How to check if a file exists in Go?

Answer by Caleb Spare posted in gonuts mailing list.

[...] It's not actually needed very often and [...] using os.Stat is easy enough for the cases where it is required.

[...] For instance: if you are going to open the file, there's no reason to check whether it exists first. The file could disappear in between checking and opening, and anyway you'll need to check the os.Open error regardless. So you simply call os.IsNotExist(err) after you try to open the file, and deal with its non-existence there (if that requires special handling).

[...] You don't need to check for the paths existing at all (and you shouldn't).

• os.MkdirAll works whether or not the paths already exist. (Also you need to check the error from that call.)

• Instead of using os.Create, you should use os.OpenFile(path, os.O_RDWR|os.O_CREATE|os.O_EXCL, 0666) . That way you'll get an error if the file already exists. Also this doesn't have a race condition with something else making the file, unlike your version which checks for existence beforehand.

Taken from: https://groups.google.com/forum/#!msg/golang-nuts/Ayx-BMNdMFo/4rL8FFHr8v4J

To check if a file doesn't exist, equivalent to Python's if not os.path.exists(filename):

if _, err := os.Stat("/path/to/whatever"); errors.Is(err, os.ErrNotExist) {
  // path/to/whatever does not exist
}

To check if a file exists, equivalent to Python's if os.path.exists(filename):

Edited: per recent comments

if _, err := os.Stat("/path/to/whatever"); err == nil {
  // path/to/whatever exists

} else if errors.Is(err, os.ErrNotExist) {
  // path/to/whatever does *not* exist

} else {
  // Schrodinger: file may or may not exist. See err for details.

  // Therefore, do *NOT* use !os.IsNotExist(err) to test for file existence


}

You should use the os.Stat() and os.IsNotExist() functions as in the following example:

func Exists(name string) (bool, error) {
    _, err := os.Stat(name)
    if err == nil {
        return true, nil
    }
    if errors.Is(err, os.ErrNotExist) {
        return false, nil
    }
    return false, err
}

edit1: fixed issue of returning true when under some circumstances.
edit2: switched to using errors.Is() from os.IsNotExist(), which many say is a best-practice and here

The first thing to consider is that it is rare that you would only want to check whether or not a file exists. In most situations, you're trying to do something with the file if it exists. In Go, any time you try to perform some operation on a file that doesn't exist, the result should be a specific error (os.ErrNotExist) and the best thing to do is check whether the return err value (e.g. when calling a function like os.OpenFile(...)) is os.ErrNotExist.

The recommended way to do this used to be:

file, err := os.OpenFile(...)
if os.IsNotExist(err) {
    // handle the case where the file doesn't exist
}

However, since the addition of errors.Is in Go 1.13 (released in late 2019), the new recommendation is to use errors.Is:

file, err := os.OpenFile(...)
if errors.Is(err, os.ErrNotExist) {
    // handle the case where the file doesn't exist
}

---

It's usually best to avoid using os.Stat to check for the existence of a file before you attempt to do something with it, because it will always be possible for the file to be renamed, deleted, etc. in the window of time before you do something with it.

However, if you're OK with this caveat and you really, truly just want to check whether a file exists without then proceeding to do something useful with it (as a contrived example, let's say that you're writing a pointless CLI tool that tells you whether or not a file exists and then exits ¯\_(ツ)_/¯), then the recommended way to do it would be:

if _, err := os.Stat(filename); errors.Is(err, os.ErrNotExist) {
    // file does not exist
} else {
    // file exists
}