Read and Write files in Golang

In this tutorial, we will learn how to read, write, and append to a file using Golang.

The built-in os package provides reading and writing files. Earlier it used to be handled by io/ioutil package.

A file must have correct file permissions to perform respective operations.

Following are the file modes(permissions) codes that we will need in the tutorial.

0000     no permissions
0700     read, write, & execute only for the owner
0770     read, write, & execute for owner and group
0777     read, write, & execute for owner, group and others
0111     execute
0222     write
0333     write & execute
0444     read
0555     read & execute
0666     read & write
0740     owner can read, write, & execute; group can only read; others have no permissions

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Write file in golang

The os.Writefile function accepts the filename or full path with extension if any, data or content in []byte format and file permission code.

Syntax

func WriteFile(name string, data []byte, perm os.FileMode) error

The below code will create a new file if it doesn't exist and write or it will override with new content to the existing file.
The 0777 file mode gives read, write and execution permission.
The data is a string converted to []byte format.

package main

import (
    "fmt"
    "log"
    "os"
)

func main() {
    data := []byte("dev.to")
    err := os.WriteFile("test.txt", data, 0777)
    if err != nil {
        log.Fatal(err)
    }
    fmt.Println("file written successfully.")
}

Read file in golang

The os.ReadFile function accepts the filename or full file path and returns []byte and error.

It will throw an error if the file doesn't exist or the file doesn't have permission to read.

Syntax

func ReadFile(name string) ([]byte, error)

package main

import (
    "fmt"
    "log"
    "os"
)

func main() {
    data, err := os.ReadFile("test.txt")
    if err != nil {
        log.Fatal(err)
    }

    fmt.Println(string(data))
}

Append to an existing file

There are 2 ways you can append content to an existing file.

1. Read and Write

In this approach, first read the file, append the content and then write the file.

package main

import (
    "fmt"
    "log"
    "os"
)

func main() {
    data, err := os.ReadFile("test.txt")
    if err != nil {
        log.Fatal(err)
    }

    fmt.Println(string(data))

    msg := []byte("\nappended message")

    data = append(data, msg...)

    err = os.WriteFile("test.txt", data, 0777)
    if err != nil {
        log.Fatal(err)
    }
    fmt.Println("file written successfully.")
}

2. Open and write

In this approach, first, open the file with the O_APPEND flag then write the content and close the file.

package main

import (
    "fmt"
    "log"
    "os"
)

func main() {
    file, err := os.OpenFile("test.txt", os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0777)
    if err != nil {
        log.Fatal(err)
    }

    defer file.Close()

    msg := []byte("\n another appended message")
    _, err = file.Write(msg)
    if err != nil {
        log.Fatal(err)
    }

    fmt.Println("file appended successfully.")
}

os.O_APPEND - File permission append
os.O_CREATE - Create a new file if it doesn't exist
os.O_WRONLY - Write only

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