Go file operations

Create Empty File

package main

import (
    "log"
    "os"
)

var (
    newFile *os.File
    err     error
)

func main() {
    newFile, err = os.Create("test.txt")
    if err != nil {
        log.Fatal(err)
    }
    log.Println(newFile)
    newFile.Close()
}

Truncate a File

package main

import (
    "log"
    "os"
)

func main() {
    // Truncate a file to 100 bytes. If file
    // is less than 100 bytes the original contents will remain
    // at the beginning, and the rest of the space is
    // filled will null bytes. If it is over 100 bytes,
    // Everything past 100 bytes will be lost. Either way
    // we will end up with exactly 100 bytes.
    // Pass in 0 to truncate to a completely empty file

    err := os.Truncate("test.txt", 100)
    if err != nil {
        log.Fatal(err)
    }
}

Get File Info

package main

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

var (
    fileInfo *os.FileInfo
    err      error
)

func main() {
    // Stat returns file info. It will return
    // an error if there is no file.
    fileInfo, err := os.Stat("test.txt")
    if err != nil {
        log.Fatal(err)
    }
    fmt.Println("File name:", fileInfo.Name())
    fmt.Println("Size in bytes:", fileInfo.Size())
    fmt.Println("Permissions:", fileInfo.Mode())
    fmt.Println("Last modified:", fileInfo.ModTime())
    fmt.Println("Is Directory: ", fileInfo.IsDir())
    fmt.Printf("System interface type: %T\n", fileInfo.Sys())
    fmt.Printf("System info: %+v\n\n", fileInfo.Sys())
}

Rename and Move a File

package main

import (
    "log"
    "os"
)

func main() {
    originalPath := "test.txt"
    newPath := "test2.txt"
    err := os.Rename(originalPath, newPath)
    if err != nil {
        log.Fatal(err)
    }
}

Delete a File

package main

import (
    "log"
    "os"
)

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

Open and Close Files

package main

import (
    "log"
    "os"
)

func main() {
    // Simple read only open. We will cover actually reading
    // and writing to files in examples further down the page
    file, err := os.Open("test.txt")
    if err != nil {
        log.Fatal(err)
    }
    file.Close()

    // OpenFile with more options. Last param is the permission mode
    // Second param is the attributes when opening
    file, err = os.OpenFile("test.txt", os.O_APPEND, 0666)
    if err != nil {
        log.Fatal(err)
    }
    file.Close()

    // Use these attributes individually or combined
    // with an OR for second arg of OpenFile()
    // e.g. os.O_CREATE|os.O_APPEND
    // or os.O_CREATE|os.O_TRUNC|os.O_WRONLY

    // os.O_RDONLY // Read only
    // os.O_WRONLY // Write only
    // os.O_RDWR // Read and write
    // os.O_APPEND // Append to end of file
    // os.O_CREATE // Create is none exist
    // os.O_TRUNC // Truncate file when opening
}

Check if File Exists

package main

import (
    "log"
    "os"
)

var (
    fileInfo *os.FileInfo
    err      error
)

func main() {
    // Stat returns file info. It will return
    // an error if there is no file.
    fileInfo, err := os.Stat("test.txt")
    if err != nil {
        if os.IsNotExist(err) {
            log.Fatal("File does not exist.")
        }
    }
    log.Println("File does exist. File information:")
    log.Println(fileInfo)
}

Check Read and Write Permissions

package main

import (
    "log"
    "os"
)

func main() {
    // Test write permissions. It is possible the file
    // does not exist and that will return a different
    // error that can be checked with os.IsNotExist(err)
    file, err := os.OpenFile("test.txt", os.O_WRONLY, 0666)
    if err != nil {
        if os.IsPermission(err) {
            log.Println("Error: Write permission denied.")
        }
    }
    file.Close()

    // Test read permissions
    file, err = os.OpenFile("test.txt", os.O_RDONLY, 0666)
    if err != nil {
        if os.IsPermission(err) {
            log.Println("Error: Read permission denied.")
        }
    }
    file.Close()
}

Change Permissions, Ownership, and Timestamps

package main

import (
    "log"
    "os"
    "time"
)

func main() {
    // Change perrmissions using Linux style
    err := os.Chmod("test.txt", 0777)
    if err != nil {
        log.Println(err)
    }

    // Change ownership
    err = os.Chown("test.txt", os.Getuid(), os.Getgid())
    if err != nil {
        log.Println(err)
    }

    // Change timestamps
    twoDaysFromNow := time.Now().Add(48 * time.Hour)
    lastAccessTime := twoDaysFromNow
    lastModifyTime := twoDaysFromNow
    err = os.Chtimes("test.txt", lastAccessTime, lastModifyTime)
    if err != nil {
        log.Println(err)
    }
}

Hard Links and Symlinks

package main

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

func main() {
    // Create a hard link
    // You will have two file names that point to the same contents
    // Changing the contents of one will change the other
    // Deleting/renaming one will not affect the other
    err := os.Link("original.txt", "original_also.txt")
    if err != nil {
        log.Fatal(err)
    }

fmt.Println("creating sym")
    // Create a symlink
    err = os.Symlink("original.txt", "original_sym.txt")
    if err != nil {
        log.Fatal(err)
    }

    // Lstat will return file info, but if it is actually
    // a symlink, it will return info about the symlink.
    // It will not follow the link and give information
    // about the real file
    // Symlinks do not work in Windows
    fileInfo, err := os.Lstat("original_sym.txt")
    if err != nil {
        log.Fatal(err)
    }
    fmt.Printf("Link info: %+v", fileInfo)

    // Change ownership of a symlink only 
    // and not the file it points to
    err = os.Lchown("original_sym.txt", os.Getuid(), os.Getgid())
    if err != nil {
        log.Fatal(err)
    }
}

Copy a File

package main

import (
    "os"
    "log"
    "io"
)

// Copy a file
func main() {
    // Open original file
    originalFile, err := os.Open("test.txt")
    if err != nil {
        log.Fatal(err)
    }
    defer originalFile.Close()

    // Create new file
    newFile, err := os.Create("test_copy.txt")
    if err != nil {
        log.Fatal(err)
    }
    defer newFile.Close()

    // Copy the bytes to destination from source
    bytesWritten, err := io.Copy(newFile, originalFile)
    if err != nil {
        log.Fatal(err)
    }
    log.Printf("Copied %d bytes.", bytesWritten)

    // Commit the file contents
    // Flushes memory to disk
    err = newFile.Sync()
    if err != nil {
        log.Fatal(err)
    }
}

Seek Positions in File

package main

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

func main() {
    file, _ := os.Open("test.txt")
    defer file.Close()

    // Offset is how many bytes to move
    // Offset can be positive or negative
    var offset int64 = 5

    // Whence is the point of reference for offset
    // 0 = Beginning of file
    // 1 = Current position
    // 2 = End of file
    var whence int = 0
    newPosition, err := file.Seek(offset, whence)
    if err != nil {
        log.Fatal(err)
    }
    fmt.Println("Just moved to 5:", newPosition)

    // Go back 2 bytes from current position
    newPosition, err = file.Seek(-2, 1)
    if err != nil {
        log.Fatal(err)
    }
    fmt.Println("Just moved back two:", newPosition)

    // Find the current position by getting the
    // return value from Seek after moving 0 bytes
    currentPosition, err := file.Seek(0, 1)
    fmt.Println("Current position:", currentPosition)

    // Go to beginning of file
    newPosition, err = file.Seek(0, 0)
    if err != nil {
        log.Fatal(err)
    }
    fmt.Println("Position after seeking 0,0:", newPosition)
}

Write Bytes to a File

package main

import (
    "os"
    "log"
)

func main() {
    // Open a new file for writing only
    file, err := os.OpenFile(
        "test.txt",
        os.O_WRONLY|os.O_TRUNC|os.O_CREATE,
        0666,
    )
    if err != nil {
        log.Fatal(err)
    }
    defer file.Close()

    // Write bytes to file
    byteSlice := []byte("Bytes!\n")
    bytesWritten, err := file.Write(byteSlice)
    if err != nil {
        log.Fatal(err)
    }
    log.Printf("Wrote %d bytes.\n", bytesWritten)
}

Quick Write to File

package main

import (
    "io/ioutil"
    "log"
)

func main() {
    err := ioutil.WriteFile("test.txt", []byte("Hi\n"), 0666)
    if err != nil {
        log.Fatal(err)
    }
}

Use Buffered Writer

package main

import (
    "log"
    "os"
    "bufio"
)

func main() {
    // Open file for writing
    file, err := os.OpenFile("test.txt", os.O_WRONLY, 0666)
    if err != nil {
        log.Fatal(err)
    }
    defer file.Close()

    // Create a buffered writer from the file
    bufferedWriter := bufio.NewWriter(file)

    // Write bytes to buffer
    bytesWritten, err := bufferedWriter.Write(
        []byte{65, 66, 67},
    )
    if err != nil {
        log.Fatal(err)
    }
    log.Printf("Bytes written: %d\n", bytesWritten)

    // Write string to buffer
    // Also available are WriteRune() and WriteByte()   
    bytesWritten, err = bufferedWriter.WriteString(
        "Buffered string\n",
    )
    if err != nil {
        log.Fatal(err)
    }
    log.Printf("Bytes written: %d\n", bytesWritten)

    // Check how much is stored in buffer waiting
    unflushedBufferSize := bufferedWriter.Buffered()
    log.Printf("Bytes buffered: %d\n", unflushedBufferSize)

    // See how much buffer is available
    bytesAvailable := bufferedWriter.Available()
    if err != nil {
        log.Fatal(err)
    }
    log.Printf("Available buffer: %d\n", bytesAvailable)

    // Write memory buffer to disk
    bufferedWriter.Flush()

    // Revert any changes done to buffer that have
    // not yet been written to file with Flush()
    // We just flushed, so there are no changes to revert
    // The writer that you pass as an argument
    // is where the buffer will output to, if you want
    // to change to a new writer
    bufferedWriter.Reset(bufferedWriter) 

    // See how much buffer is available
    bytesAvailable = bufferedWriter.Available()
    if err != nil {
        log.Fatal(err)
    }
    log.Printf("Available buffer: %d\n", bytesAvailable)

    // Resize buffer. The first argument is a writer
    // where the buffer should output to. In this case
    // we are using the same buffer. If we chose a number
    // that was smaller than the existing buffer, like 10
    // we would not get back a buffer of size 10, we will
    // get back a buffer the size of the original since
    // it was already large enough (default 4096)
    bufferedWriter = bufio.NewWriterSize(
        bufferedWriter,
        8000,
    )

    // Check available buffer size after resizing
    bytesAvailable = bufferedWriter.Available()
    if err != nil {
        log.Fatal(err)
    }
    log.Printf("Available buffer: %d\n", bytesAvailable)
}

Read up to n Bytes from File

package main

import (
    "os"
    "log"
)

func main() {
    // Open file for reading
    file, err := os.Open("test.txt")
    if err != nil {
        log.Fatal(err)
    }
    defer file.Close()

    // Read up to len(b) bytes from the File
    // Zero bytes written means end of file
    // End of file returns error type io.EOF
    byteSlice := make([]byte, 16)
    bytesRead, err := file.Read(byteSlice)
    if err != nil {
        log.Fatal(err)
    }
    log.Printf("Number of bytes read: %d\n", bytesRead)
    log.Printf("Data read: %s\n", byteSlice)
}

Read Exactly n Bytes

package main

import (
    "os"
    "log"
    "io"
)

func main() {
    // Open file for reading
    file, err := os.Open("test.txt")
    if err != nil {
        log.Fatal(err)
    }

    // The file.Read() function will happily read a tiny file in to a large
    // byte slice, but io.ReadFull() will return an
    // error if the file is smaller than the byte slice.
    byteSlice := make([]byte, 2)
    numBytesRead, err := io.ReadFull(file, byteSlice)
    if err != nil {
        log.Fatal(err)
    }
    log.Printf("Number of bytes read: %d\n", numBytesRead)
    log.Printf("Data read: %s\n", byteSlice)
}

Read At Least n Bytes

package main

import (
    "os"
    "log"
    "io"
)

func main() {
    // Open file for reading
    file, err := os.Open("test.txt")
    if err != nil {
        log.Fatal(err)
    }

    byteSlice := make([]byte, 512)
    minBytes := 8
    // io.ReadAtLeast() will return an error if it cannot
    // find at least minBytes to read. It will read as
    // many bytes as byteSlice can hold. 
    numBytesRead, err := io.ReadAtLeast(file, byteSlice, minBytes)
    if err != nil {
        log.Fatal(err)
    }
    log.Printf("Number of bytes read: %d\n", numBytesRead)
    log.Printf("Data read: %s\n", byteSlice)
}

Read All Bytes of File

package main

import (
    "os"
    "log"
    "fmt"
    "io/ioutil"
)

func main() {
    // Open file for reading
    file, err := os.Open("test.txt")
    if err != nil {
        log.Fatal(err)
    }

    // os.File.Read(), io.ReadFull(), and
    // io.ReadAtLeast() all work with a fixed
    // byte slice that you make before you read

    // ioutil.ReadAll() will read every byte
    // from the reader (in this case a file),
    // and return a slice of unknown slice
    data, err := ioutil.ReadAll(file)
    if err != nil {
        log.Fatal(err)
    }

    fmt.Printf("Data as hex: %x\n", data)
    fmt.Printf("Data as string: %s\n", data)
    fmt.Println("Number of bytes read:", len(data))
}

Quick Read Whole File to Memory

package main

import (
    "log"
    "io/ioutil"
)

func main() {
    // Read file to byte slice
    data, err := ioutil.ReadFile("test.txt")
    if err != nil {
        log.Fatal(err)
    }

    log.Printf("Data read: %s\n", data)
}

Use Buffered Reader

package main

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

func main() {
    // Open file and create a buffered reader on top
    file, err := os.Open("test.txt")
    if err != nil {
        log.Fatal(err)
    }
    bufferedReader := bufio.NewReader(file)

    // Get bytes without advancing pointer
    byteSlice := make([]byte, 5)
    byteSlice, err = bufferedReader.Peek(5)
    if err != nil {
        log.Fatal(err)
    }
    fmt.Printf("Peeked at 5 bytes: %s\n", byteSlice)

    // Read and advance pointer
    numBytesRead, err := bufferedReader.Read(byteSlice)
    if err != nil {
        log.Fatal(err)
    }
    fmt.Printf("Read %d bytes: %s\n", numBytesRead, byteSlice)

    // Ready 1 byte. Error if no byte to read
    myByte, err := bufferedReader.ReadByte()
    if err != nil {
        log.Fatal(err)
    }
    fmt.Printf("Read 1 byte: %c\n", myByte)     

    // Read up to and including delimiter
    // Returns byte slice
    dataBytes, err := bufferedReader.ReadBytes('\n') 
    if err != nil {
        log.Fatal(err)
    }
    fmt.Printf("Read bytes: %s\n", dataBytes)           

    // Read up to and including delimiter
    // Returns string   
    dataString, err := bufferedReader.ReadString('\n')
    if err != nil {
        log.Fatal(err)
    }
    fmt.Printf("Read string: %s\n", dataString)     

    // This example reads a few lines so test.txt
    // should have a few lines of text to work correct
}

Read with a Scanner

package main

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

func main() {
    // Open file and create scanner on top of it
    file, err := os.Open("test.txt")
    if err != nil {
        log.Fatal(err)
    }
    scanner := bufio.NewScanner(file)

    // Default scanner is bufio.ScanLines. Lets use ScanWords.
    // Could also use a custom function of SplitFunc type
    scanner.Split(bufio.ScanWords)

    // Scan for next token. 
    success := scanner.Scan() 
    if success == false {
        // False on error or EOF. Check error
        err = scanner.Err()
        if err == nil {
            log.Println("Scan completed and reached EOF")
        } else {
            log.Fatal(err)
        }
    }

    // Get data from scan with Bytes() or Text()
    fmt.Println("First word found:", scanner.Text())

    // Call scanner.Scan() again to find next token
}

Archive Files

// This example uses zip but standard library
// also supports tar archives
package main

import (
    "archive/zip"
    "log"
    "os"
)

func main() {
    // Create a file to write the archive buffer to
    // Could also use an in memory buffer.
    outFile, err := os.Create("test.zip")
    if err != nil {
        log.Fatal(err)
    }
    defer outFile.Close()

    // Create a zip writer on top of the file writer
    zipWriter := zip.NewWriter(outFile)


    // Add files to archive
    // We use some hard coded data to demonstrate,
    // but you could iterate through all the files
    // in a directory and pass the name and contents
    // of each file, or you can take data from your
    // program and write it write in to the archive
    // without 
    var filesToArchive = []struct {
        Name, Body string
    } {
        {"test.txt", "String contents of file"},
        {"test2.txt", "\x61\x62\x63\n"},
    }

    // Create and write files to the archive, which in turn
    // are getting written to the underlying writer to the
    // .zip file we created at the beginning
    for _, file := range filesToArchive {
            fileWriter, err := zipWriter.Create(file.Name)
            if err != nil {
                    log.Fatal(err)
            }
            _, err = fileWriter.Write([]byte(file.Body))
            if err != nil {
                    log.Fatal(err)
            }
    }

    // Clean up
    err = zipWriter.Close()
    if err != nil {
            log.Fatal(err)
    }
}

Extract Archived Files

package main

import (
    "archive/zip"
    "log"
    "io"
    "os"
    "path/filepath"
)

func main() {
    // Create a reader out of the zip archive
    zipReader, err := zip.OpenReader("test.zip")
    if err != nil {
        log.Fatal(err)
    }
    defer zipReader.Close()

    // Iterate through each file/dir found in
    for _, file := range zipReader.Reader.File {
        // Open the file inside the zip archive
        // like a normal file
        zippedFile, err := file.Open()
        if err != nil {
            log.Fatal(err)
        }
        defer zippedFile.Close()

        // Specify what the extracted file name should be.
        // You can specify a full path or a prefix
        // to move it to a different directory. 
        // In this case, we will extract the file from
        // the zip to a file of the same name.
        targetDir := "./"
        extractedFilePath := filepath.Join(
            targetDir,
            file.Name,
        )

        // Extract the item (or create directory)
        if file.FileInfo().IsDir() {
            // Create directories to recreate directory
            // structure inside the zip archive. Also
            // preserves permissions
            log.Println("Creating directory:", extractedFilePath)
            os.MkdirAll(extractedFilePath, file.Mode())
        } else {
            // Extract regular file since not a directory
            log.Println("Extracting file:", file.Name)

            // Open an output file for writing
            outputFile, err := os.OpenFile(
                extractedFilePath,
                os.O_WRONLY|os.O_CREATE|os.O_TRUNC,
                file.Mode(),
            )
            if err != nil {
                log.Fatal(err)
            }
            defer outputFile.Close()

            // "Extract" the file by copying zipped file
            // contents to the output file
            _, err = io.Copy(outputFile, zippedFile)
            if err != nil {
                log.Fatal(err)
            }
        }
    }
}

Compress a File

package main

import (
    "os"
    "compress/gzip"
    "log"
)

func main() {
    // Create .gz file to write to
    outputFile, err := os.Create("test.txt.gz")
    if err != nil {
        log.Fatal(err)
    }

    // Create a gzip writer on top of file writer
    gzipWriter := gzip.NewWriter(outputFile)
    defer gzipWriter.Close()

    // When we write to the gzip writer
    // it will in turn compress the contents
    // and then write it to the underlying
    // file writer as well
    // We don't have to worry about how all
    // the compression works since we just
    // use it as a simple writer interface
    // that we send bytes to
    _, err = gzipWriter.Write([]byte("Gophers rule!\n"))
    if err != nil {
        log.Fatal(err)
    }

    log.Println("Compressed data written to file.") 
}

Uncompress a File

package main

import (
    "compress/gzip"
    "log"
    "io"
    "os"
)

func main() {
    // Open gzip file that we want to uncompress
    // The file is a reader, but we could use any
    // data source. It is common for web servers
    // to return gzipped contents to save bandwidth
    // and in that case the data is not in a file
    // on the file system but is in a memory buffer
    gzipFile, err := os.Open("test.txt.gz")
    if err != nil {
        log.Fatal(err)
    }

    // Create a gzip reader on top of the file reader
    // Again, it could be any type reader though
    gzipReader, err := gzip.NewReader(gzipFile)
    if err != nil {
        log.Fatal(err)
    }
    defer gzipReader.Close()

    // Uncompress to a writer. We'll use a file writer
    outfileWriter, err := os.Create("unzipped.txt")
    if err != nil {
        log.Fatal(err)
    }
    defer outfileWriter.Close()

    // Copy contents of gzipped file to output file
    _, err = io.Copy(outfileWriter, gzipReader)
    if err != nil {
        log.Fatal(err)
    }
}

Temporary Files and Directories

package main

import (
     "os"
     "io/ioutil"
     "log"
     "fmt"
)

func main() {
     // Create a temp dir in the system default temp folder
     tempDirPath, err := ioutil.TempDir("", "myTempDir")
     if err != nil {
          log.Fatal(err)
     }
     fmt.Println("Temp dir created:", tempDirPath)

     // Create a file in new temp directory
     tempFile, err := ioutil.TempFile(tempDirPath, "myTempFile.txt")
     if err != nil {
          log.Fatal(err)
     }
     fmt.Println("Temp file created:", tempFile.Name())

     // ... do something with temp file/dir ...

     // Close file
     err = tempFile.Close()
     if err != nil {
        log.Fatal(err)
    }

    // Delete the resources we created
     err = os.Remove(tempFile.Name())
     if err != nil {
        log.Fatal(err)
    }
     err = os.Remove(tempDirPath)
     if err != nil {
        log.Fatal(err)
    }
}

Downloading a File Over HTTP

package main

import (
     "os"
     "io"
     "log"
     "net/http"
)

func main() {
     // Create output file
     newFile, err := os.Create("devdungeon.html")
     if err != nil {
          log.Fatal(err)
     }
     defer newFile.Close()

     // HTTP GET request devdungeon.com 
     url := "http://www.devdungeon.com/archive"
     response, err := http.Get(url)
     defer response.Body.Close()

     // Write bytes from HTTP response to file.
     // response.Body satisfies the reader interface.
     // newFile satisfies the writer interface.
     // That allows us to use io.Copy which accepts
     // any type that implements reader and writer interface
     numBytesWritten, err := io.Copy(newFile, response.Body)
     if err != nil {
          log.Fatal(err)
     }
     log.Printf("Downloaded %d byte file.\n", numBytesWritten)
}

Hashing and Checksums

package main

import (
    "crypto/md5"
    "crypto/sha1"
    "crypto/sha256"
    "crypto/sha512"
    "log"
    "fmt"
    "io/ioutil"
)

func main() {
    // Get bytes from file
    data, err := ioutil.ReadFile("test.txt")
    if err != nil {
        log.Fatal(err)
    }

    // Hash the file and output results
    fmt.Printf("Md5: %x\n\n", md5.Sum(data))
    fmt.Printf("Sha1: %x\n\n", sha1.Sum(data))
    fmt.Printf("Sha256: %x\n\n", sha256.Sum256(data))
    fmt.Printf("Sha512: %x\n\n", sha512.Sum512(data))
}

package main

import (
    "crypto/md5"
    "log"
    "fmt"
    "io"
    "os"
)

func main() {
    // Open file for reading
    file, err := os.Open("test.txt")
    if err != nil {
        log.Fatal(err)
    }
    defer file.Close()

    // Create new hasher, which is a writer interface
    hasher := md5.New()
    _, err = io.Copy(hasher, file)
    if err != nil {
        log.Fatal(err)
    }

    // Hash and print. Pass nil since
    // the data is not coming in as a slice argument
    // but is coming through the writer interface
    sum := hasher.Sum(nil)
    fmt.Printf("Md5 checksum: %x\n", sum)
}