fmt, io, os, bufio, encoding/json, net/http, time, slog, regexp, flag, embed.

`os` and Files

The os package gives you the operating-system primitives — files, environment, process info, signals.

Args and env

fmt.Println(os.Args)         // []string{program, arg1, arg2, ...}
fmt.Println(os.Getenv("HOME"))
os.Setenv("DEBUG", "1")

Reading and writing files

The simplest reads and writes:

data, err := os.ReadFile("config.toml")
if err != nil { return err }
fmt.Println(string(data))

err = os.WriteFile("out.txt", []byte("hello\n"), 0o644)

os.ReadFile and os.WriteFile are perfect for small files. For anything large or streaming, use os.Open / os.Create and a bufio.Scanner:

go playground

`bufio` — buffered I/O

| Type | Use | | ---------------- | ------------------------------------ | | bufio.Reader | Wrap an io.Reader for buffered reads. | | bufio.Writer | Wrap an io.Writer for buffered writes (call Flush at the end). | | bufio.Scanner | Line-by-line (or token-by-token) reading. |

bufio.NewScanner(os.Stdin) is the standard way to read user input from the terminal.

File modes and paths

os.MkdirAll("logs/2024", 0o755)
os.Remove("temp.txt")
os.Rename("a", "b")

import "path/filepath"
filepath.Join("dir", "file.txt")
filepath.Ext("notes.md")            // ".md"
filepath.Base("/usr/bin/go")        // "go"

filepath is the cross-platform path helper. Don't concatenate paths with / — use filepath.Join and your code works on Windows too.

Running external commands: `os/exec`

import "os/exec"

out, err := exec.Command("git", "rev-parse", "HEAD").Output()
if err != nil { return err }
fmt.Println(string(out))

exec.Command does not invoke a shell — exec.Command("ls", "-la *.go") will literally look for a flag named -la *.go. To use shell features, run exec.Command("sh", "-c", "ls -la *.go") — and beware of shell injection if any input comes from outside.

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