Os.Getenv ("") in go, cannot get custom environment variable

problem description

os.Getenv ("GOPATH") cannot be obtained, but os.Getenv ("HOME") can get

normally.

the environmental background of the problems and what methods you have tried

in the terminal, ech $GOPATH output normally, GOPATH variable is written in ~ / .bashrc file, os.Getenv ("GOPATH") cannot get
then try to write GOPATH variable in ~ / .profile, / etc/profile, / etc/environment, neither
os.Getenv (""), HOME, PATH can output normally, but custom GOPATH and GOROOT are not allowed

related codes

main.go:

package main

import (
    "os"
)

func main() {
    println(os.Getenv("HOME"))
    println(os.Getenv("GOPATH"))
}

Terminal:

zll@loong:~$ echo $GOPATH
/home/zll/go
zll@loong:~/temp$ go run main.go 
/home/zll

zll@loong:~/temp$

platform version in which the problem occurs

Ubuntu18.04
go 1.11.2

additional associations

when you use go"s third-party tool viper to parse environment variables, you can only get PATH and HOME system variables, but you can"t customize them.

package main

import (
    "fmt"

    "github.com/spf13/viper"
)

func main() {
    viper.AutomaticEnv()
    if env := viper.Get("GOPATH"); env == nil {
        println("error!")
    } else {
        fmt.Printf("%-sharpv\n", env)
    }
}

result:

error!

I wonder if there is a connection between them. Find the solution