如何在OS X上取消环境变量？

Question

我正在编写一个Makefile规则，并希望在递归调用make之前取消设置环境变量MAKEFILES。在其他BSD系统上，我是这样做的：

env -u MAKEFLAGS $(MAKE) $(SUBDIR_ARGS)

在 Linux 上，我是这样做的：

env --unset=MAKEFLAGS $(MAKE) $(SUBDIR_ARGS)

然而，无论是哪种样式，都无法在Macintosh OS X Mavericks上使用，尽管我希望其他地方使用的BSD样式可以正常工作。

使用BSD风格调用时出现的错误为：

env: illegal option -- u

如何在 OS X 上在调用命令之前取消设置环境变量，为什么其他地方使用的 BSD 样式不起作用?

这是 Mac 上 env 的手册页面:

ENV(1)                    BSD General Commands Manual                   ENV(1)

NAME
     env -- set and print environment

SYNOPSIS
     env [-i] [name=value ...] [utility [argument ...]]

DESCRIPTION
     env executes utility after modifying the environment as specified on the command line.  The option name=value specifies an environmental variable, name, with a value of value.  The option `-i' causes env to completely ignore the environment it inherits.

     If no utility is specified, env prints out the names and values of the variables in the environment, with one name=value pair per line.

DIAGNOSTICS
     If the utility is invoked, the exit status of env shall be the exit status of utility; otherwise, the env utility exits with one of the following values:

     0       The env utility completed successfully

     1-125   An error occurred in the env utility.

     126     The utility specified by utility was found, but could not be invoked.

     127     The utility specified by utility could not be found.

COMPATIBILITY
     The historic - option has been deprecated but is still supported in this implementation.

SEE ALSO
     execvp(3), environ(7)

STANDARDS
     The env utility conforms to IEEE Std 1003.2-1992 (``POSIX.2'').

BUGS
     env doesn't handle commands with equal (``='') signs in their names, for obvious reasons.

BSD                             August 27, 1993                            BSD

- WilliamKF

Answer 1

使用unset命令。来自man页面：

unset

Remove variable or function names - the opposite of set.

Syntax
  unset [-fv] [name ...]    

  Key
   -v   Each name refers to a shell variable.(default)

   -f   Each name refers to a shell function, and the function definition is removed. 

  Example

$ unset myvariable

Each unset variable or function is removed from the environment passed to subsequent commands. If any of RANDOM, SECONDS, LINENO, HISTCMD, FUNCNAME, GROUPS, or DIRSTACK are unset, they lose their special properties, even if they are subsequently reset.

The exit status is true unless a name is readonly. It is not an error for nothing to be unset.

unset is a bash built in command.

"Look Dave, I can see you're really upset about this" ~ HAL (Space 2001)

- SevenBits

Answer 2

您可以使用set命令来删除环境变量，例如：

set -e ENV_VARIABLE_NAME

- NameError