If you are sure your Recovery HD is patched to match the OS on your main system, you can snag a file and see if it works without needing to do a full reinstall of the OS or without needing to boot into Recovery HD if you are OK modifying the system as it's running.

### Use the Recovery partition

Many<sup>1</sup> system files are also included on the Recovery partition and can be copied over.

1. Mount the Recovery HD

        diskutil mount Recovery\ HD

2. Mount the BaseSystem.dmg

        hdiutil mount /Volumes/Recovery\ HD/com.apple.recovery.boot/BaseSystem.dmg

3. Copy the file

        FILE="/usr/bin/codesign"
        sudo cp -p /Volumes/OS\ X\ Base\ System/${FILE} ${FILE}

This process can be combined into a single-line command if you have to run it frequently…

    FILE="/usr/bin/codesign"; diskutil mount Recovery\ HD && hdiutil mount /Volumes/Recovery\ HD/com.apple.recovery.boot/BaseSystem.dmg && sudo cp -p /Volumes/OS\ X\ Base\ System/${FILE} ${FILE}

Even if you have deleted the `cp` binary (which could happen), there is `/usr/bin/ditto`, `/usr/bin/tar` and several other commands that can stand in for `cp`.

### Reinstall OS X

If you've got the ability to reinstall OS X through the Recovery HD, Internet Recovery or an OS X install media then reinstalling OS X will 'repair' your install and copy any missing/broken system files back to your system.

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I recommend that you do a permissions repair using Disk Utility after copying a file manually and then rebooting. If the fix works, now would be an excellent time for another backup snapshot.

<sup><sup>1</sup> Not *all* system files are on the Recovery HD, but many are. If the copying command doesn't work, check that the file exists on the Recovery HD.</sup>