You need to add Bearer in single code instead of double quote as shown below
curl https://yourinstnace.salesforce.com/services/data/v26.0/sobjects/ -H 'Authorization:Bearer 00D50000000IjCR!AR0AQLUmKfOYh5x5xEvHFZEMBqNEwakBGADwT0EFb_0Ck35r0MufEs'
When running the cURL examples for the REST resources, you may get an error on Mac and Linux systems due to the presence of the exclamation mark special character in the session ID argument.
Make sure you don't have any Escape the exclamation mark (!) special character in the session ID by inserting a backslash before it (!) when the session ID is enclosed within double quotes.