The s3cmd configuration file is named .s3cfg and it is located in the user's home directory, e.g. /home/username/ ($HOME).
On Windows the configuration file is called s3cmd.ini and it is located in %USERPROFILE% -> Application Data , usually that is c:\users\username\AppData\Roaming\s3cmd.ini
The s3cmd configuration file contains all s3cmd settings. This includes the Amazon access key and secret key for s3cmd to use to connect to Amazon S3.
A basic configuration file is created automatically when you first issue the s3cmd --configure command after installation. You will be asked a few questions about your Amazon access key and secret key and other settings you wish to use, and then s3cmd will save that information in a new config file.
Other advanced settings can be changed (if needed) by editing the config file manually. Some of the settings contain the default values for s3cmd to use. For instance, you could change the multipart_chunk_size_mb default value from 15 to 5, and that would become the new default value for the s3cmd option --multipart-chunk-size-mb.
The following is an example of a s3cmd config file: