curl/docs/cmdline-opts/_VARIABLES.md

VARIABLES

curl supports command line variables (added in 8.3.0). Set variables with --variable name=content or --variable name@file (where file can be stdin if set to a single dash (-)).

Variable contents can be expanded in option parameters using {{name}} if the option name is prefixed with --expand-. This gets the contents of the variable name inserted, or a blank if the name does not exist as a variable. Insert {{ verbatim in the string by prefixing it with a backslash, like \{{.

You an access and expand environment variables by first importing them. You can select to either require the environment variable to be set or you can provide a default value in case it is not already set. Plain --variable %name imports the variable called 'name' but exits with an error if that environment variable is not already set. To provide a default value if it is not set, use --variable %name=content or --variable %name@content.

Example. Get the USER environment variable into the URL, fail if USER is not set:

--variable '%USER'
--expand-url = "https://example.com/api/{{USER}}/method"

When expanding variables, curl supports a set of functions that can make the variable contents more convenient to use. It can trim leading and trailing white space with trim, it can output the contents as a JSON quoted string with json, URL encode the string with url or base64 encode it with b64. To apply functions to a variable expansion, add them colon separated to the right side of the variable. Variable content holding null bytes that are not encoded when expanded cause error.

Example: get the contents of a file called $HOME/.secret into a variable called "fix". Make sure that the content is trimmed and percent-encoded when sent as POST data:

--variable %HOME
--expand-variable fix@{{HOME}}/.secret
--expand-data "{{fix:trim:url}}"
https://example.com/

Command line variables and expansions were added in 8.3.0.