At the time of this writing, 7-zip was still not available in 64-bit Cygwin.
Here’s how to work around this issue by using the Windows 7z.exe from Cygwin. NOTE: This method will not show percent complete (no progress report).
1) install the 64-bit 7zip for Windows from http://www.7-zip.org/
2) in Cygwin, type
nano ~/.bashrc and at the bottom, paste in the line:
alias 7z="/cygdrive/c/Program\ Files/7-Zip/7z.exe"
3) close and reopen Cygwin and you’ll find that you can 7-zip files as usual.
say you have a big file named huge.dat. To archive this file type:
7z a huge.7z huge.dat
to extract the file type
7z e huge.7z