How do you check whether a file is a hard link in Go? os.FileMode only has a mode for symlinks, not hard links.
I had an idea that unfortunately doesn't work:
package main
func main() {
filename := os.Args[1]
var hardlink bool
link, _ := os.Readlink(filename)
fi, _ := os.Stat(filename)
mode := string(fi.Mode().String()[0])
if link != "" && mode != "L" {
hardlink = true
}
fmt.Printf("%v is hard link? %v
", filename, hardlink)
}
This^ doesn't work because os.Readlink reads only symlinks, not hard links.
I found a somewhat related answer:
Counting hard links to a file in Go
But this answer shows how to find the number of hard links to a file, not whether a file itself is a hard link.
I'm guessing that the syscall package used in that answer or, better yet, the sys package has a way to test whether a file's a hard link. Does anyone know to do this? (I have trouble understanding those packages because they're so low-level.)
I should add the reason why I'd like to check this. I'm trying to make a function to create a tar archive of a directory [using filepath.Walk()]. In this function, when I create the *tar.Header for a file, I set a value to *tar.Header.Typeflag.
For example, if fi is a file's *os.FileInfo variable and hdr is the *tar.Header variable for that file's place in a new tar archive, it looks like this:
if fi.Mode().IsDir() {
hdr.Typeflag = tar.TypeDir
}
In the tar package, the modes for hard links and regular files are distinct, TypeLink and TypeReg, but this isn't the case in the os package. So running this won't set the correct Typeflag:
hdr.Mode = int64(fi.Mode())
Figured it out from an example in Docker's source code:
https://github.com/docker/docker/blob/master/pkg/archive/archive.go
https://github.com/docker/docker/blob/master/pkg/archive/archive_unix.go
package main
import (
"errors"
"fmt"
"log"
"os"
"syscall"
)
func main() {
filename := os.Args[1]
// 'os.Lstat()' reads the link itself.
// 'os.Stat()' would read the link's target.
fi, err := os.Lstat(filename)
if err != nil {
log.Fatal(err)
}
// https://github.com/docker/docker/blob/master/pkg/archive/archive_unix.go
// in 'func setHeaderForSpecialDevice()'
s, ok := fi.Sys().(*syscall.Stat_t)
if !ok {
err = errors.New("cannot convert stat value to syscall.Stat_t")
log.Fatal(err)
}
// The index number of this file's inode:
inode := uint64(s.Ino)
// Total number of files/hardlinks connected to this file's inode:
nlink := uint32(s.Nlink)
// True if the file is a symlink.
if fi.Mode()&os.ModeSymlink != 0 {
link, err := os.Readlink(fi.Name())
if err != nil {
log.Fatal(err)
}
fmt.Printf("%v is a symlink to %v on inode %v.
", filename, link, inode)
os.Exit(0)
}
// Otherwise, for hardlinks:
fmt.Printf("The inode for %v, %v, has %v hardlinks.
", filename, inode, nlink)
if nlink > 1 {
fmt.Printf("Inode %v has %v other hardlinks besides %v.
", inode, nlink, filename)
} else {
fmt.Printf("%v is the only hardlink to inode %v.
", filename, inode)
}
}
Files are hard links to inodes, you can have multiple hard links to inodes and the file system will not distinguish between them. If you have two files that point to the same inode on disk and remove one the file will still exist. Space is only reclaimed by the file system once all links to the inode have been deleted.
You can use ls -i to determine the index, and stats <path to file> to work out how many links to the inode there are in your file system.
The term "hard link" is a bit of a misnomer. The way files work in most filesystems is that a given file (the thing identified by a path, like /foo) is actually just a pointer to a structure called an "inode." Inodes are the structures on disk that actually represent the contents and metadata of the file. When a file is a "hard link" to another file, it just means that they both point to the same inode. The system will keep track of the number of files pointing to a given inode, and make sure not to delete the inode until all files pointing to it have been deleted.
Thus, the question "is this file a hard link" doesn't really make sense. What does make sense is the question, "are these two files hard links to each other," or, more accurately, "are these two files pointers to the same inode." If you want to answer that question in Go, all you need is os.SameFile.