Cygwin uses a different path scheme than Windows. If you use a Cygwin shell with Cygwin programs, then everything works well. But if you use a Cygwin shell as your default shell (which I do, because face it, cmd.exe sucks) and you’re running non-Cygwin programs, you can run into trouble because the shell may send Cygwin-style paths for filenames to Windows programs that don’t understand them.

For instance today I was using a Cygwin bash shell with a non-Cygwin Perforce client (i.e.: “p4”).

$ p4 open foo.cpp 
Path '/cygdrive/c/dev/project/...\foo.cpp' is not under 
client's root 'C:\dev\project'.

p4 is a regular (non-Cygwin) program and thus it doesn’t understand the Cygwin path that gets sent to it.

One way to fix this would be to find a version of p4 that was compiled against Cygwin.

Another way is to use the cygpath command to convert the Cygwin path to a Windows path.

$ p4 open $(cygpath -aw foo.cpp) 
//depot/project/foo.cpp#2 - currently opened for edit

The -a option tells it to generate an absolute path and the -w option tells it to convert from a Cygwin path to a Windows path.