应该回显 Hello World ，但没有。
如果我做

  HELLO ='Hello World'bash -c'echo $ HELLO'
  

它按预期打印 Hello World （感谢这个信息的答案）。

我在这里缺少什么？ p>

干杯，
尼克拉斯

解决方案

情况下，您的当前shell在运行命令之前扩展$ HELLO变量。而您当前的shell中没有设置HELLO变量。

env HELLO ='Hello World'echo $ HELLO / p>

将这样做：

• 展开任何给定的变量，在这种情况下为$ HELLO


• 运行env与3个参数'HELLO = Hello World'，'echo'和''（一个空字符串，因为在当前shell中没有设置HELLO变量）


• env命令将在其环境中运行并设置HELLO ='Hello World'


• env将使用参数'（空字符串）运行echo / li>
  

如你所见，当前的shell扩展了$ HELLO变量，没有设置。

HELLO ='Hello World'bash -c'echo $ HELLO'

会做这个：

• 设置变量 HELLO ='Hello World / li>
  
• 运行bash与2个参数'-c'和'echo $ HELLO'


• 最后一个参数用单引号括起来，其中的任何内容都不会展开


• 新的bash依次运行命令 echo $ HELLO


• 要在新的bash子shell中运行echo $ HELLO，bash首先扩展它可以使用的任何东西，在这种情况下，$ HELLO，并将父shell设置为Hello World我们。


• subshel​​l运行echo'Hello World'

如果你试图做例如这个：

env HELLO ='Hello World'echo'$ HELLO'

• 当前的shell将扩展任何可以的东西，这是没有，因为$ HELLO被包含在单引号


• 运行env与3参数'HELLO = Hello World'，'echo'和'$ HELLO'


• env命令将在其环境中运行并设置HELLO ='Hello World'


• env将使用参数'$ HELLO'运行echo。
  

在这种情况下，没有shell可以扩展$ HELLO，所以echo接收字符串 $ HELLO 并打印出来。变量扩展仅由shell完成。

I am trying to invoke a shell command with a modified environment via the command env.

According to the manual

env HELLO='Hello World' echo $HELLO

should echo Hello World, but it doesn't. If I do

HELLO='Hello World' bash -c 'echo $HELLO'

it prints Hello World as expected (thanks to this answer for this info).

What am I missing here?

Cheers, Niklas

解决方案

It's because in your first case, your current shell expands the $HELLO variable before running the commands. And there's no HELLO variable set in your current shell.

env HELLO='Hello World' echo $HELLO

will do this:

• expand any variables given, in this case $HELLO
• run env with the 3 arguments 'HELLO=Hello World', 'echo' and '' (an empty string, since there's no HELLO variable set in the current shell)
• The env command will run and set the HELLO='Hello World' in its environment
• env will run echo with the argument '' (an empty string)

As you see, the current shell expanded the $HELLO variable, which isn't set.

HELLO='Hello World' bash -c 'echo $HELLO'

will do this:

• set the variable HELLO='Hello World for the following command
• run bash with the 2 arguments '-c' and 'echo $HELLO'
• since the last argument is enclosed in single quotes, anything inside it is not expanded
• the new bash in turn will run the command echo $HELLO
• To run echo $HELLO in the new bash sub-shell, bash first expands anything it can, $HELLO in this case, and the parent shell set that to "Hello World" for us.
• The subshell runs echo 'Hello World'

If you tried to do e.g. this:

env HELLO='Hello World' echo '$HELLO'

• The current shell would expand anything it can, which is nothing since $HELLO is enclosed in single quotes
• run env with the 3 arguments 'HELLO=Hello World', 'echo' and '$HELLO'
• The env command will run and set the HELLO='Hello World' in its environment
• env will run echo with the argument '$HELLO'

In this case, there's no shell that will expand the $HELLO, so echo receives the string $HELLO and prints out that. Variable expansion is done by shells only.

这篇关于使用env为bash中的一个程序调用设置环境变量的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！