Practical Examples for mkdir and curl

mkdir

  • Creating a single directory:
mkdir new_directory
  • Creating multiple directories at once:
mkdir -p path/to/nested/directory

This creates all the parent directories if they don't exist.

  • Creating multiple directories in the current directory:
mkdir dir1 dir2 dir3

curl

Basic Usage:

  • Downloading a web page:
curl https://www.example.com
  • Downloading a file:
curl -o file.txt https://www.example.com/file.txt
  • Downloading a file using the filename from the URL:
curl -O https://www.example.com/file.txt
  • Following redirects:
curl -L https://www.example.com/redirect

Sending Headers:

  • Sending an Authorization header:
curl -H "Authorization: Bearer your_token" https://api.example.com

Making POST Requests:

  • Sending a POST request with data:
curl -X POST -d "name=John&email=john.doe@example.com" https://api.example.com/users
  • Sending a POST request with a file:
curl -X POST -F "file=@file.txt" https://api.example.com/upload

Using Variables:

  • Storing a URL in a variable:
url="https://www.example.com"
curl $url
  • Escaping special characters:
url="https://www.example.com/search?query=hello%20world"
curl $url

Error Handling:

  • Failing on non-200 status codes:
curl -f https://www.example.com/nonexistent_page
  • Outputting status codes with -w:
curl -w "%{http_code}" https://www.example.com

Retry Flags:

  • Retrying failed requests:
curl --retry 3 --connect-timeout 10 https://www.example.com

Piping Output:

  • Downloading a script and executing it:
curl -o script.sh https://www.example.com/script.sh
bash script.sh

Handling Multi-File Transfers:

  • Stopping the process on the first failure:
curl --fail-early -o file1.txt https://www.example.com/file1.txt -o file2.txt https://www.example.com/file2.txt

// Start of Selection Remember:

  • -o is generally preferred over redirection operators (>) for writing to files, as it allows for retry flags.
  • Use -w to cleanly output status codes instead of parsing command-line output.
  • Be aware of status codes in multi-file transfers.

These examples illustrate the versatility of mkdir and curl for managing directories and interacting with web resources. Experiment with different options and combinations to tailor them to your specific needs.

curl returns success, even on HTTP status code failures (e.g., 404, 500, etc.) by default and will only fail if it can’t connect to the server or the server doesn’t exist. You can make it fail on non-200 status codes with -f.

If the URL ends with a “/” do not use -O; otherwise, it won’t be able to write to the file. Prefer -o and specify your own filename.

You’re right, and I appreciate the clarification. Let’s break down the combination of -f, -s, and -S in the curl command curl -fsSL example.com for a better understanding:

Basically, -f says that if the server returns a 404, then curl returns a non-zero exit code. Technically, this isn’t a failure of the server because the server did respond with something, so therefore curl returns exit code 0 (success) because the request was successful, but the server said it wasn’t found (successfully returned an error).

  • -f or --fail: Tells curl to fail and return a non-zero exit status for server errors (HTTP responses like 4xx or 5xx). When this option is used, curl will not output the error page content but will exit with an error status.
  • -s or --silent: Suppresses all output, including error messages and the progress meter. It’s typically used to make curl quiet in scripts or when you only care about the fetched content or the exit status. This is useful; otherwise, your logs can get messy because the progress bar gets chopped up.
  • -S or --show-error: When used with -s, this option tells curl to still show error messages even though it’s in silent mode. Normally, -s suppresses error messages, but -S overrides this for error messages while keeping other outputs (like the progress meter) suppressed.

So, when you use -fsS together, it means:

  • curl will fail and return a non-zero status on server errors (-f).
  • curl will not output the regular HTTP content or progress meter (-s).
  • However, it will still show error messages if there are any (-S).

In essence, this combination makes curl operate quietly in normal circumstances but still report if something goes wrong, such as an HTTP error. It’s a common pattern used in scripting where you want minimal output except for indicating when errors occur.

You’re right to be cautious about using curl -J. While it seems convenient for grabbing content with a specific filename, it comes with several caveats that can lead to unexpected behavior and problems.

Here’s a breakdown of the issues:

  1. Filename Changes: The biggest caveat is that the -J flag relies on the server’s Content-Disposition header to determine the filename. Servers can have different configurations and may not always provide this header, or they may provide an inconsistent filename depending on the request. This means your downloaded file might not have the expected name, potentially overwriting existing files with unintended content.

  2. Server-Side Inconsistencies: Even if the Content-Disposition header is present, the filename it provides can change depending on factors like:

    • Dynamically generated content: For example, a website might create a new file name each time you request it, so -J would give you a different filename every time.
    • Server configuration: Different servers might have different settings for the Content-Disposition header.
    • File extensions: The server might not include the file extension in the header, making it difficult to determine the file type.

  3. Potential Security Issues: Using -J can expose you to potential security vulnerabilities if you’re not careful. If a malicious server sends a Content-Disposition header with a filename that tries to access sensitive system files or directories, curl -J could unknowingly execute commands or provide access to sensitive data.

Alternatives to curl -J:

Instead of relying on -J, it’s usually safer to:

  • Use -o and specify the filename yourself: This gives you complete control over the filename and avoids the potential for unintended consequences.
  • Extract the filename from the Content-Disposition header: You can use curl’s output parsing features to extract the filename from the Content-Disposition header and then use it with -o. This is more complex but can provide more robust filename handling.
  • Combine -J with -O: While this is not recommended for the same reasons as using -J alone, it could be used if you want to be sure to use the exact filename provided in the Content-Disposition header (if available) and need to avoid potential filename collisions.

In Summary:

While curl -J might seem tempting for its simplicity, the potential for unexpected behavior and security vulnerabilities outweigh the benefits. It’s generally safer to avoid -J and use alternative methods to control filenames and ensure predictable and secure downloads. Remember to always be mindful of the origin of the files you are downloading and take appropriate precautions to protect your system from malicious content.