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This article mainly explains "what is the method of obtaining and setting environment variables by Go". The content of the explanation in the article is simple and clear, and it is easy to learn and understand. Please follow the editor's train of thought to study and learn "what is the method of obtaining and setting environment variables by Go".

01 start with the installation of Go

In fact, not only the installation of Go, but also other languages will have similar problems. In general, after installing Go, it is recommended that the go executable be configured into the PATH environment variable.

For example, the value of my local PATH environment variable:

$echo $PATH/Users/xuxinhua/.go/bin:/usr/local/sbin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:/Applications/Wireshark.app/Contents/MacOS:/Users/xuxinhua/.cargo/bin:/Users/xuxinhua/bin:/usr/local/git/bin:/Users/xuxinhua/.composer/vendor/bin:/Users/xuxinhua/go/bin

So what is the purpose of the PATH environment variable?

In a nutshell, you don't need to enter an absolute path when you execute a command at the terminal. For example, Go is installed in the ~ / .go/bin directory. If we want to execute the Go command, we have to do something like this:

$/ .go/bin/go version

Isn't it troublesome?! However, after adding the ~ / .go/bin directory to the PATH environment variable, you can directly execute the Go command like this:

$go version

This is what the PATH environment variable does: it tells you where to find the command to execute.

So what is the role of environmental variables? The explanation of environmental variables in the encyclopedia:

Environment variables (environment variables) generally refer to some parameters used in the operating system to specify the operating environment, such as temporary folder location and system folder location.

Processes also have their own environment variables, which are generally inherited from the parent process or can be artificially specified. For example, when a terminal runs a program, you can pass it environment variables:

$NAME=polarisxu. / xxx

The value of polarisxu can be obtained through NAME in the process.

Environmental variables can be said to be everywhere, and most of the time we just don't think about it.

Note: because of the mechanism of the PATH environment variable, in Shell, Dockerfile, etc., you need to always be aware of what the value of the PATH environment variable is and whether it contains your command path. For such scenarios, it may be better to write the absolute path rather than rely on PATH.

02 how Go uses environment variables

Many large applications are configured using environment variables (of course, other ways of configuration are supported, such as flag). Environment variables as configuration options greatly simplify the deployment of applications. These are also common in cloud infrastructure.

In general, based on the configuration of environment variables, the program will have a default value if the environment variable is not set.

In the Go language, the API related to environment variables is mainly in the os package. The API below is annotated.

/ / Environ returns all environment variables as key=value. Func Environ () [] string// ExpandEnv replaces ${var} or $var in the string based on the value of the current environment variable. / / A reference to a pair of undefined variables is replaced by an empty string. Func ExpandEnv (s string) string// Getenv retrieves the value of the environment variable corresponding to the key key. / / returns an empty string if the environment variable does not exist. / / to distinguish between null and unset values, use LookupEnv. The func Getenv (key string) string// LookupEnv retrieves the value of the environment variable corresponding to the key key. / / if the environment variable exists, the corresponding value is returned (possibly empty), and the Boolean value is true. / / otherwise, the return value will be empty and the Boolean value will be false. Func LookupEnv (key string) (string, bool) / / Setenv sets the value of the environment variable corresponding to the key key. / / an error will be returned if there is an error. Func Setenv (key, value string) error// Unsetenv unsets a single environment variable. Func Unsetenv (key string) error// Clearenv removes all environment variables. Func Clearenv ()

In addition, there is a LookPath function in os/exec that is related to the PATH environment variable:

/ / search the directory corresponding to the PATH environment variable for the executable file named file. / / if the file contains /, the PATH is not searched, but the normal path is searched. / / the result returned may be an absolute path or a relative path relative to the current directory. Func LookPath (file string) (string, error)

Now, take a look at how these API are used through an example.

/ / main.gopackage mainimport ("fmt"os") func main () {name: = os.Getenv ("NAME") fmt.Println ("name is:", name)}

Then run:

$NAME=polarisxu go run main.goname is: polarisxu

If the previous NAME=polarisxu does not exist, the returned name is an empty string. What should I do if I want to have default values?

Package mainimport ("fmt"os") func main () {name: = GetenvDefault ("NAME", "xuxinhua") fmt.Println ("name is:", name)} func GetenvDefault (key, defVal string) string {val, ok: = os.LookupEnv (key) if ok {return val} return defVal}

You can find out whether the environment variable is set through os.LookupEnv. The result of running go run main.go at this point is: name is: xuxinhua.

These are the API that are commonly used in Go to get environment variables.

Other API is unlikely to be used. There are two API worth mentioning: Environ () and ExpandEnv ().

As mentioned earlier, the process inherits the environment variables from the parent process. The most important thing here is the PATH environment variable. Sometimes, when we execute an external program through the os/exec package, we may prompt that the command cannot be found, and we need to make sure that the PATH is correct. Maybe PATH under Shell contains the directory where the command is located, but the process may not. We can output all the environment variables in the program:

Envs: = os.Environ () for _, env: = range envs {fmt.Println (env)}

One line is a complete environment variable, such as LANG=zh_CN.UTF-8.

Take another look at the ExpandEnv () function. There is the following code: (other codes related to main are omitted)

Host: = os.ExpandEnv ("127.0.0.1:$PORT") fmt.Println (host)

The form of IP:PORT is common. Usually, we do string concatenation: host + ":" + port. With os.ExpandEnv, there is no need for concatenation. It replaces $PORT with the value of os.Getenv ("PORT").

Thank you for your reading, the above is the content of "what is the method for Go to obtain and set environment variables". After the study of this article, I believe you have a deeper understanding of what is the method for Go to obtain and set environment variables, and the specific use needs to be verified in practice. Here is, the editor will push for you more related knowledge points of the article, welcome to follow!

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