EC
os.ErrExist
GoERRORNotableFilesystem

file already exists

Quick Answer

Use errors.Is(err, os.ErrExist) to skip creation when the file already exists.

What this means

Returned when an OS operation fails because the target file or directory already exists. Common with os.Mkdir and exclusive creates.

Why it happens
  1. 1Creating a file with os.O_EXCL when it already exists
  2. 2Calling os.Mkdir on an existing directory path

Fix

Guard with errors.Is

Guard with errors.Is
err := os.Mkdir("dir", 0755)
if err != nil && !errors.Is(err, os.ErrExist) {
    log.Fatal(err)
}

Why this works

Ignoring os.ErrExist allows idempotent directory creation.

Code examples
Detectgo
err := os.Mkdir("out", 0755)
if errors.Is(err, os.ErrExist) {
    fmt.Println("already exists")
}
MkdirAll avoids errorgo
// MkdirAll never returns ErrExist
err := os.MkdirAll("a/b/c", 0755)
Exclusive creatego
f, err := os.OpenFile("lock",
    os.O_CREATE|os.O_EXCL, 0600)
Sources
Official documentation ↗

Go standard library

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