12. Identify particular types of files

12.1. Introduction

In lot of cases, you we have to identify the type of file processed by MFDATA In most cases, you may want to identify the type of file your MFDATA plugin have to to deal with.

In order to do this, you configure the switch_logical_condition parameter in your plugin config.ini file, e.g. to identifiy JPEG files

switch_logical_condition = ( x['latest.switch.main.system_magic'].startswith(b'JPEG image') )

By default, MDFATA uses the default Linux magic file in its attempt to identify the type of a file. For further information about magic, see https://en.wikipedia.org/wiki/File_(command) or http://man7.org/linux/man-pages/man3/libmagic.3.html.

To locate the magic file path, enter:

file --version

Output:

file-5.11
magic file from /etc/magic:/usr/share/misc/magic

So it uses both the /etc/magic file, and the /usr/share/misc/magic. The magic file contains lines describing magic numbers, which identify particular types of files.

In order to identify type which are not in the default magic file or if the file identification doesn’t suit you, you may:

  • either create your own magic file with your identification rule(s) and put it in you plugin directory. This will be available only in your plugin context.** This is the recommended way in order to manage custom magic file**.
  • or add the your identification rule(s) in the /etc/magic file. This will be available in the Linux system.

12.2. Example:

Identify grib files.

By default grib file are identified as data:

file example.grib

Output:

example.grib: data

Create a new magic file in your plugin directory and add grib identification rules:

# GRIB
0   string  GRIB    GRIB file

Save the magic file and enter the file with -m {magic_file) where magic_file is the path to your magic file:

file example.grib -m magic

Output:

example.grib: GRIB file

For further information, check Linux documentation (Linux man file and man magic commands)

CAUTION: so that the switch plugin can detect the magic file stored in your plugin root directory, the magic file must be named magic.

From now, you are able to identify GRIB files in your plugin, by setting the switch_logical_condition parameter in your plugin config.ini file:

switch_logical_condition = ( x['latest.switch.main.{your_plugin_name}_magic'].startswith(b'GRIB file') )

or

switch_logical_condition = ( x['latest.switch.main.{your_plugin_name}_magic'].contains(b'GRIB file') )

where {your_plugin_name} is the name of your plugin.

CAUTION: Because you create a custom magic file, the condition must be set on latest.switch.main.{your_plugin_name}_magic instead of latest.switch.main.system_magic.