git difference on parameter one dash and two dashes
That's not really git specific. Many programs use the following convention:
- single-letter parameter: one dash
- multi-letter parameter: two dashes
This is handy, because it allows you to specify many single-letter parmeters at once with a single dash and all letters of the parameters you need: ls -al is equivalent to ls -a -l.
Often, one-letter parameters are the most used ones and can have a longer equivalent with two dashes: for example git add -v and git add --verbose mean the same.
Git follows the GNU-adjusted POSIX conventions for command line arguments. Short, one letter options start with a single dash, long options start with two.
Note that the linked page claims that this is the POSIX ("unix") standard, but that's not true. --option is a format pioneered by GNU.
There are alternatives for commands. You may prefer using the long version to avoid a typo :). Also the short version can be combined. Once through the command line parser, there's no difference.