Changing file extensions for all files in a directory on OS X
Solution 1:
You could use something like this:
for old in *.txt; do mv $old `basename $old .txt`.md; done
Make a copy first!
Solution 2:
Alternatively, you could install the ren (rename) utility
brew install ren
ren '*.txt' '#1.md'
If you want to rename files with prefix or suffix in file names
ren 'prefix_*.txt' 'prefix_#1.md'