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Created February 4, 2016 08:30
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os.IsExist(err) vs os.IsNotExist(err)
Watch out, os.IsExist(err) != !os.IsNotExist(err)
They are error checkers, so use them only when err != nil, and you want to handle
specific errors in a different way!
Their main purpose is to wrap around OS error messages for you, so you don't have to test
for Windows/Unix/Mobile/other OS error messages for "file exists/directory exists" and
"file does not exist/directory does not exist"
This way you can handle such an error, while still failing on other unexpected errors like "permission denied",
"filesystem error", "wrong filename" error, etc.
// We want to create a symlink, but the target path exists already :
if _, err := os.Symlink("/old/path", "/path/to/whatever"); os.IsExist(err) {
// error happened, can't Symlink
// can't create symlink because /path/to/whatever already exists
// We want to stat a file, but it doesn't exist :
if _, err := os.Stat("/path/to/whatever"); os.IsNotExist(err) {
// error happened, can't Stat!
// /path/to/whatever does not exist
// BUT :
// Anti-pattern : We want to stat a file, and continue if it exists :
if _, err := os.Stat("/file/that/exists"); os.IsExist(err) {
// will never trigger!
// why? because os.Stat runs normally if file exists.
// it's expected behaviour for os.Stat, so it doesn't throw an error
// os.IsExist() does not receive an error ( it's nil ), so it can't tell you
// if the error message was "file not found"
// Instead :
// We want to stat a file, and continue if it exists :
_, err := os.Stat("/file/that/exists");
if err != nil {
if os.IsNotExist(err) {
// file does not exist, do something
} else {
// more serious errors
// file exists.. continue with code here, or in else statement, or specify if err == nil { // do something }
os.IsExist(err) is good for cases when you expect the file to not exist yet,
but the file actually exists :
os.Symlink("/path/that/exists", "/path/to/symlink/target")
// os.IsExist(err) will trigger when target exists already
// os.IsExist(err) will trigger because target path already exists
os.OpenFile(target, os.O_RDWR|os.O_CREATE|os.O_EXCL, 0600)
// os.IsExist(err) will trigger when target exists because O_EXCL means that file should not exist yet
os.IsNotExist(err) is good for more common cases where you expect the file to exists,
but it actually doesn't exist :
os.OpenFile() // without os.O_EXCL
// etc...
// More reading :
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Seekload commented Mar 9, 2021

Good! Thanks!

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Good! Thanks!

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ByZain commented Sep 23, 2022

helpful!! Thanks

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Zar-H commented Jan 19, 2024

thanks a lot,

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