If your filenames contain no whitepace and you don’t have any subdirectories, you can use a simple for loop:
$ for FILENAME in *; do mv $FILENAME Unix_$FILENAME; done
Otherwise use the convenient rename command (which is a perl script) – although it might not be available out of the box on every Unix (e.g. OS X doesn’t come with rename).
A short overview at debian-administration.org:
- Easily renaming multiple files
If your filenames contain whitespace it’s easier to use find, on Linux the following should work:
$ find . -type f -name '*' -printf "echo mv '%h/%f' '%h/Unix_%f\n'" | sh
On BSD systems, there is no -printf option, unfortunately. But GNU findutils should be installable (on e.g. Mac OS X with brew install findutils).
$ gfind . -type f -name '*' -printf "mv \"%h/%f\" \"%h/Unix_%f\"\n" | sh