This is one of the often forgotten little gems in the curl arsenal of command line options. --write-out or just -w for short, writes out information after a transfer has completed and it has a large range of variables that you can include in the output, variables that have been set with values and information from the transfer.

You tell curl to write a string just by passing that string to this option:

curl -w "formatted string" http://example.com/

…and you can also have curl read that string from a given file instead if you prefix the string with '@':

curl -w @filename http://example.com/

…or even have curl read the string from stdin if you use '-' as filename:

curl -w @- http://example.com/

The variables that are available are accessed by writing %{variable_name} in the string and that variable will then be substituted by the correct value. To output a normal '%' you just write it as '%%'. You can also output a newline by using '\n', a carriage return with '\r' and a tab space with '\t'.

(The %-symbol is special on the Windows command line, where all occurrences of % must be doubled when using this option.)

As an example, we can output the Content-Type and the response code from an HTTP transfer, separated with newlines and some extra text like this:

curl -w "Type: %{content_type}\nCode: %{response_code}\n" http://example.com

This feature writes the output to stdout so you probably want to make sure that you do not also send the downloaded content to stdout as then you might have a hard time to separate out the data.

Available --write-out variables

Some of these variables are not available in really old curl versions.

• %{content_type} the Content-Type of the requested document, if there was any.

• %{errormsg} the error message from the transfer. Empty if no error occured. (Introduced in 7.75.0)

• %{exitcode} the numerical exit code from the transfer. 0 if no error occured. (Introduced in 7.75.0)

• %{filename_effective} the ultimate filename that curl writes out to. This is only meaningful if curl is told to write to a file with the --remote-name or --output option. It's most useful in combination with the --remote-header-name option.

• %{ftp_entry_path} the initial path curl ended up in when logging on to the remote FTP server.

• %{http_code} the old variable name for what is now known as response_code.

• %{http_connect} the numerical code that was found in the last response (from a proxy) to a curl CONNECT request.

• %{http_version} The http version that was used.

• %{json} all write-out variables as a single JSON object.

• %{local_ip} the IP address of the local end of the most recently done connection—can be either IPv4 or IPv6

• %{local_port} the local port number of the most recently made connection

• %{method} HTTP method the most recent request used

• %{num_connects} the number of new connects made in the recent transfer.

• %{num_headers} number of response headers in the last responsea

• %{num_redirects} number of redirects that were followed in the request.

• %{onerror} if the transfer ended with an error, show the rest of the string, otherwise stop here. (Introduced in 7.75.0)

• %{proxy_ssl_verify_result} the result of the SSL peer certificate verification that was requested when communicating with a proxy. 0 means the verification was successful.

• %{redirect_url} the actual URL a redirect would take you to when an HTTP request was made without -L to follow redirects.

• %{remote_ip} the remote IP address of the most recently made connection—can be either IPv4 or IPv6.

• %{remote_port} the remote port number of the most recently made connection.

• %{response_code} the numerical response code that was found in the last transfer.

• %{scheme} scheme used in the previous URL

• %{size_download} the total number of bytes that were downloaded.

• %{size_header} the total number of bytes of the downloaded headers.

• %{size_request} the total number of bytes that were sent in the HTTP request.

• %{size_upload} the total number of bytes that were uploaded.

• %{speed_download} the average download speed that curl measured for the complete download in bytes per second.

• %{speed_upload} the average upload speed that curl measured for the complete upload in bytes per second.

• %{ssl_verify_result} the result of the SSL peer certificate verification that was requested. 0 means the verification was successful.

• %{stderr} - makes the rest of the output get written to stderr.

• %{stdout} - makes the rest of the output get written to stdout.

• %{time_appconnect} the time, in seconds, it took from the start until the SSL/SSH/etc connect/handshake to the remote host was completed.

• %{time_connect} the time, in seconds, it took from the start until the TCP connect to the remote host (or proxy) was completed.

• %{time_namelookup} the time, in seconds, it took from the start until the name resolving was completed.

• %{time_pretransfer} the time, in seconds, it took from the start until the file transfer was just about to begin. This includes all pre-transfer commands and negotiations that are specific to the particular protocol(s) involved.

• %{time_redirect} the time, in seconds, it took for all redirection steps including name lookup, connect, pre-transfer and transfer before the final transaction was started. time_redirect the complete execution time for multiple redirections.

• %{time_starttransfer} the time, in seconds, it took from the start until the first byte was just about to be transferred. This includes time_pretransfer and also the time the server needed to calculate the result.

• %{time_total} the total time, in seconds, that the full operation lasted. The time will be displayed with millisecond resolution.

• %{url} the URL used in the transfer. (Introduced in 7.75.0)

• %{url_effective} the URL that was fetched last. This is particularly meaningful if you have told curl to follow Location: headers (with -L).

• %{urlnum} the 0-based numerical index of the URL used in the transfer. (Introduced in 7.75.0)