Exactly what happens when you replace
quiet splash or
splash quiet (the order doesn't matter) by
single depends on the distribution. Most distributions will ask for a root password.
If you don't remember the root password, or you just want to boot in the most minimal way, you can replace
quiet splash (and
$vt_handoff, for that matter) by
init=/bin/bash. The line should look like
linux /vmlinuz-… root=… ro init=/bin/bash
The amount of whitespace between the parts doesn't matter, just leave at least one space wherever there was one before. The parts that I replaced by
… above do matter, you must leave what was there before. Remove everything except for the leading word
linux, the word after that,
ro, and add
When you boot, you'll get a bash command line, running as root. When you have physical access, the only security that could prevent you from getting in is encryption. (If your system has full-disk encryption, you will need to enter the encryption password.)
At this command line, run the following commands:
mount -o remount,rw /
Then you can view and modify the user database. The main user database file is
/etc/passwd. It contains user names (for both physical users and system accounts), but passwords are in a different file
/etc/shadow. Both files are human-readable up to a point. You cannot recover passwords though; if you've forgotten a password, all you can do is change it.
The following command lists accounts that have a password:
grep -v ':[*!]:' /etc/shadow
(Type it carefully, it's pretty sensitive to the exact punctuation.) The first part of each line, before the first
: sign, is the username.
If you want to change the password for an account, run
rob is the username.
Once you've noted the username and changed the password if desired, run
mount -o remount,ro /