err := os.RemoveAll(filePath)
if err != nil {
fmt.Println("cannot delete the file", err)
return nil, err
}
Currently the above code is deleting the files having the read only(tried for 0444) file permissions.
Are there any particular file permissions, which won't allow me to delete the files or the folder?
The key thing is to have write permissions on the directory that contains the files as this example shows
package main
import (
"fmt"
"os"
)
func dostuff(p os.FileMode, n string) {
err := os.Mkdir(n, 0700) // make as writable
_, err = os.Create(n + "/a")
_, err = os.Create(n + "/b")
os.Chmod(n, p) // alter permissions to see what happens
err = os.RemoveAll(n)
if err != nil {
fmt.Println("cannot delete the file", err)
}
}
func main() {
dostuff(0700, "writeallowed")
dostuff(0400, "readonly")
}
Related
I have a file of huge size for example 100MB, I need to chunk it into 4 25MB files using golang.
The thing here is, if i use go routine and read the file, the order of the data inside the files are not preserved. the code i used is
package main
import (
"bufio"
"fmt"
"log"
"os"
"sync"
"github.com/google/uuid"
)
func main() {
file, err := os.Open("sampletest.txt")
if err != nil {
log.Fatal(err)
}
defer file.Close()
lines := make(chan string)
// start four workers to do the heavy lifting
wc1 := startWorker(lines)
wc2 := startWorker(lines)
wc3 := startWorker(lines)
wc4 := startWorker(lines)
scanner := bufio.NewScanner(file)
go func() {
defer close(lines)
for scanner.Scan() {
lines <- scanner.Text()
}
if err := scanner.Err(); err != nil {
log.Fatal(err)
}
}()
writefiles(wc1, wc2, wc3, wc4)
}
func writefile(data string) {
file, err := os.Create("chunks/" + uuid.New().String() + ".txt")
if err != nil {
fmt.Println(err)
}
defer file.Close()
file.WriteString(data)
}
func startWorker(lines <-chan string) <-chan string {
finished := make(chan string)
go func() {
defer close(finished)
for line := range lines {
finished <- line
}
}()
return finished
}
func writefiles(cs ...<-chan string) {
var wg sync.WaitGroup
output := func(c <-chan string) {
var d string
for n := range c {
d += n
d += "\n"
}
writefile(d)
wg.Done()
}
wg.Add(len(cs))
for _, c := range cs {
go output(c)
}
go func() {
wg.Wait()
}()
}
Here using this code my file got split into 4 equal files, but the order in it is not preserved.
I am very new to golang, any suggestions are highly appreciated.
I took this code from some site and tweaked here and there to meet my requirements.
I took this code from some site and tweaked here and there to meet my requirements.
Based on your statement, you should be able to modify the code from running concurrently to sequentially, it's faaar easier than applying concurrent aspect to existing code.
The work is basically just: remove the concurrent part.
Anyway, below is a simple example of how to achieve what you want. I use your code as the base, and then I remove everything related to concurrent process.
package main
import (
"bufio"
"fmt"
"log"
"os"
"strings"
"github.com/google/uuid"
)
func main() {
split := 4
file, err := os.Open("file.txt")
if err != nil {
log.Fatal(err)
}
defer file.Close()
scanner := bufio.NewScanner(file)
texts := make([]string, 0)
for scanner.Scan() {
text := scanner.Text()
texts = append(texts, text)
}
if err := scanner.Err(); err != nil {
log.Fatal(err)
}
lengthPerSplit := len(texts) / split
for i := 0; i < split; i++ {
if i+1 == split {
chunkTexts := texts[i*lengthPerSplit:]
writefile(strings.Join(chunkTexts, "\n"))
} else {
chunkTexts := texts[i*lengthPerSplit : (i+1)*lengthPerSplit]
writefile(strings.Join(chunkTexts, "\n"))
}
}
}
func writefile(data string) {
file, err := os.Create("chunks-" + uuid.New().String() + ".txt")
if err != nil {
fmt.Println(err)
}
defer file.Close()
file.WriteString(data)
}
Here is a simple file splitter. You can handle the leftovers yourself, I added the leftover bytes to 5th file.
package main
import (
"bufio"
"fmt"
"os"
)
func main() {
file, err := os.Open("sample-text-file.txt")
if err != nil {
panic(err)
}
defer file.Close()
// to divide file in four chunks
info, _ := file.Stat()
chunkSize := int(info.Size() / 4)
// reader of chunk size
bufR := bufio.NewReaderSize(file, chunkSize)
// Notice the range over slice of len 5, after 4 leftover will be written to 5th file
for i := range [5]int{} {
reader := make([]byte, chunkSize)
rlen, err := bufR.Read(reader)
fmt.Println("Read: ", rlen)
if err != nil {
panic(err)
}
writeFile(i, rlen, &reader)
}
}
// Notice bufW as a pointer to avoid exchange of big byte slices
func writeFile(i int, rlen int, bufW *[]byte) {
fname := fmt.Sprintf("file_%v", i)
f, err := os.Create(fname)
defer f.Close()
w := bufio.NewWriterSize(f, rlen)
wbytes := *(bufW)
wLen, err := w.Write(wbytes[:rlen])
if err != nil {
panic(err)
}
fmt.Println("Wrote ", wLen, "to", fname)
w.Flush()
}
I have written a function to read zip archive tomap[string]*zip.File.
func ReadZip(file string) (map[string]*zip.File, error) {
r, err := zip.OpenReader(file)
if err != nil {
return nil, err
}
defer r.Close()
files := make(map[string]*zip.File)
for _, f := range r.File {
files[f.Name] = f
}
return files, nil
}
But when i try to open file infoRC, err := f["info.json"].Open() arises error
read file.zip: bad file descriptor.
Is there better way to read zip archive?
Once ReadCloser.Close is called, any of the *zip.File structs are invalid:
Close closes the Zip file, rendering it unusable for I/O.
You need to either:
Keep r open as long as you want to read the ZIP entries, or
make an in-memory/temporary file copy of all of the zip file contents
An example of the latter option:
func ReadZip(file string) (map[string][]byte, error) {
r, err := zip.OpenReader(file)
if err != nil {
return nil, err
}
defer r.Close()
files := make(map[string][]byte)
for _, f := range r.File {
fc, err := f.Open()
if err != nil {
return nil, err
}
contents, err := ioutil.ReadAll(fc)
fc.Close()
if err != nil {
return nil, err
}
files[f.Name] = contents
}
return files, nil
}
Is there any simple/fast way to copy a file in Go?
I couldn't find a fast way in the Doc's and searching the internet doesn't help as well.
Warning: This answer is mainly about adding a hard link to a file, not about copying the contents.
A robust and efficient copy is conceptually simple, but not simple to implement due to the need to handle a number of edge cases and system limitations that are imposed by the target operating system and it's configuration.
If you simply want to make a duplicate of the existing file you can use os.Link(srcName, dstName). This avoids having to move bytes around in the application and saves disk space. For large files, this is a significant time and space saving.
But various operating systems have different restrictions on how hard links work. Depending on your application and your target system configuration, Link() calls may not work in all cases.
If you want a single generic, robust and efficient copy function, update Copy() to:
Perform checks to ensure that at least some form of copy will succeed (access permissions, directories exist, etc.)
Check to see if both files already exist and are the same using
os.SameFile, return success if they are the same
Attempt a Link, return if success
Copy the bytes (all efficient means failed), return result
An optimization would be to copy the bytes in a go routine so the caller doesn't block on the byte copy. Doing so imposes additional complexity on the caller to handle the success/error case properly.
If I wanted both, I would have two different copy functions: CopyFile(src, dst string) (error) for a blocking copy and CopyFileAsync(src, dst string) (chan c, error) which passes a signaling channel back to the caller for the asynchronous case.
package main
import (
"fmt"
"io"
"os"
)
// CopyFile copies a file from src to dst. If src and dst files exist, and are
// the same, then return success. Otherise, attempt to create a hard link
// between the two files. If that fail, copy the file contents from src to dst.
func CopyFile(src, dst string) (err error) {
sfi, err := os.Stat(src)
if err != nil {
return
}
if !sfi.Mode().IsRegular() {
// cannot copy non-regular files (e.g., directories,
// symlinks, devices, etc.)
return fmt.Errorf("CopyFile: non-regular source file %s (%q)", sfi.Name(), sfi.Mode().String())
}
dfi, err := os.Stat(dst)
if err != nil {
if !os.IsNotExist(err) {
return
}
} else {
if !(dfi.Mode().IsRegular()) {
return fmt.Errorf("CopyFile: non-regular destination file %s (%q)", dfi.Name(), dfi.Mode().String())
}
if os.SameFile(sfi, dfi) {
return
}
}
if err = os.Link(src, dst); err == nil {
return
}
err = copyFileContents(src, dst)
return
}
// copyFileContents copies the contents of the file named src to the file named
// by dst. The file will be created if it does not already exist. If the
// destination file exists, all it's contents will be replaced by the contents
// of the source file.
func copyFileContents(src, dst string) (err error) {
in, err := os.Open(src)
if err != nil {
return
}
defer in.Close()
out, err := os.Create(dst)
if err != nil {
return
}
defer func() {
cerr := out.Close()
if err == nil {
err = cerr
}
}()
if _, err = io.Copy(out, in); err != nil {
return
}
err = out.Sync()
return
}
func main() {
fmt.Printf("Copying %s to %s\n", os.Args[1], os.Args[2])
err := CopyFile(os.Args[1], os.Args[2])
if err != nil {
fmt.Printf("CopyFile failed %q\n", err)
} else {
fmt.Printf("CopyFile succeeded\n")
}
}
import (
"io/ioutil"
"log"
)
func checkErr(err error) {
if err != nil {
log.Fatal(err)
}
}
func copy(src string, dst string) {
// Read all content of src to data, may cause OOM for a large file.
data, err := ioutil.ReadFile(src)
checkErr(err)
// Write data to dst
err = ioutil.WriteFile(dst, data, 0644)
checkErr(err)
}
If you are running the code in linux/mac, you could just execute the system's cp command.
srcFolder := "copy/from/path"
destFolder := "copy/to/path"
cpCmd := exec.Command("cp", "-rf", srcFolder, destFolder)
err := cpCmd.Run()
It's treating go a bit like a script, but it gets the job done. Also, you need to import "os/exec"
Starting with Go 1.15 (Aug 2020), you can use File.ReadFrom:
package main
import "os"
func main() {
r, err := os.Open("in.txt")
if err != nil {
panic(err)
}
defer r.Close()
w, err := os.Create("out.txt")
if err != nil {
panic(err)
}
defer w.Close()
w.ReadFrom(r)
}
Perform the copy in a stream, using io.Copy.
Close all opened file descriptors.
All errors that should be checked are checked, including the errors in deferred (*os.File).Close calls.
Gracefully handle multiple non-nil errors, e.g. non-nil errors from both io.Copy and (*os.File).Close.
No unnecessary complications that were present in other answers, such as calling Close twice on the same file but ignoring the error on one of the calls.
No unnecessary stat checks for existence or for file type. These checks aren't necessary: the future open and read operations will return an error anyway if it's not a valid operation for the type of file. Secondly, such checks are prone to races (e.g. the file might be removed in the time between stat and open).
Accurate doc comment. See: "file", "regular file", and behavior when dstpath exists. The doc comment also matches the style of other functions in package os.
// Copy copies the contents of the file at srcpath to a regular file at dstpath.
// If dstpath already exists and is not a directory, the function truncates it.
// The function does not copy file modes or file attributes.
func Copy(srcpath, dstpath string) (err error) {
r, err := os.Open(srcpath)
if err != nil {
return err
}
defer r.Close() // ok to ignore error: file was opened read-only.
w, err := os.Create(dstpath)
if err != nil {
return err
}
defer func() {
e := w.Close()
// Report the error from Close, if any.
// But do so only if there isn't already
// an outgoing error.
if e != nil && err == nil {
err = e
}
}()
_, err = io.Copy(w, r)
return err
}
In this case there are a couple of conditions to verify, I prefer non-nested code
func Copy(src, dst string) (int64, error) {
src_file, err := os.Open(src)
if err != nil {
return 0, err
}
defer src_file.Close()
src_file_stat, err := src_file.Stat()
if err != nil {
return 0, err
}
if !src_file_stat.Mode().IsRegular() {
return 0, fmt.Errorf("%s is not a regular file", src)
}
dst_file, err := os.Create(dst)
if err != nil {
return 0, err
}
defer dst_file.Close()
return io.Copy(dst_file, src_file)
}
If you are on windows, you can wrap CopyFileW like this:
package utils
import (
"syscall"
"unsafe"
)
var (
modkernel32 = syscall.NewLazyDLL("kernel32.dll")
procCopyFileW = modkernel32.NewProc("CopyFileW")
)
// CopyFile wraps windows function CopyFileW
func CopyFile(src, dst string, failIfExists bool) error {
lpExistingFileName, err := syscall.UTF16PtrFromString(src)
if err != nil {
return err
}
lpNewFileName, err := syscall.UTF16PtrFromString(dst)
if err != nil {
return err
}
var bFailIfExists uint32
if failIfExists {
bFailIfExists = 1
} else {
bFailIfExists = 0
}
r1, _, err := syscall.Syscall(
procCopyFileW.Addr(),
3,
uintptr(unsafe.Pointer(lpExistingFileName)),
uintptr(unsafe.Pointer(lpNewFileName)),
uintptr(bFailIfExists))
if r1 == 0 {
return err
}
return nil
}
Code is inspired by wrappers in C:\Go\src\syscall\zsyscall_windows.go
Here is an obvious way to copy a file:
package main
import (
"os"
"log"
"io"
)
func main() {
sFile, err := os.Open("test.txt")
if err != nil {
log.Fatal(err)
}
defer sFile.Close()
eFile, err := os.Create("test_copy.txt")
if err != nil {
log.Fatal(err)
}
defer eFile.Close()
_, err = io.Copy(eFile, sFile) // first var shows number of bytes
if err != nil {
log.Fatal(err)
}
err = eFile.Sync()
if err != nil {
log.Fatal(err)
}
}
You can use "exec".
exec.Command("cmd","/c","copy","fileToBeCopied destinationDirectory") for windows
I have used this and its working fine. You can refer manual for more details on exec.
I looked up golang.org/pkg/os/#File , but still have no idea.
Seems there is no way to get file length, did I miss something?
How to get file length in Go?
(*os.File).Stat() returns a os.FileInfo value, which in turn has a Size() method. So, given a file f, the code would be akin to
fi, err := f.Stat()
if err != nil {
// Could not obtain stat, handle error
}
fmt.Printf("The file is %d bytes long", fi.Size())
If you don't want to open the file, you can directly call os.Stat instead.
fi, err := os.Stat("/path/to/file")
if err != nil {
return err
}
// get the size
size := fi.Size()
Slightly more verbose answer:
file, err := os.Open( filepath )
if err != nil {
log.Fatal(err)
}
fi, err := file.Stat()
if err != nil {
log.Fatal(err)
}
fmt.Println( fi.Size() )
Calling os.Stat as sayed by #shebaw (at least in UNIX OS) is more efficient, cause stat() is a Unix system call that returns file attributes about an inode, and is not necessary to deal with open the file.
NOTE: Using other method can lead to too many open files in multithread/concurrency application, due to the fact that you open the file for query the stats
Here the benchmark
func GetFileSize1(filepath string) (int64, error) {
fi, err := os.Stat(filepath)
if err != nil {
return 0, err
}
// get the size
return fi.Size(), nil
}
func GetFileSize2(filepath string) (int64, error) {
f, err := os.Open(filepath)
if err != nil {
return 0, err
}
defer f.Close()
fi, err := f.Stat()
if err != nil {
return 0, err
}
return fi.Size(), nil
}
BenchmarkGetFileSize1-8 704618 1662 ns/op
BenchmarkGetFileSize2-8 199461 5668 ns/op
I'm trying to map an array to a file via Mmap, the array could be any type, like float64. In C, I find this one. After reading some texts, I wrote this sample. I don't know if it is correct, and it is not writing the values to the file. If I increase the size of array a lot, e.g from 1000 to 10000, it crashes. If someone know how to do that in the correctly way, please, tell me.
Thanks!
For example, revising your sample program,
package main
import (
"fmt"
"os"
"syscall"
"unsafe"
)
func main() {
const n = 1e3
t := int(unsafe.Sizeof(0)) * n
map_file, err := os.Create("/tmp/test.dat")
if err != nil {
fmt.Println(err)
os.Exit(1)
}
_, err = map_file.Seek(int64(t-1), 0)
if err != nil {
fmt.Println(err)
os.Exit(1)
}
_, err = map_file.Write([]byte(" "))
if err != nil {
fmt.Println(err)
os.Exit(1)
}
mmap, err := syscall.Mmap(int(map_file.Fd()), 0, int(t), syscall.PROT_READ|syscall.PROT_WRITE, syscall.MAP_SHARED)
if err != nil {
fmt.Println(err)
os.Exit(1)
}
map_array := (*[n]int)(unsafe.Pointer(&mmap[0]))
for i := 0; i < n; i++ {
map_array[i] = i * i
}
fmt.Println(*map_array)
err = syscall.Munmap(mmap)
if err != nil {
fmt.Println(err)
os.Exit(1)
}
err = map_file.Close()
if err != nil {
fmt.Println(err)
os.Exit(1)
}
}