Extract boundaries from OpenStreetMap (as lines, to keep individual tags, like maritime=*)

boundary.ini

# put here the name of keys for ways that are assumed to be polygons if they are closed
# see http://wiki.openstreetmap.org/wiki/Map_Features
closed_ways_are_polygons=aeroway,amenity,boundary,building,craft,geological,historic,landuse,leisure,military,natural,office,place,shop,sport,tourism

# comment to avoid laundering of keys ( ':' turned into '_' )
attribute_name_laundering=yes

# uncomment to report all nodes, including the ones without any (significant) tag
report_all_nodes=yes

# uncomment to report all ways, including the ones without any (significant) tag
report_all_ways=yes


[multipolygons]
# common attributes
# note: for multipolygons, osm_id=yes instanciates a osm_id field for the id of relations
# and a osm_way_id field for the id of closed ways. Both fields are exclusively set.
osm_id=yes
osm_version=no
osm_timestamp=no
osm_uid=no
osm_user=no
osm_changeset=no

# keys to report as OGR fields
attributes=name,name:fr,name:en,name:ar,name:es,boundary,border_type,admin_level,maritime
# keys that should NOT be reported in the "other_tags" field
ignore=area,created_by,converted_by,source,time,ele,note,openGeoDB:,fixme,FIXME
# uncomment to avoid creation of "other_tags" field
other_tags=no


[multilinestrings]
# common attributes
# note: for multipolygons, osm_id=yes instanciates a osm_id field for the id of relations
# and a osm_way_id field for the id of closed ways. Both fields are exclusively set.
osm_id=yes
osm_version=no
osm_timestamp=no
osm_uid=no
osm_user=no
osm_changeset=no

# keys to report as OGR fields
attributes=name,name:fr,name:en,name:ar,name:es,boundary,border_type,admin_level,maritime
# keys that should NOT be reported in the "other_tags" field
ignore=area,created_by,converted_by,source,time,ele,note,openGeoDB:,fixme,FIXME
# uncomment to avoid creation of "other_tags" field
other_tags=no

[lines]
# common attributes
# note: for multipolygons, osm_id=yes instanciates a osm_id field for the id of relations
# and a osm_way_id field for the id of closed ways. Both fields are exclusively set.
osm_id=yes
osm_version=no
osm_timestamp=no
osm_uid=no
osm_user=no
osm_changeset=no

# keys to report as OGR fields
attributes=name,name:fr,name:en,name:ar,name:es,boundary,border_type,admin_level,maritime
# keys that should NOT be reported in the "other_tags" field
ignore=area,created_by,converted_by,source,time,ele,note,openGeoDB:,fixme,FIXME
# uncomment to avoid creation of "other_tags" field
other_tags=no

extract.py

"""
Filter all objects with a coastline tag.

This example shows how to write objects to a file.

We need to go twice over the file. First read the ways, filter the ones
we are interested in and remember the nodes required. Then, in a second
run all the relevant nodes and ways are written out.
"""

import osmium
import sys


class WayFilter(osmium.SimpleHandler):

    def __init__(self):
        super().__init__()
        self.ways = set()

    def relation(self, rel):
        if (rel.tags.get('boundary') == 'administrative' and
           rel.tags.get('admin_level') == '2'):
            for way in rel.members:
                self.ways.add(way.ref)


class NodeFilter(osmium.SimpleHandler):

    def __init__(self, ways):
        super().__init__()
        self.nodes = set()
        self.ways = ways

    def way(self, way):
        if way.id in self.ways:
            for node in way.nodes:
                self.nodes.add(node.ref)


class BoundaryWriter(osmium.SimpleHandler):

    def __init__(self, writer, ways, nodes):
        super().__init__()
        self.writer = writer
        self.nodes = nodes
        self.ways = ways

    def node(self, node):
        if node.id in self.nodes:
            self.writer.add_node(node)

    def way(self, way):
        if way.id in self.ways:
            self.writer.add_way(way)


if __name__ == '__main__':
    if len(sys.argv) != 3:
        print("Usage: python extract.py <infile> <outfile>")
        sys.exit(-1)

    # go through the relations to find all relevant ways
    ways = WayFilter()
    ways.apply_file(sys.argv[1])

    # go through the ways to find all relevant nodes
    nodes = NodeFilter(ways.ways)
    nodes.apply_file(sys.argv[1])

    # go through the file again and write out the data
    writer = osmium.SimpleWriter(sys.argv[2])
    BoundaryWriter(writer, ways.ways, nodes.nodes).apply_file(sys.argv[1])

    writer.close()

run.sh

# Get the planet (38G)
 wget https://ftp5.gwdg.de/pub/misc/openstreetmap/planet.openstreetmap.org/pbf/planet-latest.osm.pbf

# Install Osmium
git clone https://github.com/osmcode/libosmium.git
git clone https://github.com/osmcode/osmium-tool.git
cd osmium-tool/
mkdir build
cd build
cmake ..
make

# Filter boundaries
./osmium tags-filter ../../planet-latest.osm.pbf r/admin_level=2 -o ../../boundary.osm.pbf

# Extract ways
python extract.py boundary.osm.pbf boundary_lines.osm.pbf

# Transform to SHP with ogr2ogr
ogr2ogr --config OSM_CONFIG_FILE boundary.ini -skipfailures boundary.shp  boundary_lines.osm.pbf -sql "SELECT * FROM multipolygons WHERE admin_level='2'" -lco ENCODING=UTF-8