Querying from the command line

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The default Comunica query engine that exposes most standard features is Comunica SPARQL, which uses the package name @comunica/actor-init-sparql. In this guide, we will install it globally, and show how it can be invoked from the command line.

This guide discusses querying over Linked Data on the Web, exposed via URLs. Click here to query over local RDF files instead.

1. Installation

Since Comunica runs on Node.js, make sure you have Node.js installed on your machine.

Next, we can install Comunica SPARQL on our machine:

$ npm install -g @comunica/actor-init-sparql

2. SPARQL querying over one source

After installing Comunica SPARQL, you will be given access to several commands including comunica-sparql, which allows you to execute SPARQL queries from the command line.

This command requires one or more URLs to be provided as sources to query over. As last argument, as SPARQL query string can be provided.

For example, the following query retrieves the first 100 triples from DBpedia:

$ comunica-sparql https://fragments.dbpedia.org/2016-04/en \
    "SELECT * WHERE { ?s ?p ?o } LIMIT 100"

Given a URL, Comunica will automatically detect the type of source and handle it accordingly.

As output, a JSON array of bindings for the selected variables will be returned:

[
{"?s":"https://fragments.dbpedia.org/2016-04/en#dataset","?p":"http://www.w3.org/1999/02/22-rdf-syntax-ns#type","?o":"http://rdfs.org/ns/void#datasource"},
{"?s":"https://fragments.dbpedia.org/2016-04/en#dataset","?p":"http://www.w3.org/1999/02/22-rdf-syntax-ns#type","?o":"http://www.w3.org/ns/hydra/core#Collection"},
{"?s":"https://fragments.dbpedia.org/2016-04/en#dataset","?p":"http://www.w3.org/ns/hydra/core#search","?o":"https://fragments.dbpedia.org/2016-04/en#triplePattern"}
...

3. Query file input

Since SPARQL queries can sometimes become very large, it is possible to supply them via a local file using the -f option.

Assuming a file path/myquery.sparql exists, we can query over it as follows:

$ comunica-sparql https://fragments.dbpedia.org/2016-04/en -f path/myquery.sparql

4. SPARQL querying over multiple sources

One key feature of Comunica is its ability to query over multiple sources. For this, you can just supply any number of URLs as arguments. Just make sure that the last argument remains your query.

$ comunica-sparql https://fragments.dbpedia.org/2016-04/en \
    https://www.rubensworks.net/ \
    https://ruben.verborgh.org/profile/ \
    "SELECT * WHERE { ?s ?p ?o } LIMIT 100"

5. SPARQL CONSTRUCT and ASK

Next to SPARQL SELECT queries, it is also possible to execute CONSTRUCT queries to produce RDF triples:

$ comunica-sparql https://fragments.dbpedia.org/2016-04/en \
    "CONSTRUCT WHERE { ?s ?p ?o } LIMIT 100"

<http://0-access.newspaperarchive.com.topcat.switchinc.org/Viewer.aspx?img=8716084> <http://dbpedia.org/property/accessdate> "2010-04-21"^^<http://www.w3.org/2001/XMLSchema#date>;
    <http://dbpedia.org/property/date> "1939-01-02"^^<http://www.w3.org/2001/XMLSchema#date>;
    <http://dbpedia.org/property/format> "PDF";
    <http://dbpedia.org/property/isCitedBy> <http://dbpedia.org/resource/List_of_Attorneys_General_of_Wisconsin>;
    <http://dbpedia.org/property/location> "Sheboygan, Wisconsin";
    <http://dbpedia.org/property/page> "1";
...

ASK queries will produce a boolean output:

$ comunica-sparql https://fragments.dbpedia.org/2016-04/en \
    "ASK { ?s ?p ?o }"

true

6. Changing result format

SELECT queries will be printed as JSON by default, and CONSTRUCT queries as RDF TriG. This can be overridden using the -t option.

For example, displaying results as SPARQL JSON results:

$ comunica-sparql https://fragments.dbpedia.org/2016-04/en \
    "SELECT * WHERE { ?s ?p ?o } LIMIT 100" \
    -t 'application/sparql-results+json'

{"head": {"vars":["s","p","o"]},
"results": { "bindings": [
{"s":{"value":"http://0-access.newspaperarchive.com.lib.utep.edu/us/mississippi/biloxi/biloxi-daily-herald/1899/05-06/page-6?tag=tierce+wine&rtserp=tags/tierce-wine?page=2","type":"uri"},"p":{"value":"http://dbpedia.org/property/date","type":"uri"},"o":{"value":"1899-05-06","type":"literal","datatype":"http://www.w3.org/2001/XMLSchema#date"}},
{"s":{"value":"http://0-access.newspaperarchive.com.lib.utep.edu/us/mississippi/biloxi/biloxi-daily-herald/1899/05-06/page-6?tag=tierce+wine&rtserp=tags/tierce-wine?page=2","type":"uri"},"p":{"value":"http://dbpedia.org/property/isCitedBy","type":"uri"},"o":{"value":"http://dbpedia.org/resource/Tierce_(unit)","type":"uri"}},
{"s":{"value":"http://0-access.newspaperarchive.com.lib.utep.edu/us/mississippi/biloxi/biloxi-daily-herald/1899/05-06/page-6?tag=tierce+wine&rtserp=tags/tierce-wine?page=2","type":"uri"},"p":{"value":"http://dbpedia.org/property/newspaper","type":"uri"},"o":{"value":"Biloxi Daily Herald","type":"literal"}},
{"s":{"value":"http://0-access.newspaperarchive.com.lib.utep.edu/us/mississippi/biloxi/biloxi-daily-herald/1899/05-06/page-6?tag=tierce+wine&rtserp=tags/tierce-wine?page=2","type":"uri"},"p":{"value":"http://dbpedia.org/property/page","type":"uri"},"o":{"value":"6","type":"literal"}},
{"s":{"value":"http://0-access.newspaperarchive.com.lib.utep.edu/us/mississippi/biloxi/biloxi-daily-herald/1899/05-06/page-6?tag=tierce+wine&rtserp=tags/tierce-wine?page=2","type":"uri"},"p":{"value":"http://dbpedia.org/property/title","type":"uri"},"o":{"value":"A New System of Weights and Measures","type":"literal"}},
{"s":{"value":"http://0-access.newspaperarchive.com.lib.utep.edu/us/mississippi/biloxi/biloxi-daily-herald/1899/05-06/page-6?tag=tierce+wine&rtserp=tags/tierce-wine?page=2","type":"uri"},"p":{"value":"http://dbpedia.org/property/url","type":"uri"},"o":{"value":"http://0-access.newspaperarchive.com.lib.utep.edu/us/mississippi/biloxi/biloxi-daily-herald/1899/05-06/page-6?tag=tierce+wine&rtserp=tags/tierce-wine?page=2","type":"uri"}},
...

All available formats can be printed via comunica-sparql --listformats

7. Learn more

This guide only discussed the basic functionality of comunica-sparql. You can learn more options by invoking the help command:

$ comunica-sparql --help

Usage:
  comunica-sparql http://fragments.dbpedia.org/2016-04/en [-q] 'SELECT * WHERE { ?s ?p ?o }'
  comunica-sparql http://fragments.dbpedia.org/2016-04/en [-q] '{ hero { name friends { name } } }' -i graphql
  comunica-sparql http://fragments.dbpedia.org/2016-04/en [-f] query.sparql
  comunica-sparql http://fragments.dbpedia.org/2016-04/en https://query.wikidata.org/sparql ...
  comunica-sparql hypermedia@http://fragments.dbpedia.org/2016-04/en sparql@https://query.wikidata.org/sparql ...

Options:
  -q            evaluate the given SPARQL query string
  -f            evaluate the SPARQL query in the given file
  -c            use the given JSON configuration file (e.g., config.json)
  -t            the MIME type of the output (e.g., application/json)
  -i            the query input format (e.g., graphql, defaults to sparql)
  -b            base IRI for the query (e.g., http://example.org/)
  -l            sets the log level (e.g., debug, info, warn, ... defaults to warn)
  -d            sets a datetime for querying Memento-enabled archives
  -p            delegates all HTTP traffic through the given proxy (e.g. http://myproxy.org/?uri=)
  --lenient     if failing requests and parsing errors should be logged instead of causing a hard crash
  --help        print this help message
  --listformats prints the supported MIME types
  --version     prints version information