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/query-sparql.
In this guide, we will install it globally, and show how it can be invoked from the command line.
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/query-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"
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"}}, ...
comunica-sparql --listformats
7. Printing the query plan
Using the --explain option, the query plan can be printed via different explain modes.
8. Learn more
This guide only discussed the basic functionality of comunica-sparql.
You can learn more options by invoking the help command:
$ comunica-sparql evaluates SPARQL queries Recommended options: -q, --query Evaluate the given SPARQL query string [string] -f, --file Evaluate the SPARQL query in the given file [string] -i, --inputType Query input format (e.g., graphql, sparql) [string] [default: "sparql"] -t, --outputType MIME type of the output (e.g., application/json) [string] Options: -c, --context Use the given JSON context string or file (e.g., config.json) [string] --to Destination for update queries [string] -b, --baseIRI base IRI for the query (e.g., http://example.org/) [string] -d, --dateTime Sets a datetime for querying Memento-enabled archives [string] -l, --logLevel Sets the log level (e.g., debug, info, warn, ...) [string] [default: "warn"] --lenient If failing requests and parsing errors should be logged instead of causing a hard crash [boolean] -v, --version Prints version information [boolean] --showStackTrace Prints the full stacktrace when errors are thrown [boolean] --httpTimeout HTTP requests timeout in milliseconds [number] --httpBodyTimeout Makes the HTTP timeout take into account the response body stream read [boolean] --httpRetryCount The number of retries to perform on failed fetch requests [number] --httpRetryDelay The number of milliseconds to wait between fetch retries [number] --httpRetryOnServerError If fetch should be retried on 5xx server error responses, instead of being resolved. [boolean] --unionDefaultGraph If the default graph should also contain the union of all named graphs [boolean] -p, --proxy Delegates all HTTP traffic through the given proxy (e.g. http://myproxy.org/?uri=) [string] --listformats Prints the supported MIME types [boolean] --explain Print the query plan [string] [choices: "parsed", "logical", "physical"] --localizeBlankNodes If blank nodes should be localized per bindings entry [boolean] -r, --recoverBrokenLinks Use the WayBack machine to recover broken links [boolean] [default: false] Examples: comunica-sparql https://fragments.dbpedia.org/2016-04/en -q 'SELECT * { ?s ?p ?o }' comunica-sparql https://fragments.dbpedia.org/2016-04/en -f query.sparql comunica-sparql https://fragments.dbpedia.org/2016-04/en https://query.wikidata.org/sparql ... comunica-sparql hypermedia@https://fragments.dbpedia.org/2016-04/en sparql@https://query.wikidata.org/sparql ...