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Write out

--write-out or just -w for short, outputs text and information after a transfer is completed. It offers a large range of variables that you can include in the output, variables that have been set with values and information from the transfer.
Instruct curl to output a string by passing plain text 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.
Variable
Description
content_type
Content-Type of the requested document, if there was any.
errormsg
Error message from the transfer. Empty if no error occurred. (Introduced in 7.75.0)
exitcode
Numerical exit code from the transfer. 0 if no error occurred. (Introduced in 7.75.0)
filename_effective
The ultimate filename that curl writes out to. Practical 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 former 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
IP address of the local end of the most recently used connection - can be either IPv4 or IPv6
local_port
Local port number of the most recently used connection
method
HTTP method the most recent request used
num_connects
Number of new connects made in the recent transfer.
num_headers
Number of response headers in the last response
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 used 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
Total number of bytes that were downloaded.
size_header
Total number of bytes of the downloaded headers.
size_request
Total number of bytes that were sent in the HTTP request.
size_upload
Total number of bytes that were uploaded.
speed_download
Average download speed that curl measured for the complete download in bytes per second.
speed_upload
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
0-based numerical index of the URL used in the transfer. (Introduced in 7.75.0)