The Old Monk

There's not much info on the interwebs on why ssh authentication via pam_mysql fails, and there are definitely some misleading answers like

when logging in via ssh, the ssh daemon checks some files in ~/,
so it can be done if the user already have a valid home directory.

from this thread.

The real reason why it doesn't successfully authenticate is because ssh does a getpwent() call to check if the user exists on the system. If you're using just a PAM solution (which doesn't provide accounts), that call fails if there isn't any such local user. sshd then sets the password to '^H ^M INCORRECT' before passing it to PAM, which obviously thinks it's the wrong password. The simple (and probably unscalable - but then you should be using something like nss_mysql) solution is to add local accounts to the machines for these users. That will make the getpwent() call succeed, and sshd will authenticate successfully via PAM.

I've mentioned this earlier.

A few shell tricks that I picked up.
Variable manipulation:

gera@gera-laptop:~$ BAR=bar
gera@gera-laptop:~$ echo $FOO

gera@gera-laptop:~$ echo $BAR
bar
gera@gera-laptop:~$ echo ${FOO:-baz}
baz
gera@gera-laptop:~$ echo $FOO

gera@gera-laptop:~$ echo ${FOO:-$BAR}
bar
gera@gera-laptop:~$ echo $FOO

gera@gera-laptop:~$ echo ${FOO:=newvalue}
newvalue
gera@gera-laptop:~$ echo $FOO
newvalue
gera@gera-laptop:~$ echo ${BAR:=newvalue}
bar
gera@gera-laptop:~$ echo $BAR
bar

More here.

Here strings - which are like here docs.

gera@gera-laptop:~$ cat <<END
> foo
> bar
> baz spam eggs
> END
foo
bar
baz spam eggs
gera@gera-laptop:~$ cat <<<"foo bar baz spam eggs"
foo bar baz spam eggs

Although you can do the same with a piped echo, the here string saves you a process (the echo). Still, I'm looking for a more convincing use case.

gera@gera-laptop:~$ echo "foo bar baz spam eggs" | cat
foo bar baz spam eggs