将值附加到go中环境变量的现有值

Question

如何将另一个值附加到go环境变量的现有值？如果CGO_CXXFLAGS 的值为“-I/blah/blah”由于以下不起作用$ go env -w CGO_CXXFLAGS="$CGO_CXXFLAGS -I/foo/bar"我想找到一个正确的方法让CGO_CXXFLAGS具有值“-I/blah/blah -I/foo/bar”通过避免简单地重新设置所有值加上新值，例如：$ go env -w CGO_CXXFLAGS="-I/blah/blah -I/foo/bar"

Answer 1

由于go env <var_name> 输出变量的有效设置（即不一定是默认值），获取环境变量当前默认值的可靠方法是使用子 shell（假设是 Bash shell）：

$ (unset <var_name>; go env <var_name>)

然后，要设置一个新值，您需要使用命令替换：

$ go env -w <var_name>="$(unset <var_name>; go env <var_name>)<value_to_append>"

有关工作示例，请参阅以下带注释的日志：

# Write an initial default.

$ go env -w CGO_CXXFLAGS=-I/blah/blah

$ go env CGO_CXXFLAGS

-I/blah/blah

# Keep in mind that the default can be overridden by a currently set 

# environment variable.

$ export CGO_CXXFLAGS="-I/override/default"

$ go env CGO_CXXFLAGS

-I/override/default

# To print the default value even when it is being overridden,

# in a subshell, unset the environment variable and then run 'go env'. 

$ (unset CGO_CXXFLAGS; go env CGO_CXXFLAGS)

-I/blah/blah

# To set a new default value utilizing the existing default value.

#

# Note: if the default isn't being overridden, no warning will be printed.

$ go env -w CGO_CXXFLAGS="$(unset CGO_CXXFLAGS; go env CGO_CXXFLAGS) -I/foo/bar"

warning: go env -w CGO_CXXFLAGS=... does not override conflicting OS environment variable

# State after change...

# Default value

$ (unset CGO_CXXFLAGS; go env CGO_CXXFLAGS)

-I/blah/blah -I/foo/bar

# Overridden value

$ go env CGO_CXXFLAGS

-I/override/default

或者，您可以找到具有默认值的文件并直接解析它（go env GOENV会告诉您文件位置），但我认为暂时取消设置环境变量并让其完成go env工作会更容易。

PowerShell 中的等价物：

# Write default.

PS > go env -w CGO_CXXFLAGS=-I/blah/blah

PS > go env CGO_CXXFLAGS

-I/blah/blah

# Override default.

PS > $Env:CGO_CXXFLAGS = "-I/override/default"

PS > go env CGO_CXXFLAGS

-I/override/default

# Print default.

PS > Start-Job {$Env:CGO_CXXFLAGS=''; go env CGO_CXXFLAGS} | Receive-Job -Wait

-I/blah/blah

# Append to default.

PS > go env -w CGO_CXXFLAGS="$(Start-Job {$Env:CGO_CXXFLAGS=''; go env CGO_CXXFLAGS} | Receive-Job -Wait) -I/foo/bar"

warning: go env -w CGO_CXXFLAGS=... does not override conflicting OS environment variable

# Final state.

PS > go env CGO_CXXFLAGS

-I/override/default

PS > Start-Job {$Env:CGO_CXXFLAGS=''; go env CGO_CXXFLAGS} | Receive-Job -Wait

-I/blah/blah -I/foo/bar