$ for i in *.log; do mv -- "$i" "${i%.log}.txt"; done
$ for i in $( ls ); do mv $i $i.old; done
$ for i in $( ls ); do mv $i $i.txt; done
for f in *.jpg; do mv "$f" "$(echo "$f" | sed s/IMG/VACATION/)"; done
rename 's/^/MyVacation2011_/g' *.jpg