How to Use io.ReadAll vs os.ReadFile in Go

Use os.ReadFile to read an entire file from the filesystem into a byte slice, and use io.ReadAll to read all data from any io.Reader (like a network connection, archive entry, or pipe) into a byte slice. os.ReadFile is a convenience wrapper that opens the file, reads it, and closes it, while io.ReadAll only handles the reading part of an already-open stream.

// Read a file from disk
content, err := os.ReadFile("config.json")

// Read data from an io.Reader (e.g., a tar entry or network stream)
reader := getReader() // returns io.Reader
data, err := io.ReadAll(reader)

Use os.ReadFile when you have a filename and want the whole file's content immediately. Use io.ReadAll when you already have an open data stream (like a file handle, network connection, or archive entry) and need to read everything from it. Think of os.ReadFile as a complete vending machine that takes a coin and gives you a snack, while io.ReadAll is just the hand that grabs the snack once the machine has already opened.