Created
February 4, 2016 08:30
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os.IsExist(err) vs os.IsNotExist(err)
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/* | |
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.Mkdir(target) | |
// 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.Chdir() | |
os.Stat() | |
os.Open() | |
os.OpenFile() // without os.O_EXCL | |
os.Chmod() | |
os.Chown() | |
os.Close() | |
os.Read() | |
os.ReadAt() | |
os.ReadDir() | |
os.Readdirnames() | |
os.Seek() | |
os.Truncate() | |
os.Write() | |
os.WriteAt() | |
os.WriteString() | |
// etc... | |
// More reading : | |
// http://stackoverflow.com/a/12518877 | |
// http://stackoverflow.com/questions/25939584/file-both-exists-and-not-exists-in-go/25939743#25939743 | |
// | |
// https://github.com/golang/go/search?utf8=%E2%9C%93&q=IsExist | |
// https://golang.org/pkg/os/#IsExist | |
// | |
// http://stackoverflow.com/search?q=os.IsExist | |
// http://stackoverflow.com/search?q=os.IsNotExist |
Thanks
helpful!! Thanks
thanks a lot,
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Good! Thanks!