Git Bash operate with cygwin

Git bash, and related tools in msysgit package, are pure Windows executables.

That's why you cannot run Cygwin tools into Git bash, since these are not completely windows programs (they need the cygwin.dll, see @fvu comment below).

If you want to use Git with Cygwin tools, you have to install the git package for Cygwin.

You might just need to add the cygwin bin directory to your $PATH variable in git bash.

Despite what everyone else seems to think they know, Git Bash is actually pretty much repackaged Cygwin. Viz: https://github.com/msysgit/msys/tree/master/winsup/cygwin