Cara Menggunakan curl untuk memuat turun fail dari baris arahan Linux

curlPerintah Linux boleh melakukan lebih banyak daripada memuat turun fail. Ketahui apa curlyang mampu, dan bila anda harus menggunakannya dan bukannya wget.

curl vs. wget: Apakah Perbezaannya?

Orang sering berjuang untuk mengenal pasti kekuatan relatif wgetdan curlperintah. Perintah tersebut mempunyai beberapa pertindihan fungsi. Mereka masing-masing dapat mengambil fail dari lokasi terpencil, tetapi di situlah kesamaan berakhir.

wgetadalah alat yang hebat untuk memuat turun kandungan dan fail. Ia boleh memuat turun fail, halaman web, dan direktori. Ini mengandungi rutin cerdas untuk melintasi pautan di laman web dan memuat turun kandungan secara berulang-ulang di seluruh laman web. Ia tidak dapat ditandingi sebagai pengurus muat turun baris perintah.

curlmemenuhi keperluan yang sama sekali berbeza. Ya, ia dapat mengambil fail, tetapi tidak dapat menavigasi secara berulang-ulang laman web yang mencari kandungan untuk diambil. Apa yang curlsebenarnya dilakukan ialah membiarkan anda berinteraksi dengan sistem jarak jauh dengan membuat permintaan ke sistem tersebut, dan mengambil dan memaparkan respons mereka kepada anda. Respons tersebut mungkin berupa kandungan dan file halaman web, tetapi juga dapat berisi data yang disediakan melalui layanan web atau API sebagai hasil dari "pertanyaan" yang diajukan oleh permintaan curl.

Dan curltidak terhad kepada laman web. curlmenyokong lebih daripada 20 protokol, termasuk HTTP, HTTPS, SCP, SFTP, dan FTP. Boleh dikatakan, kerana pengendalian paip Linux yang unggul, curldapat lebih mudah disatukan dengan perintah dan skrip lain.

Pengarang curlmempunyai laman web yang menerangkan perbezaan yang dilihatnya antara curldan wget.

Memasang keriting

Dari komputer yang digunakan untuk meneliti artikel ini, Fedora 31 dan Manjaro 18.1.0 curl telah dipasang. curlterpaksa dipasang pada Ubuntu 18.04 LTS. Di Ubuntu, jalankan arahan ini untuk memasangnya:

sudo apt-get curl pemasangan

Versi keriting

The --versionjenama pilihan  curlmelaporkan versi. Ia juga menyenaraikan semua protokol yang disokongnya.

keriting - penukaran

Mengambil Halaman Web

Sekiranya kita menunjuk curllaman web, ia akan mendapatkannya semula untuk kita.

curl //www.bbc.com

Tetapi tindakan lalai adalah membuangnya ke tetingkap terminal sebagai kod sumber.

Hati-hati : Sekiranya anda tidak memberitahu curlanda mahu sesuatu disimpan sebagai fail, ia akan selalu membuangnya ke tetingkap terminal. Sekiranya fail yang diambilnya adalah fail binari, hasilnya tidak dapat diramalkan. Shell mungkin cuba menafsirkan beberapa nilai bait dalam fail binari sebagai watak kawalan atau urutan pelarian.

Menyimpan Data ke Fail

Mari beritahu curl untuk mengarahkan output ke fail:

curl //www.bbc.com> bbc.html

Kali ini kami tidak melihat maklumat yang diambil, ia dihantar terus ke fail untuk kami. Kerana tidak ada output tetingkap terminal untuk ditampilkan, curlmenghasilkan sekumpulan maklumat kemajuan.

Ini tidak melakukan ini pada contoh sebelumnya kerana maklumat kemajuan akan tersebar di seluruh kod sumber halaman web, jadi curlsecara automatik menekannya.

Dalam contoh ini,  curlmengesan bahawa output sedang diarahkan ke fail dan bahawa selamat untuk menghasilkan maklumat kemajuan.

Maklumat yang diberikan adalah:

• % Jumlah : Jumlah keseluruhan yang akan diambil.
• % Diterima : Nilai peratusan dan sebenar data yang diambil setakat ini.
• % Xferd : Peratusan dan sebenar yang dihantar, jika data sedang dimuat naik.
• Beban Kelajuan Purata: Kelajuan muat turun purata.
• Muat naik Kelajuan Purata: Kelajuan muat naik purata.
• Jumlah Masa : Anggaran jangka masa pemindahan.
• Time Spent: The elapsed time so far for this transfer.
• Time Left: The estimated time left for the transfer to complete
• Current Speed: The current transfer speed for this transfer.

Because we redirected the output from curl to a file, we now have a file called “bbc.html.”

Double-clicking that file will open your default browser so that it displays the retrieved web page.

Note that the address in the browser address bar is a local file on this computer, not a remote website.

We don’t have to redirect the output to create a file. We can create a file by using the -o (output) option, and telling curl to create the file. Here we’re using the -o option and providing the name of the file we wish to create “bbc.html.”

curl -o bbc.html //www.bbc.com

Using a Progress Bar To Monitor Downloads

To have the text-based download information replaced by a simple progress bar, use the -# (progress bar) option.

curl -x -o bbc.html //www.bbc.com

Restarting an Interrupted Download

It is easy to restart a download that has been terminated or interrupted. Let’s start a download of a sizeable file. We’ll use the latest Long Term Support build of Ubuntu 18.04. We’re using the --output option to specify the name of the file we wish to save it into: “ubuntu180403.iso.”

curl --output ubuntu18043.iso //releases.ubuntu.com/18.04.3/ubuntu-18.04.3-desktop-amd64.iso

The download starts and works its way towards completion.

If we forcibly interrupt the download with Ctrl+C , we’re returned to the command prompt, and the download is abandoned.

To restart the download, use the -C (continue at) option. This causes curl to restart the download at a specified point or offset within the target file. If you use a hyphen - as the offset, curl will look at the already downloaded portion of the file and determine the correct offset to use for itself.

curl -C - --output ubuntu18043.iso //releases.ubuntu.com/18.04.3/ubuntu-18.04.3-desktop-amd64.iso

The download is restarted. curl reports the offset at which it is restarting.

Retrieving HTTP headers

With the -I (head) option, you can retrieve the HTTP headers only. This is the same as sending the HTTP HEAD command to a web server.

curl -I www.twitter.com

This command retrieves information only; it does not download any web pages or files.

Downloading Multiple URLs

Using xargs we can download multiple URLs at once. Perhaps we want to download a series of web pages that make up a single article or tutorial.

Copy these URLs to an editor and save it to a file called “urls-to-download.txt.” We can use xargs to treat the content of each line of the text file as a parameter which it will feed to curl, in turn.

//tutorials.ubuntu.com/tutorial/tutorial-create-a-usb-stick-on-ubuntu#0 //tutorials.ubuntu.com/tutorial/tutorial-create-a-usb-stick-on-ubuntu#1 //tutorials.ubuntu.com/tutorial/tutorial-create-a-usb-stick-on-ubuntu#2 //tutorials.ubuntu.com/tutorial/tutorial-create-a-usb-stick-on-ubuntu#3 //tutorials.ubuntu.com/tutorial/tutorial-create-a-usb-stick-on-ubuntu#4 //tutorials.ubuntu.com/tutorial/tutorial-create-a-usb-stick-on-ubuntu#5

This is the command we need to use to have xargs pass these URLs to curl one at a time:

xargs -n 1 curl -O < urls-to-download.txt

Note that this command uses the -O (remote file) output command, which uses an uppercase “O.” This option causes curl to save the retrieved  file with the same name that the file has on the remote server.

The -n 1 option tells xargs to treat each line of the text file as a single parameter.

When you run the command, you’ll see multiple downloads start and finish, one after the other.

Checking in the file browser shows the multiple files have been downloaded. Each one bears the name it had on the remote server.

RELATED:How to Use the xargs Command on Linux

Downloading Files From an FTP Server

Using curl with a File Transfer Protocol (FTP) server is easy, even if you have to authenticate with a username and password. To pass a username and password with curl use the -u (user) option, and type the username, a colon “:”, and the password. Don’t put a space before or after the colon.

This is a free-for-testing FTP server hosted by Rebex. The test FTP site has a pre-set username of “demo”, and the password is “password.” Don’t use this type of weak username and password on a production or “real” FTP server.

curl -u demo:password ftp://test.rebex.net

curl figures out that we’re pointing it at an FTP server, and returns a list of the files that are present on the server.

The only file on this server is a “readme.txt” file, of 403 bytes in length. Let’s retrieve it. Use the same command as a moment ago, with the filename appended to it:

curl -u demo:password ftp://test.rebex.net/readme.txt

The file is retrieved and curl displays its contents in the terminal window.

In almost all cases, it is going to be more convenient to have the retrieved file saved to disk for us, rather than displayed in the terminal window. Once more we can use the -O (remote file) output command to have the file saved to disk, with the same filename that it has on the remote server.

curl -O -u demo:password ftp://test.rebex.net/readme.txt

The file is retrieved and saved to disk. We can use ls to check the file details. It has the same name as the file on the FTP server, and it is the same length, 403 bytes.

ls -hl readme.txt

RELATED:How to Use the FTP Command on Linux

Sending Parameters to Remote Servers

Some remote servers will accept parameters in requests that are sent to them. The parameters might be used to format the returned data, for example, or they may be used to select the exact data that the user wishes to retrieve. It is often possible to interact with web application programming interfaces (APIs) using curl.

As a simple example, the ipify website has an API can be queried to ascertain your external IP address.

curl //api.ipify.org

By adding the format parameter to the command, with the value of “json” we can again request our external IP address, but this time the returned data will be encoded in the JSON format.

curl //api.ipify.org?format=json

Here’s another example that makes use of a Google API. It returns a JSON object describing a book. The parameter you must provide is the International Standard Book Number (ISBN) number of a book. You can find these on the back cover of most books, usually below a barcode. The parameter we’ll use here is “0131103628.”

curl //www.googleapis.com/books/v1/volumes?q=isbn:0131103628

The returned data is comprehensive:

Sometimes curl, Sometimes wget

If I wanted to download content from a website and have the tree-structure of the website searched recursively for that content, I’d use wget.

If I wanted to interact with a remote server or API, and possibly download some files or web pages, I’d use curl. Especially if the protocol was one of the many not supported by wget.