psql -U postgres

# Postgres "psql not recognized as an internal or external command"

# Just an update because I was trying it on Windows 10 you do need to set the 
# path to the following: 
# ;C:\Program Files\PostgreSQL\9.5\bin ;C:\Program Files\PostgreSQL\9.5\lib

# You can do that either through the CMD by using set PATH [the path] or from my
# computer => properties => advanced system settings 
# => Environment Variables => System Variables

# Then search for path.

# Important: don't replace the PATHs that are already there just add one 
# beside them as follows 
# ;C:\Program Files\PostgreSQL\9.5\bin ;C:\Program Files\PostgreSQL\9.5\lib

# Please note: On windows 10, if you follow this: 
# computer => properties => advanced system settings 
# => Environment Variables => System Variables> select PATH, 
# you actually get the option to add new row. Click Edit, add 
# the /bin and /lib folder locations and save changes.

# Then close your command prompt if it's open and then start it again 
# try psql --version If it gives you an answer then you are good to go if 
# not try echo %PATH% and see if the path you set was added or not and if 
# it's added is it added correctly or not.

psql -U postgres
# enter your password and you are in