Change spaces with underscore in filename

Question

I have a multiples files with space, and I want replace this for a underscore with command line (Mac)

xxx xxx.jpg -> xxx_xxx.jpg

It's possible with only line command?

Answer 1

For all files in current directory

for i in *;do mv "$i" "${i// /_}";done

If you only want to match files with spaces (to prevent tons of error messages when it tries to move files into themselves) though you could use extended glob

shopt -s extglob
for i in +(* *);do mv "$i" "${i// /_}";done