File Handling (Input/Output) in Go (Golang)

Introduction to File Handling

File handling refers to the process of creating, reading, writing, updating, and deleting files stored on a computer’s file system. In Go, file handling is part of standard input/output (I/O) operations and is handled mainly using the os, io, and bufio packages.

File handling allows programs to:

• Store data permanently
• Read configuration files
• Process logs
• Handle large datasets
• Exchange data between programs

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Packages Used for File Handling in Go

Go provides powerful built-in packages for file I/O:

Package	Purpose
os	File creation, opening, deleting
io	Low-level I/O primitives
bufio	Buffered I/O (efficient reading/writing)
fmt	Formatted input/output
ioutil (deprecated)	Older file utilities (replaced by os & io)

---

Creating a File in Go

Using os.Create()

os.Create() creates a new file.
If the file already exists, it truncates (clears) the file.

package main

import (
	"os"
)

func main() {
	file, err := os.Create("example.txt")
	if err != nil {
		panic(err)
	}
	defer file.Close()
}

Explanation

• Returns a file pointer
• Must always close the file
• defer file.Close() ensures cleanup

---

Opening an Existing File

Using os.Open()

Used for read-only access.

file, err := os.Open("example.txt")
if err != nil {
	panic(err)
}
defer file.Close()

---

Using os.OpenFile()

Allows full control over file access modes.

file, err := os.OpenFile(
	"example.txt",
	os.O_APPEND|os.O_CREATE|os.O_WRONLY,
	0644,
)

File Flags

Flag	Meaning
os.O_RDONLY	Read only
os.O_WRONLY	Write only
os.O_RDWR	Read & write
os.O_CREATE	Create if not exists
os.O_APPEND	Append to file
os.O_TRUNC	Truncate file

---

Writing to a File

Writing Using WriteString()

file, _ := os.Create("data.txt")
defer file.Close()

file.WriteString("Hello, Go File Handling\n")

---

Writing Using fmt.Fprintln()

fmt.Fprintln(file, "This is a line written using fmt")

---

Writing Using bufio.Writer (Recommended)

Buffered writing is faster.

writer := bufio.NewWriter(file)
writer.WriteString("Buffered writing in Go\n")
writer.Flush()

---

Reading from a File

Reading Using os.ReadFile() (Simple)

data, err := os.ReadFile("example.txt")
if err != nil {
	panic(err)
}
fmt.Println(string(data))

Best for small files.

---

Reading Using bufio.Scanner (Line by Line)

file, _ := os.Open("example.txt")
defer file.Close()

scanner := bufio.NewScanner(file)
for scanner.Scan() {
	fmt.Println(scanner.Text())
}

Used for:

• Log files
• Text processing
• Line-based input

---

Reading Using bufio.Reader

reader := bufio.NewReader(file)
line, _ := reader.ReadString('\n')
fmt.Println(line)

---

Appending to a File

Appending adds content without deleting existing data.

file, _ := os.OpenFile(
	"example.txt",
	os.O_APPEND|os.O_WRONLY,
	0644,
)
defer file.Close()

file.WriteString("New appended line\n")

---

Deleting a File

Use os.Remove().

err := os.Remove("example.txt")
if err != nil {
	panic(err)
}

---

Checking if a File Exists

if _, err := os.Stat("example.txt"); err == nil {
	fmt.Println("File exists")
} else {
	fmt.Println("File does not exist")
}

---

Getting File Information

Use os.Stat().

info, _ := os.Stat("example.txt")

fmt.Println("Name:", info.Name())
fmt.Println("Size:", info.Size())
fmt.Println("Permissions:", info.Mode())
fmt.Println("Modified:", info.ModTime())

---

File Permissions in Go

Permissions use Unix-style notation.

Value	Meaning
0644	Owner read/write, others read
0755	Executable permissions
0600	Owner only

---

Handling Errors in File I/O

Error handling is critical in Go.

file, err := os.Open("missing.txt")
if err != nil {
	fmt.Println("Error:", err)
	return
}

Go forces explicit error checking, improving reliability.

---

Copying File Contents

source, _ := os.Open("a.txt")
defer source.Close()

destination, _ := os.Create("b.txt")
defer destination.Close()

io.Copy(destination, source)

---

Reading User Input and Writing to File

reader := bufio.NewReader(os.Stdin)
fmt.Print("Enter text: ")
input, _ := reader.ReadString('\n')

file, _ := os.Create("user.txt")
defer file.Close()

file.WriteString(input)

---

File Handling Best Practices

• Always close files
• Use defer
• Prefer buffered I/O
• Handle errors properly
• Avoid reading large files fully into memory

---

Common Mistakes in Go File Handling

• Forgetting file.Close()
• Ignoring errors
• Using deprecated ioutil package
• Overwriting files unintentionally
• Not flushing buffered writers

---

Practical Example: Log File Writer

file, _ := os.OpenFile("log.txt",
	os.O_APPEND|os.O_CREATE|os.O_WRONLY,
	0644)

defer file.Close()

log := bufio.NewWriter(file)
log.WriteString("Application started\n")
log.Flush()

---

Summary

• Go provides robust file I/O using os, io, and bufio
• Supports file creation, reading, writing, appending, and deletion
• Explicit error handling ensures safety
• Buffered I/O improves performance
• Essential for backend systems, CLI tools, and system programs