Start by moving your com.apple.Terminal.plist file out of the Preferences folder
Quit from the Terminal app
Finder -> Go -> Go to folder -> ~/Library/Preferences
The ~ is the Tilda character often found on the upper left corner of your keyboard.
Relaunch Terminal
If that does not clear up the problem.
Look for one of the following files in your home directory (which is going to be fun trying to see as they are not normally displayed by the Finder)
- .bash_profile
- .bash_login
- .profile
- .bashrc
But you can most likely see them if you use
Terminal -> Shell -> New Command -> ls -a
By default none of these files exist, so you may not find them. But if you do fine one of them you want to rename so that the shell will not look at them
Terminal -> Shell -> New Command -> mv .bash_profile save.bash_profile
Terminal -> Shell -> New Command -> mv .bash_login save.bash_login
Terminal -> Shell -> New Command -> mv .profile save.profile
Terminal -> Shell -> New Command -> mv .bashrc save.bashrc
Obviously, only use one of these commands if the file in question exists.
Quit Terminal
Relaunch Terminal
If that does not work, then create a new account, as an experiment, and try Terminal in the new account
System Preferences -> Users & Groups
Click on the padlock in the lower left-hand corner if the padlock is locked
Click on the [+] to add a new account.
Again, this is an experiment to see if the problem continues.
Then let us know the results of the experiment, as we will still want to figure out what is going wrong in your normal account.