Mastering curl on Linux: Download Files and Consume APIs

The curl command on Linux can be used to download files, as well as communicate with Web APIs directly from the Linux shell. It is a very versatile command for programmers to test access to APIs of software and applications hosted on the web.

The basic syntax of curl is:

$ curl URL

Example:

$ curl https://google.com<HTML><HEAD><meta http-equiv="content-type" content="text/html;charset=utf-8"><TITLE>301 Moved</TITLE></HEAD><BODY><H1>301 Moved</H1>The document has moved<A HREF="https://www.google.com/">here</A>.</BODY></HTML>

This way, it sends a GET request to the specified server, on the specified page. You can pass GET parameters directly in the URL using the separators "?" and "&".

$ curl http://api.servidor.com/myapi/2.5/tempo?id=217

When passing more than one parameter in the URL, especially when using the symbol "&", you must use single quotes around the URL:

$ curl 'http://api.servidor.com/myapi/2.5/tempo?id=217&user=uira'

Curl allows you to specify a timeout in seconds for which it will wait for a server response, using the "-m" option:

$ curl -m 3  'http://api.servidor.com/myapi/2.5/tempo?id=2172797&user=uira'

You can make a POST request using curl, with the "-X" option:

$ curl -X POST  'http://api.servidor.com/myapi/2.5/tempo'

The -X option allows the methods GET (default), POST and PUT.

To send data in the POST request, you must use the "-d" option. The data must be in URL encoded format.

$ curl -d 'id=217&user=uira' -X POST 'http://api.servidor.com/myapi/2.5/tempo'

You can also separate the data into multiple "-d" options:

$ curl -d id=217  -d user=uira  -X POST 'http://api.servidor.com/myapi/2.5/tempo'

You can also specify a file containing the data to send via POST, using the "-d" option with "@":

$ curl -d @arquivo -X POST 'http://api.servidor.com/myapi/2.5/tempo'

If the site requires simple HTTP authentication, the "-u" option can send a username and password:

$ curl -u uira@senha -d id=217  -d user=uira  -X POST 'http://api.servidor.com/myapi/2.5/tempo'

To send a JSON file, you need to specify the HTTP header, using the "-H" option:

$ curl -H "Accept: application/json" -d @arquivojson  -X PUT 'http://api.servidor.com/myapi/2.5/tempo'

The "-i" option allows you to see the HTTP response from the server:

$ curl -i 'https://api.openweathermap.org/data/2.5/weather?q=Belo%20Horizonte,PT&appid=9908ae7bbb3c530f54efdec77ac3ccde'
HTTP/1.1 200 OK
Server: openresty
Date: Tue, 12 May 2020 22:13:25 GMT
Content-Type: application/json; charset=utf-8
Content-Length: 473
Connection: keep-alive
X-Cache-Key: /data/2.5/weather?q=belo%20horizonte,pt
Access-Control-Allow-Origin: *
Access-Control-Allow-Credentials: true
Access-Control-Allow-Methods: GET, POST

{"coord":{"lon":-7.42,"lat":39.31},"weather":[{"id":801,"main":"Clouds","description":"few clouds","icon":"02n"}],"base":"stations","main":{"temp":286.83,"feels_like":285.15,"temp_min":286.48,"temp_max":288.15,"pressure":1008,"humidity":87},"visibility":10000,"wind":{"speed":3.1,"deg":220},"clouds":{"all":20},"dt":1589321586,"sys":{"type":1,"id":6402,"country":"PT","sunrise":1589260722,"sunset":1589312008},"timezone":3600,"id":2270968,"name":"Belo Horizonte","cod":200}

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