6. Go Run

The go run command quickly compiles and runs a Go package. The compiled binary is not saved in your working directory.

I typically use go run to do local testing, scripting and debugging.

Assignment

• Inside hellogo, create a new file called main.go. Conventionally, the file in the main package that contains the main() function is called main.go.

• Paste the code shown in Step 2 below into your file.

• Run the code from the root of the main package and submit the CLI tests.

1

Create main.go

Make sure you're in the hellogo directory:

pwd
# Should show: .../hellogo

Create the file:

touch main.go

Or create it with your text editor.

2

Add the code

Open main.go and paste:

main.go

package main

import "fmt"

func main() {
	fmt.Println("hello world")
}

3

Run it

go run main.go

Expected output:

hello world

4

Run Boot.dev Tests

Run the tests from the root of the main package:

bootdev test

go run usage & behavior (detailed)

usage: go run [build flags] [-exec xprog] package [arguments...]

Run compiles and runs the named main Go package. Typically the package is specified as a list of .go source files from a single directory, but it may also be an import path, file system path, or pattern matching a single known package, as in 'go run .' or 'go run my/cmd'.

If the package argument has a version suffix (like @latest or @v1.0.0), "go run" builds the program in module-aware mode, ignoring the go.mod file in the current directory or any parent directory, if there is one. This is useful for running programs without affecting the dependencies of the main module.

If the package argument doesn't have a version suffix, "go run" may run in module-aware mode or GOPATH mode, depending on the GO111MODULE environment variable and the presence of a go.mod file. See 'go help modules' for details. If module-aware mode is enabled, "go run" runs in the context of the main module.

By default, 'go run' runs the compiled binary directly: 'a.out arguments...'. If the -exec flag is given, 'go run' invokes the binary using xprog: 'xprog a.out arguments...'. If the -exec flag is not given, GOOS or GOARCH is different from the system default, and a program named go_$GOOS_$GOARCH_exec can be found on the current search path, 'go run' invokes the binary using that program, for example 'go_js_wasm_exec a.out arguments...'. This allows execution of cross-compiled programs when a simulator or other execution method is available.

By default, 'go run' compiles the binary without generating the information used by debuggers, to reduce build time. To include debugger information in the binary, use 'go build'.

The exit status of Run is not the exit status of the compiled binary.

For more about build flags, see 'go help build'. For more about specifying packages, see 'go help packages'.

See also: go build.

What is go run?

• Compiles and runs Go code in one command

• Doesn't save the compiled binary (temporary)

• Used for testing, debugging, and scripting

Your Project Structure Now

~/workspace/hellogo/
├── go.mod
└── main.go

Understanding the Code

package main

• Declares this file belongs to the main package

• The main package is special - it's executable

import "fmt"

• Imports the fmt package from the standard library

• Gives access to Println

func main() {
	fmt.Println("hello world")
}

• main() is the program entry point

• fmt.Println("hello world") prints "hello world" to the console

go run vs go build

go run:

go run main.go
# Compiles and runs immediately
# No binary file saved

go build:

go build
# Creates executable binary: hellogo (or hellogo.exe on Windows)
# Doesn't run it

Common Commands

go run main.go        # Compile and run
go build              # Compile to binary
go help run           # Show help for run command

Tips:

• go run is great for quick testing.

• Use go build when you want to distribute your program.

• The main package must have a main() function to be executable.

• You can execute go help run in your shell to read the official documentation.