在go中对环境变量的现有值追加值

Question

如何将另一个值附加到go的现有值环境变量?

如果CGO_CXXFLAGS值"-I/blah/blah">

因为下面的代码不起作用

$ go env -w CGO_CXXFLAGS="$CGO_CXXFLAGS -I/foo/bar"

我想找到一个合适的方法来CGO_CXXFLAGS值"-I/blah/blah -I/foo/bar">

通过避免简单地重新设置所有值和新值，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)，因此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>"

请看下面带注释的日志示例:

# 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，您可以使用默认值找到文件并直接解析它(go env GOENV将告诉您文件位置)，但我认为暂时取消设置环境变量并让go env做这项工作更容易。

---

在PowerShell中等效:

# 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