Can't assign to PS1 on OS X bash
I would check you rc files to make sure PS1 isn't being defined readonly in them.
try the following:
user@server$ PS1='oh yeah :'
oh yeah :readonly PS1
oh yeah :PS1='different PS1 :'
bash: PS1: readonly variable
oh yeah :bash
user@server$ PS1='something else '
something else
try
grep -E 'PS1=' ~/.bashrc <other rc files>
Have you tried
export PS1='oh yeah'
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