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How does curl print to terminal while piping?

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Understanding how curl prints to the terminal while piping is vital for anyone working with this powerful tool in a Unix-like environment. This article will delve into the details, explaining the concept and providing examples for a clearer understanding.

Quick Answer

When using curl to download a file and piping it to another command or redirecting it to a file, the progress data is printed to the terminal because it is sent to the standard error (stderr) stream instead of the standard output (stdout) stream. To suppress both the progress data and the file contents, you can redirect stderr to the same destination as stdout using the 2>&1 operator. Alternatively, you can use the -s or --silent option with curl to suppress both the progress data and error messages.

Introduction to curl

curl is a command-line tool used to transfer data to or from a server. It supports a multitude of protocols, including HTTP, HTTPS, FTP, and many more. You can use curl to download files, submit form data, or even scrape data from websites.

Understanding Standard Streams

Before we delve into how curl prints to the terminal while piping, it’s essential to understand the concept of standard streams in Unix-like systems. There are three standard streams: standard input (stdin), standard output (stdout), and standard error (stderr).

  • Standard Input (stdin): This is the data input into the command-line interface.
  • Standard Output (stdout): This is the primary output of a program.
  • Standard Error (stderr): This is an independent output stream for error messages.

By default, both stdout and stderr print their output to the terminal.

How curl Prints to Terminal

When you use curl to download a file and pipe it to another command or redirect it to a file, you may still see output in your terminal. This happens because curl prints its progress data to stderr instead of stdout.

For example:

curl http://www.archive.org/stream/Pi_to_100000000_places/pi.txt > /dev/null

In this command, the > operator redirects the contents of the file being downloaded (stdout) to /dev/null, effectively discarding it. However, the progress data is still printed to the terminal because it is sent to stderr.

Redirecting stderr

If you want to suppress both the progress data and the file contents, you can use the following command:

curl http://www.archive.org/stream/Pi_to_100000000_places/pi.txt > /dev/null 2>&1

Here, 2>&1 redirects stderr to the same destination as stdout, which is /dev/null in this case. The 2 represents stderr, and 1 represents stdout. The > operator is used for redirection.

Using Silent Mode

Alternatively, you can use the -s or --silent option with curl to suppress both the progress data and error messages:

curl -s http://www.archive.org/stream/Pi_to_100000000_places/pi.txt | some_other_command > some_file

In this case, the -s option makes curl run in silent mode, suppressing any progress data or error messages. The file contents are then piped to some_other_command or redirected to some_file.

Conclusion

In summary, curl prints its progress data to stderr, so redirecting or suppressing stdout alone will not hide the progress data. To redirect or suppress both the progress data and the file contents, you need to redirect stderr as well using 2>&1 or 2> /dev/null. Understanding these concepts will help you better manage the output of your curl commands and create more efficient scripts.

What is the purpose of `curl`?

curl is a command-line tool used to transfer data to or from a server. It supports various protocols such as HTTP, HTTPS, FTP, and more. It can be used for downloading files, submitting form data, or scraping data from websites.

Why does `curl` still print to the terminal while piping?

curl prints its progress data to stderr instead of stdout while piping. stderr is not affected by redirection or piping, so the progress data is still visible in the terminal.

How can I redirect or suppress both the progress data and file contents when using `curl`?

To redirect or suppress both the progress data and file contents, you can use the following command: curl http://example.com/file.txt > /dev/null 2>&1. This redirects stderr to the same destination as stdout, effectively discarding both outputs.

Is there a way to make `curl` run in silent mode?

Yes, you can use the -s or --silent option with curl to suppress both the progress data and error messages. For example: curl -s http://example.com/file.txt | some_other_command > some_file. The -s option makes curl run in silent mode.

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