2004-06-20 Yoshinori K. Okuji <okuji@enbug.org>
This is a big change on saving a default entry. This change makes it possible to set up a quite robust system using GRUB. Now we do not use the second sector of Stage 2 to store an entry number but use the file /boot/grub/default. This file must be generated by grub-set-default, although this file is plain-text. * util/grub-set-default.in: New file. * util/grub-install.in (grub_set_default): New variable. Use /grub instead of /boot/grub on OpenBSD as well as NetBSD. Run grub-set-default to make a default file. * util/Makefile.am (sbin_SCRIPTS): Added grub-set-default. * stage2/stage2.c (run_menu): Change the fallback handling to support multiple fallback entries. (cmain): Likewise. Also, get a saved entry from a default file if possible, before reading a config file. * stage2/shared.h (DEFAULT_FILE_BUF): New macro. (DEFAULT_FILE_BUFLEN): Likewise. (CMDLINE_BUF): Set to DEFAULT_FILE_BUF + DEFAULT_FILE_BUFLEN. (MENU_BUFLEN): Set to 0x8000 + PASSWORD_BUF - MENU_BUF. (fallback_entry): Removed. (fallback_entries): Declared. (fallback_entryno): Likewise. (MAX_FALLBACK_ENTRIES): New macro. * stage2/cmdline.c (run_script): Use FALLBACK_ENTRYNO instead of FALLBACK_ENTRY. * stage2/builtins.c (fallback_entry): Removed. (fallback_entryno): New variable. (fallback_entries): Likewise. (init_config): Initialize FALLBACK_ENTRYNO and FALLBACK_ENTRIES. (fallback_func): Rewritten completely. (savedefault_func): Likewise. * docs/grub.texi (grub-set-default): New direntry. (Installation): Describe grub-set-default for manual installations. (Making your system robust): New section. (Booting once-only): New subsection. (Booting fallback systems): Likewise. (fallback): Describe multiple fallback entries. (savedefault): Describe an optional argument. (Invoking grub-set-default): New chapter. (Future): Replaced with a description about GRUB 2. * configure.ac (AC_CONFIG_FILES): Added util/grub-set-default.
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docs/grub.texi
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docs/grub.texi
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@ -27,6 +27,8 @@
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* grub-terminfo: (grub)Invoking grub-terminfo. Generate a terminfo
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command from a
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terminfo name
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* grub-set-default: (grub)Invoking grub-set-default. Set a default boot
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entry
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* mbchk: (grub)Invoking mbchk. Check for the format of a Multiboot kernel
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@end direntry
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@ -113,6 +115,7 @@ This edition documents version @value{VERSION}.
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* Invoking grub-install:: How to use the GRUB installer
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* Invoking grub-md5-crypt:: How to generate a cryptic password
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* Invoking grub-terminfo:: How to generate a terminfo command
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* Invoking grub-set-default:: How to set a default boot entry
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* Invoking mbchk:: How to use the Multiboot checker
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* Obtaining and Building GRUB:: How to obtain and build GRUB
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* Reporting bugs:: Where you should send a bug report
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@ -475,11 +478,14 @@ if, by any chance, your hard drive becomes unusable (unbootable).
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GRUB comes with boot images, which are normally put in the directory
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@file{/usr/share/grub/i386-pc}. If you do not use grub-install, then
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you need to copy the files @file{stage1}, @file{stage2}, and
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@file{*stage1_5} to the directory @file{/boot/grub}. Hereafter, the
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directory where GRUB images are initially placed (normally
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@file{/usr/share/grub/i386-pc}) will be called the @dfn{image
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directory}, and the directory where the boot loader needs to find them
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(usually @file{/boot/grub}) will be called the @dfn{boot directory}.
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@file{*stage1_5} to the directory @file{/boot/grub}, and run the
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@command{grub-set-default} (@pxref{Invoking grub-set-default}) if you
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intend to use @samp{default saved} (@pxref{default}) in your
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configuration file. Hereafter, the directory where GRUB images are
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initially placed (normally @file{/usr/share/grub/i386-pc}) will be
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called the @dfn{image directory}, and the directory where the boot
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loader needs to find them (usually @file{/boot/grub}) will be called
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the @dfn{boot directory}.
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@menu
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* Creating a GRUB boot floppy::
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@ -734,6 +740,7 @@ magic.
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@menu
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* General boot methods:: How to boot OSes with GRUB generally
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* OS-specific notes:: Notes on some operating systems
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* Making your system robust:: How to make your system robust
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@end menu
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@ -1068,6 +1075,184 @@ grub> @kbd{boot}
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@end example
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@node Making your system robust
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@section How to make your system robust
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When you test a new kernel or a new OS, it is important to make sure
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that your computer can boot even if the new system is unbootable. This
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is crucial especially if you maintain servers or remote systems. To
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accomplish this goal, you need to set up two things:
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@enumerate
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@item
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You must maintain a system which is always bootable. For instance, if
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you test a new kernel, you need to keep a working kernel in a
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different place. And, it would sometimes be very nice to even have a
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complete copy of a working system in a different partition or disk.
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@item
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You must direct GRUB to boot a working system when the new system
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fails. This is possible with the @dfn{fallback} system in GRUB.
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@end enumerate
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The former requirement is very specific to each OS, so this
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documentation does not cover that topic. It is better to consult some
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backup tools.
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So let's see the GRUB part. There are two possibilities: one of them
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is quite simple but not very robust, and the other is a bit complex to
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set up but probably the best solution to make sure that your system
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can start as long as GRUB itself is bootable.
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@menu
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* Booting once-only::
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* Booting fallback systems::
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@end menu
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@node Booting once-only
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@subsection Booting once-only
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You can teach GRUB to boot an entry only at next boot time. Suppose
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that your have an old kernel @file{old_kernel} and a new kernel
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@file{new_kernel}. You know that @file{old_kernel} can boot
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your system correctly, and you want to test @file{new_kernel}.
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To ensure that your system will go back to the old kernel even if the
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new kernel fails (e.g. it panics), you can specify that GRUB should
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try the new kernel only once and boot the old kernel after that.
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First, modify your configuration file. Here is an example:
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@group
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@example
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default saved # This is important!!!
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timeout 10
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title the old kernel
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root (hd0,0)
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kernel /old_kernel
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savedefault
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title the new kernel
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root (hd0,0)
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kernel /new_kernel
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savedefault 0 # This is important!!!
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@end example
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@end group
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Note that this configuration file uses @samp{default saved}
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(@pxref{default}) at the head and @samp{savedefault 0}
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(@pxref{savedefault}) in the entry for the new kernel. This means
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that GRUB boots a saved entry by default, and booting the entry for the
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new kernel saves @samp{0} as the saved entry.
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With this configuration file, after all, GRUB always tries to boot the
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old kernel after it booted the new one, because @samp{0} is the entry
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of @code{the old kernel}.
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The next step is to tell GRUB to boot the new kernel at next boot
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time. For this, execute @command{grub-set-default} (@pxref{Invoking
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grub-set-default}):
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@example
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# @kbd{grub-set-default 1}
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@end example
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This command sets the saved entry to @samp{1}, that is, to the new
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kernel.
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This method is useful, but still not very robust, because GRUB stops
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booting, if there is any error in the boot entry, such that the new
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kernel has an invalid executable format. Thus, it it even better to
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use the @dfn{fallback} mechanism of GRUB. Look at next subsection for
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this feature.
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@node Booting fallback systems
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@subsection Booting fallback systems
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GRUB supports a fallback mechanism of booting one or more other
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entries if a default boot entry fails. You can specify multiple
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fallback entries if you wish.
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Suppose that you have three systems, @samp{A}, @samp{B} and
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@samp{C}. @samp{A} is a system which you want to boot by
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default. @samp{B} is a backup system which is supposed to boot
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safely. @samp{C} is another backup system which is used in case where
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@samp{B} is broken.
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Then you may want GRUB to boot the first system which is bootable
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among @samp{A}, @samp{B} and @samp{C}. A configuration file can be
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written in this way:
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@group
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@example
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default saved # This is important!!!
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timeout 10
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fallback 1 2 # This is important!!!
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title A
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root (hd0,0)
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kernel /kernel
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savedefault fallback # This is important!!!
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title B
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root (hd1,0)
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kernel /kernel
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savedefault fallback # This is important!!!
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title C
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root (hd2,0)
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kernel /kernel
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savedefault
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@end example
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@end group
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Note that @samp{default saved} (@pxref{default}), @samp{fallback 1 2}
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and @samp{savedefault fallback} are used. GRUB will boot a saved entry
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by default and save a fallback entry as next boot entry with this
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configuration.
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When GRUB tries to boot @samp{A}, GRUB saves @samp{1} as next boot
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entry, because the command @command{fallback} specifies that @samp{1}
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is the first fallback entry. The entry @samp{1} is @samp{B}, so GRUB
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will try to boot @samp{B} at next boot time.
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Likewise, when GRUB tries to boot @samp{B}, GRUB saves @samp{2} as
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next boot entry, because @command{fallback} specifies @samp{2} as next
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fallback entry. This makes sure that GRUB will boot @samp{C} after
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booting @samp{B}.
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It is noteworthy that GRUB uses fallback entries both when GRUB
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itself fails in booting an entry and when @samp{A} or @samp{B} fails
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in starting up your system. So this solution ensures that your system
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is started even if GRUB cannot find your kernel or if your kernel
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panics.
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However, you need to run @command{grub-set-default} (@pxref{Invoking
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grub-set-default}) when @samp{A} starts correctly or you fix @samp{A}
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after it crashes, since GRUB always sets next boot entry to a fallback
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entry. You should run this command in a startup script such as
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@file{rc.local} to boot @samp{A} by default:
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@example
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# @kbd{grub-set-default 0}
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@end example
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where @samp{0} is the number of the boot entry for the system
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@samp{A}.
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If you want to see what is current default entry, you can look at the
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file @file{/boot/grub/default} (or @file{/grub/default} in
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some systems). Because this file is plain-text, you can just
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@command{cat} this file. But it is strongly recommended @strong{not to
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modify this file directly}, because GRUB may fail in saving a default
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entry in this file, if you change this file in an unintended
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manner. Therefore, you should use @command{grub-set-default} when you
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need to change the default entry.
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@node Configuration
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@chapter Configuration
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@ -1952,12 +2137,13 @@ default entry is the entry saved with the command
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@node fallback
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@subsection fallback
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@deffn Command fallback num
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@deffn Command fallback num...
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Go into unattended boot mode: if the default boot entry has any errors,
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instead of waiting for the user to do something, immediately start
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over using the @var{num} entry (same numbering as the @code{default}
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command (@pxref{default})). This obviously won't help if the machine was
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rebooted by a kernel that GRUB loaded.
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rebooted by a kernel that GRUB loaded. You can specify multiple
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fallback entry numbers.
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@end deffn
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@ -2962,8 +3148,9 @@ derived from attempting the mount will @emph{not} work correctly.
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@node savedefault
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@subsection savedefault
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@deffn Command savedefault
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Save the current menu entry as a default entry. Here is an example:
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@deffn Command savedefault num
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Save the current menu entry or @var{num} if specified as a default
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entry. Here is an example:
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@example
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@group
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@ -2984,7 +3171,13 @@ savedefault
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@end example
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With this configuration, GRUB will choose the entry booted previously as
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the default entry. See also @ref{default}.
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the default entry.
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You can specify @samp{fallback} instead of a number. Then, next
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fallback entry is saved. Next fallback entry is chosen from fallback
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entries. Normally, this will be the first entry in fallback ones.
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See also @ref{default} and @ref{Invoking grub-set-default}.
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@end deffn
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@ -3559,6 +3752,68 @@ You must specify one argument to this command. For example:
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@end example
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@node Invoking grub-set-default
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@chapter Invoking grub-set-default
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The program @command{grub-set-default} sets the default boot entry for
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GRUB. This automatically creates a file named @file{default} under
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your GRUB directory (i.e. @file{/boot/grub}), if it is not
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present. This file is used to determine the default boot entry when
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GRUB boots up your system when you use @samp{default saved} in your
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configuration file (@pxref{default}), and to save next default boot
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entry when you use @samp{savedefault} in a boot entry
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(@pxref{savedefault}).
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@command{grub-set-default} accepts the following options:
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@table @option
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@item --help
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Print a summary of the command-line options and exit.
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@item --version
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Print the version information and exit.
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@item --root-directory=@var{dir}
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Use the directory @var{dir} instead of the root directory
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(i.e. @file{/}) to define the location of the default file. This
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is useful when you mount a disk which is used for another system.
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@end table
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You must specify a single argument to @command{grub-set-default}. This
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argument is normally the number of a default boot entry. For example,
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if you have this configuration file:
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@group
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@example
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default saved
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timeout 10
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title GNU/Hurd
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root (hd0,0)
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...
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title GNU/Linux
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root (hd0,1)
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...
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@end example
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@end group
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and if you want to set the next default boot entry to GNU/Linux, you
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may execute this command:
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@example
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@kbd{grub-set-default 1}
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@end example
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Because the entry for GNU/Linux is @samp{1}. Note that entries are
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counted from zero. So, if you want to specify GNU/Hurd here, then you
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should specify @samp{0}.
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This feature is very useful if you want to test a new kernel or to
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make your system quite robust. @xref{Making your system robust}, for
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more hints about how to set up a robust system.
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@node Invoking mbchk
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@chapter Invoking mbchk
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@ -3695,27 +3950,12 @@ Once we get your report, we will try to fix the bugs.
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@node Future
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@chapter Where GRUB will go
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Here are some ideas of what might happen in the future:
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@itemize @bullet
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@item
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Support dynamic loading.
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@item
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Add real memory management.
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@item
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Add a real scripting language.
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@item
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Support internationalization.
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@item
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Support other architectures than i386-pc.
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@end itemize
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See the file @file{TODO} in the source distribution, for more
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information.
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We started the next generation of GRUB, GRUB 2. This will include
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internationalization, dynamic module loading, real memory management,
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multiple architecture support, a scripting language, and many other
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nice feature. If you are interested in the development of GRUB 2, take
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a look at @uref{http://www.gnu.org/software/grub/grub.html, the
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homepage}.
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@c Separate the programming guide.
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@ -1,4 +1,4 @@
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@set UPDATED 11 May 2004
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@set UPDATED-MONTH May 2004
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@set UPDATED 20 June 2004
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@set UPDATED-MONTH June 2004
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@set EDITION 0.95
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@set VERSION 0.95
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@ -1,4 +1,4 @@
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@set UPDATED 11 May 2004
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@set UPDATED-MONTH May 2004
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@set UPDATED 20 June 2004
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@set UPDATED-MONTH June 2004
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@set EDITION 0.95
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@set VERSION 0.95
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