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Is it possible to checkout a branch using the command line (terminal on mac) knowing just:

ssh://git@dev.somename.se:8000/abc/somename.git

Answers

You have to clone this repository first using

git clone ssh://git@dev.somename.se:8000/abc/somename.git

This will be creating a local working copy of this project and will be on the default branch of it what is probably 'master'

You can verify which branch you are on using

git status

If you want a list of the available branches (in the remote repo) execute

git branch -r

If you want another remote branch available on your local copy you can use

git checkout --track origin/remotebranchname

The --track parameter will tie your local branch and remote branch together so you won't have to be give it explicitly when pushing your code for example.