Terminal

Try this first: Quit the Terminal app, then to your user Library and delete the Terminal .plist file. You can do that by copying and pasting the following in the Finder's Go menu > Go to Folder... field:

~/Library/Preferences/com.apple.Terminal.plist

A Finder window will open with that file already selected. Drag it to the Trash. Launch Terminal again and see if it's fixed.

If that doesn't help, please return to your Terminal's Preferences, and instead of using the default login shell, select Command (complete path), and make it look like my example. In other words type /bin/bash where yours shows TUSHAR.

Just to be clear, I understand when you open the Terminal app it returns with that [Process completed] message. Open a new window with Shell > New Window > Solid Colors.

Does the same thing occur?

Your default login shell is messed up. Reinstall macOS.

Open Terminal's Preferences > General.

Make it look like this

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