I am trying to understand what are the differences of boot messages verbosity levels for the kernel field in grub.conf
From my research, there appear to be three levels:
quiet
verbose
debug
I have also found documents that specify removing quiet from the kernel field. If this is done, is there a fourth default level or does the messaging level default to verbose or debug?
quiet and debug are Linux kernel command line parameters. They are not specific to GRUB.
See https://www.kernel.org/doc/Documentation/kernel-parameters.txt
quiet just disables most kernel log messages. Removing quiet reverts level of kernel log messages back to default.
verbose is a hack on some Linux distributions. If present on the kernel command line, one of the initialization files - such as /etc/rc.sysinit, or /etc/rc.d/rc.sysinit - does something like the following:
if /bin/grep -q " verbose" /proc/cmdline; then /bin/dmesg -n 8; fi