How to Precisely Set File Permissions in Go using `os.FileMode`?

Instantiating os.FileMode for File Creation/Updating

Instantiating os.FileMode correctly ensures proper permission settings for files during creation or update. Many tutorials resort to setting permission bits directly, but a more precise approach involves utilizing the os.FileMode type.

Custom Constants for Permission Setting

In the absence of predefined constants in os or syscall, you can declare your own:

const (
    OS_READ = 04
    OS_WRITE = 02
    OS_EX = 01
    ...
    OS_ALL_RWX = OS_ALL_RW | OS_ALL_X
)

Example of Precision Permission Setting

Here's an example demonstrating the usage of custom constants:

func FileWrite(path string, r io.Reader, uid, gid int, perms string) (int64, error) {
    fileMode := os.FileMode(0)
    if perms[0] == '6' {
        fileMode |= OS_USER_RW
    } else if perms[0] == '7' {
        fileMode |= OS_USER_RWX
    }
    ...
    // Continue setting file mode based on the remaining characters of `perms`
    ...
    w, err := os.OpenFile(path, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, fileMode)
    ...
}

Benefits of Custom Constant Approach

Custom constants provide:

• Clear and portable permission-setting mechanism
• Elimination of magic numbers and potential confusion
• Flexibility to manipulate permissions based on desired scenarios

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