File Systems

package main

import (
	"fmt"
	"os"
)

func main() {

	file, e := os.Stat("/some/path")
	if e != nil {
		panic(e)
	}

	mode := file.Mode()
	if mode.IsDir() {
		fmt.Println("directory")
	} else if mode.IsRegular() {
		fmt.Println("file")
	}
}

package main

import (
	"fmt"
	"os"
)

func exists(path string) bool {
	if _, err := os.Stat(path); os.IsNotExist(err) {
		return false
	}
	return true
}

func main() {
	fmt.Println(exists("./my/file.txt"))
}

package main

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

func check(e error) {
    if e != nil {
        panic(e)
    }
}

func main() {

    // Perhaps the most basic file reading task is slurping a file’s entire contents into memory.
    dat, err := ioutil.ReadFile("/tmp/dat")
    check(err)
    fmt.Print(string(dat))

    // You’ll often want more control over how and what parts of a file are read.
    // For these tasks, start by Opening a file to obtain an os.File value.
    f, err := os.Open("/tmp/dat")
    check(err)

    // Read some bytes from the beginning of the file.
    // Allow up to 5 to be read but also note how many actually were read.
    b1 := make([]byte, 5)
    n1, err := f.Read(b1)
    check(err)
    fmt.Printf("%d bytes: %s\n", n1, string(b1[:n1]))

    // You can also Seek to a known location in the file and Read from there.
    o2, err := f.Seek(6, 0)
    check(err)
    b2 := make([]byte, 2)
    n2, err := f.Read(b2)
    check(err)
    fmt.Printf("%d bytes @ %d: ", n2, o2)
    fmt.Printf("%v\n", string(b2[:n2]))

    // The io package provides some functions that may be helpful for file reading.
    // For example, reads like the ones above can be more robustly implemented with ReadAtLeast.
    o3, err := f.Seek(6, 0)
    check(err)
    b3 := make([]byte, 2)
    n3, err := io.ReadAtLeast(f, b3, 2)
    check(err)
    fmt.Printf("%d bytes @ %d: %s\n", n3, o3, string(b3))

    // There is no built-in rewind, but Seek(0, 0) accomplishes this.
    _, err = f.Seek(0, 0)
    check(err)

    // The bufio package implements a buffered reader that may be useful both for its efficiency
    // with many small reads and because of the additional reading methods it provides.
    r4 := bufio.NewReader(f)
    b4, err := r4.Peek(5)
    check(err)
    fmt.Printf("5 bytes: %s\n", string(b4))

    f.Close()
}

package main

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

func check(e error) {
    if e != nil {
        panic(e)
    }
}

func main() {

    // To start, here’s how to dump a string (or just bytes) into a file.
    d1 := []byte("hello\ngo\n")
    err := ioutil.WriteFile("/tmp/dat1", d1, 0644)
    check(err)

    // For more granular writes, open a file for writing.
    f, err := os.Create("/tmp/dat2")
    check(err)

    // It’s idiomatic to defer a Close immediately after opening a file.
    defer f.Close()

    // You can Write byte slices as you’d expect.
    d2 := []byte{115, 111, 109, 101, 10}
    n2, err := f.Write(d2)
    check(err)
    fmt.Printf("wrote %d bytes\n", n2)

    // A WriteString is also available.
    n3, err := f.WriteString("writes\n")
    fmt.Printf("wrote %d bytes\n", n3)

    // Issue a Sync to flush writes to stable storage.
    f.Sync()

    // bufio provides buffered writers in addition to the buffered readers we saw earlier.
    w := bufio.NewWriter(f)
    n4, err := w.WriteString("buffered\n")
    fmt.Printf("wrote %d bytes\n", n4)

    // Use Flush to ensure all buffered operations have been applied to the underlying writer.
    w.Flush()

}

package main

import (
    "fmt"
    "path/filepath"
    "strings"
)

func main() {

    // Join should be used to construct paths in a portable way.
    // It takes any number of arguments and constructs a hierarchical path from them.
    p := filepath.Join("dir1", "dir2", "filename")
    fmt.Println("p:", p)

    // You should always use Join instead of concatenating /s or \s manually.
    // In addition to providing portability, Join will also normalize paths by
    // removing superfluous separators and directory changes.
    fmt.Println(filepath.Join("dir1//", "filename"))
    fmt.Println(filepath.Join("dir1/../dir1", "filename"))

    // Dir and Base can be used to split a path to the directory and the file.
    // Alternatively, Split will return both in the same call.
    fmt.Println("Dir(p):", filepath.Dir(p))
    fmt.Println("Base(p):", filepath.Base(p))

    // We can check whether a path is absolute.
    fmt.Println(filepath.IsAbs("dir/file"))
    fmt.Println(filepath.IsAbs("/dir/file"))

    filename := "config.json"

    // Some file names have extensions following a dot.
    // We can split the extension out of such names with Ext.
    ext := filepath.Ext(filename)
    fmt.Println(ext)

    // To find the file’s name with the extension removed, use strings.TrimSuffix.
    fmt.Println(strings.TrimSuffix(filename, ext))

    // Rel finds a relative path between a base and a target.
    // It returns an error if the target cannot be made relative to base.
    rel, err := filepath.Rel("a/b", "a/b/t/file")
    if err != nil {
        panic(err)
    }
    fmt.Println(rel)

    rel, err = filepath.Rel("a/b", "a/c/t/file")
    if err != nil {
        panic(err)
    }
    fmt.Println(rel)
}

package main

import (
    "fmt"
    "io/ioutil"
    "os"
    "path/filepath"
)

func check(e error) {
    if e != nil {
        panic(e)
    }
}

func main() {

    // Create a new sub-directory in the current working directory.
    err := os.Mkdir("subdir", 0755)
    check(err)

    // When creating temporary directories, it’s good practice to defer their removal.
    // os.RemoveAll will delete a whole directory tree (similarly to rm -rf).
    defer os.RemoveAll("subdir")

    createEmptyFile := func(name string) {
        d := []byte("")
        check(ioutil.WriteFile(name, d, 0644))
    }

    createEmptyFile("subdir/file1")

    // We can create a hierarchy of directories, including parents with MkdirAll.
    // This is similar to the command-line mkdir -p.
    err = os.MkdirAll("subdir/parent/child", 0755)
    check(err)

    createEmptyFile("subdir/parent/file2")
    createEmptyFile("subdir/parent/file3")
    createEmptyFile("subdir/parent/child/file4")

    // ReadDir lists directory contents, returning a slice of os.FileInfo objects.
    c, err := ioutil.ReadDir("subdir/parent")
    check(err)

    fmt.Println("Listing subdir/parent")
    for _, entry := range c {
        fmt.Println(" ", entry.Name(), entry.IsDir())
    }

    // Chdir lets us change the current working directory, similarly to cd.
    err = os.Chdir("subdir/parent/child")
    check(err)

    // Now we’ll see the contents of subdir/parent/child when listing the current directory.
    c, err = ioutil.ReadDir(".")
    check(err)

    fmt.Println("Listing subdir/parent/child")
    for _, entry := range c {
        fmt.Println(" ", entry.Name(), entry.IsDir())
    }

    err = os.Chdir("../../..")
    check(err)

    // We can also visit a directory recursively, including all its sub-directories.
    // Walk accepts a callback function to handle every file or directory visited.
    fmt.Println("Visiting subdir")
    err = filepath.Walk("subdir", visit)
}

func visit(p string, info os.FileInfo, err error) error {
    if err != nil {
        return err
    }
    fmt.Println(" ", p, info.IsDir())
    return nil
}

package main

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

func check(e error) {
    if e != nil {
        panic(e)
    }
}

func main() {

    // The easiest way to create a temporary file is by calling ioutil.TempFile.
    // It creates a file and opens it for reading and writing. We provide "" as the first argument,
    // so ioutil.TempFile will create the file in the default location for our OS.
    f, err := ioutil.TempFile("", "sample")
    check(err)

    // Display the name of the temporary file.
    // On Unix-based OSes the directory will likely be /tmp.
    // The file name starts with the prefix given as the second argument to ioutil.TempFile
    // and the rest is chosen automatically to ensure that concurrent calls will always
    // create different file names.
    fmt.Println("Temp file name:", f.Name())

    // Clean up the file after we’re done. The OS is likely to clean up temporary files by itself after some time,
    // but it’s good practice to do this explicitly.
    defer os.Remove(f.Name())

    _, err = f.Write([]byte{1, 2, 3, 4})
    check(err)
}

package main

import (
    "fmt"
    "io/ioutil"
    "os"
    "path/filepath"
)

func check(e error) {
    if e != nil {
        panic(e)
    }
}

func main() {

    // If we intend to write many temporary files, we may prefer to create a temporary directory.
    // ioutil.TempDir’s arguments are the same as TempFile’s, but it returns a directory name rather than an open file.
    dname, err := ioutil.TempDir("", "sampledir")
    fmt.Println("Temp dir name:", dname)

    defer os.RemoveAll(dname)

    fname := filepath.Join(dname, "file1")
    err = ioutil.WriteFile(fname, []byte{1, 2}, 0666)
    check(err)
}

package main

import (
	"fmt"
	"path/filepath"
)

func main() {
	fmt.Println(filepath.Ext("hello.go"))
}

package main

import (
	"fmt"
	"io/ioutil"
)

func main() {

	files, err := ioutil.ReadDir("./some/dir")
	if err == nil {
		panic("oops")
	}
	for _, file := range files {
		fmt.Println(file.Name())
	}
}

package main

import (
	"os"
)

func main() {
	err := os.Remove("./some/file.txt")
	if err != nil {
		panic(err)
	}
}