Shell - Replace substring in a variable

scnzzh@zubt1:~/zzh$ str='I love python and python.'
scnzzh@zubt1:~/zzh$ echo ${str/'python'/'shell'}
I love shell and python.
scnzzh@zubt1:~/zzh$ echo ${str/python/shell}
I love shell and python.
scnzzh@zubt1:~/zzh$ echo ${str//python/shell}
I love shell and shell.

To replace all occurrences, use ${parameter//pattern/string}.