Process completed immediately after opening Terminal and impossible to add commands
Problem fixed after following this:

1. Go to Terminal Preferences &gt; Open Shells with : enter "/bin/bash -x"
2. On the terminal : sudo nano .bash_profile 
3. Delete the whole content of the bash_profile, then Ctrl + X (exit) Yes, Enter
4. Go to Terminal Preferences &gt; Open Shells with : enter "/bin/bash"


someone guided me to that. For some reason, my .bash_profile was kind of doing a loop... 
Apparently, I also had another bash_profile document somewhere else...
I won't be able to give more details for now as the person left but I will add more info here once I got them. Hope this will help others.

Thanks a lot to @nohillside and @klanomath for your help!! I appreciate a lot.