Writing files

Write the whole file

The simplest way to write data to a file:

d := []byte("content of the file")
err := ioutil.WriteFile("foo.txt", d, 0644)
if err != nil {
    log.Fatalf("ioutil.WriteFile failed with '%s'\n", err)
}

Open file for writing

f, err := os.Create("foo.txt")
if err != nil {
    log.Fatalf("os.Open failed with '%s'\n", err)
}

Create returns *os.File which implements io.Writer and io.Closer interfaces.

If file doesn’t exist, it’ll be created.

If file does exist, it’ll be truncated.

You should always close files to avoid leaking file descriptors.

Be aware that Close on a file can return an error so for a robust code you should check for errors from Close.

Writes can be buffered and Close might need to flush remaining cached bytes to disk. That might fail and return an error.

Open file for appending

f, err := os.OpenFile(filePath, os.O_WRONLY | os.O_APPEND | os.O_CREATE, 0666)
if err != nil {
    log.Fatalf("os.Open failed with '%s'\n", err)
}

Second argument to os.OpenFile is a flag that determines the exact mode for opening the file.

When you open for reading, use os.Open.

When you open for writing to a new file, use os.Create.

When you open for appending to existing file, use os.OpenFile with the following flags: * os.O_WRONLY. Could also be os.RDWR if we also to both read and write * os.O_APPEND means that if file exists, we’ll append * os.O_CREATE means that if file doesn’t exist, we’ll create it. Without this flag opening non-existing file would fail

Write to a file

d := []byte("data to write")
nWritten, err := f.Write(d)

File permissions when creating files

When you create a new file with os.Create or os.OpenFile, you need to provide file permissions for the new file.

For most cases 0644 is a good choice.

Those are Unix style permissions in octal format.

Let’s deconstruct parts of 0644

• 0 means this is a number in octal format. That means that each number is in 0-7 range (compare to regular decimal notation where each number is in 0-9 range)
• 6 is permissions for the creator of the file. It’s a bitmask 4 + 2. 4 means read permissions, 2 means write permission. When combined it means read/write access
• 4 is permissions for the group that user belongs to. I.e. every user that also belongs to the same group will have those permissions. 4 means only read permissions
• last 4 is for everyone. Again, 4 means only read permissions
• for completeness, 1 represents executable permissions so if this is meant to be an executable file, the permissions flag would be 0755 i.e. for each user/group/anonymous component we add 1 to set executable bit

Those permissions are native to Unix and Mac OS but they map loosely on Windows.

To manage Windows permissions you need to use Windows API.