In the following command to change the Bash prompt of a user in bash terminal

PS1='[d]u@h:w$'

This command throws an error “command not found” if –> I leave a whitespace before the “=” or after the “=”.

I was expecting all of the following:

↣ PS1 = '[d]u@h:w$'
↣ PS1 ='[d]u@h:w$'
↣ PS1= '[d]u@h:w$'
↣ PS1='[d]u@h:w$'

to produce the same result but only the 4th one works fine and top3 throws error – “command not found”.