Get a writable temporary directory in Elixir

When you need to write a temporary file to disk it is common to assume a particular path exists based on the operating system the app is running on:

tmp_file = Path.join("/tmp", filename)
File.write!(tmp_file)

However depending on the configuration of the app as it is deployed in production, it may not have access to the /tmp directory. Fortunately Elixir provides a function that returns a writable path for temporary files:

dir = System.tmp_dir!()
tmp_file = Path.join(dir, filename)
File.write!(tmp_file)

From the docs (h System.tmp_dir):

Writable temporary directory.

Returns a writable temporary directory. Searches for directories in the following order:

    1. the directory named by the TMPDIR environment variable
    2. the directory named by the TEMP environment variable
    3. the directory named by the TMP environment variable
    4. C:\TMP on Windows or /tmp on Unix
    5. as a last resort, the current working directory

Returns nil if none of the above are writable.

I prefer to use the "bang" version System.tmp_dir! described as:

Same as tmp_dir/0 but raises RuntimeError instead of returning nil if no temp dir is set.

On my machine it returns something like this:

"/var/folders/66/dj7rwns53vn4db4_1npvqtrh0000gn/T/"