The installer for 'El Capitan' stops and says
"File system verify or repair failed. Quit the installer to restart your computer and try again."
It shows a "Restart" button. When I restart, it gets back to exactly the same place again.
How do I quit the installer so it doesn't start again upon boot?
I tried holding command-s on restart (from here), but that did nothing.
I do not have a recent system disk available.
Edit: I shut down completely. Let it stay powered off for 30 seconds. Restart holding down command-r (as noted here and in one answer below). It chimes, goes through 3/4 of one progress bar, starts another progress bar, goes through half, then says "Install OS X" and looks exactly like the (failing) 'El Capitan' upgrade. It has ignored my command-r I believe. This page says "It's possible that you didn't press Command-R early enough. Restart and try again." I think that is unlikely, since I held it down right from the start.
Edit 2: Restart has a menu, with "Disk Utilities", "Terminal", etc. When I try to quit the installer, it wants me to choose another startup disk. Once it didn't like "Macintosh HD" as a choice, but then on a second round it was fine. When I try to repair the disk, I get "file system check exit code is 8", so I think the disk is hard to repair, (perhaps because it is mounted?). I have an external drive, but it has the backup, so I don't know how to make it a bootable drive. In /var/log/install.log there is "WARNING: 997: Failure to setup sound, err = -50", but I think that is not important.
The really weird thing is looking in /var/log/install.log it says (paraphrased without all the dates, etc. because I can't type it all):
The volume Macintosh HD appears to be OK. File system check exit code is 0. Repair completed successfully. ... Attaching disk image /Volumes/Mac OS X Install DVD 1/BaseSystem.dmg Evaluating 1 disks OS X Base System is a valid target. ... Shrinking host partition and creating new recovery partition ... Verifying file system. Using live mode. Performing live verification. Checking Journaled HFS Plus volume. Checking extents overflow file. ... Checking volume bitmap. The volume Macintosh HD could not be verified completely. File system check exit code is 8. Detaching Base System disk image. Evaluating 1 disks. .. Ensure recovery failed. Operation: Ensure recovery system failed, Failure Reason: Error Domain=com.apple.DiskManagement Code=-69845 "File system verify or repair failed."
It makes it look like maybe the downloaded image is corrupted? But why would it have started the install if the image was corrupted? And why is it ignoring my command-r upon restart?
Edit 3: I managed to choose "Macintosh HD" as the boot drive, and boot back into the old system. It seems fine, but I still have no idea how to upgrade successfully.
Edit 4: I am upgrading from snow leopard, so a lot of these options won't work. I appreciate everyone's help. The answer to this question was, "Notice there is an installer menu. Choose quit from the installer menu. If it won't allow you to choose 'Macintosh HD', reboot a few times until it does." If someone submits that, I'll accept it.