For example:

00:00:00 to 06:00:00 -> Slitaz
06:00:01 to 13:00:00 -> Ubuntu
13:00:01 to 19:00:00 -> Fedora
19:00:01 to 23:59:59 -> openSUSE

Can grub change default 'entry' automatically?

  • 1
    In theory, if you can use date and time related commands on /boot/grub/grub.cfg, choosing a grub entry for a given hour should be possible just by editing the files on /etc/grub.d. But I think the date/time before grub loads wouldn't even be a calibrated source (like NTP, for example). So that would leave you with the PC internal clock.
    – edwin
    Jul 22, 2013 at 1:54

3 Answers 3


First, run grep -E '(menuentry |submenu )' /boot/grub/grub.cfg to get a list of your grub menu entries. You should see something like:

menuentry 'Ubuntu' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-simple-7820cc72-dac4-447f-bba0-996ed1a12fa5' {
submenu 'Advanced options for Ubuntu' $menuentry_id_option 'gnulinux-advanced-7820cc72-dac4-447f-bba0-996ed1a12fa5' {
    menuentry 'Ubuntu, with Linux 3.16.0-28-generic' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-3.16.0-28-generic-advanced-7820cc72-dac4-447f-bba0-996ed1a12fa5' {
    menuentry 'Ubuntu, with Linux 3.16.0-28-generic (recovery mode)' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-3.16.0-28-generic-recovery-7820cc72-dac4-447f-bba0-996ed1a12fa5' {
    menuentry 'Ubuntu, with Linux 3.16.0-25-generic' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-3.16.0-25-generic-advanced-7820cc72-dac4-447f-bba0-996ed1a12fa5' {
    menuentry 'Ubuntu, with Linux 3.16.0-25-generic (recovery mode)' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-3.16.0-25-generic-recovery-7820cc72-dac4-447f-bba0-996ed1a12fa5' {
menuentry 'Memory test (memtest86+)' {
menuentry 'Memory test (memtest86+, serial console 115200)' {
menuentry 'Windows 7 (loader) (on /dev/sda2)' --class windows --class os $menuentry_id_option 'osprober-chain-C84087BD4087B12C' {

Here you can see my first menu entry is Ubuntu, followed by a Advanced options for Ubuntu submenu (with four other entries), 2 memory tests and, at last, Windows 7.

If we create a file named /boot/grub/custom.cfg, it will be loaded after /boot/grub/grub.cfg, so we can easily change GRUB's default configuration.

I used GRUB's module datehook to get the current time.


# This module creates special variables that return the current date/time
insmod datehook

# Add and extra 0 to minutes if it's less than 10 (force a 2-digit minute) 
if [ $MINUTE -lt 10 ]; then PADDING="0"; else PADDING=""; fi

# Boot "Ubuntu" from midnight to 5:59AM
if [ $TIME -ge 0 -a $TIME -lt 559 ]; then
    set default="Ubuntu"

# Boot "Windows 7" from 6AM to 4:59PM
if [ $TIME -ge 600 -a $TIME -lt 1659 ]; then
    set default="Windows 7 (loader) (on /dev/sda2)"

# If you want to boot an entry that's inside a submenu,
# you have to prepend its name with the submenu position, starting from 0.
# Boot "Ubuntu, with kernel 3.16.0-25-generic" from 5PM to 11:59PM
if [ $TIME -ge 1700 -a $TIME -lt 2359 ]; then
    set default="1>Ubuntu, with Linux 3.16.0-25-generic"

The module datehook makes available the variables: DAY, HOUR, MINUTE, MONTH, SECOND, WEEKDAY and YEAR, which return the actual date/time values based on hardware clock.

Let's take if [ $TIME -ge 600 -a $TIME -lt 1659 ]; then as an example. It means: if the current time is greater than or equal to 6AM and less than 4:59PM (16:59) then execute the next command (set default="Windows 7 (loader) (on /dev/sda2)"), which set the default variable with the Windows 7 menu entry name taken from that grep command above.

The last if block exemplifies the selection of a submenu entry. In that case "Ubuntu, with Linux 3.16.0-25-generic" lies inside a submenu that is the second entry in the main menu. As an entry position in a menu starts from 0, the menu entry named "Ubuntu" is 0 and the "Advanced options for Ubuntu" submenu is 1, that's why I had to add 1> before the entry name: set default="1>Ubuntu, with Linux 3.16.0-25-generic".

There no need to run update-grub.

The hardware clock may be unreliable, specially if the battery is dead. Also, enter BIOS setup and check the time. If it is UTC you'll have to change the time range in the script.


For the beginning, run the following command in terminal:

grep -E '^menuentry|^submenu' /boot/grub/grub.cfg | cut -d '"' -f2 | cut -d "'" -f2

This will return a list of your grub menu entries. I assume that in your case this list is something like this:

Advanced options
Memory test (memtest86+)
Memory test (memtest86+, serial console 115200)

Now, for each of these entries you should assign a number in ascending order starting with 0 (for "Slitaz" - 0, for "Advanced options" - 1 and so on). You will use this numbers to set default entry in grub menu.

Next, and the last thing, you should edit /boot/grub/grub.cfg file as follow:

  • From terminal open in gedit the file using:

    sudo -H gedit /boot/grub/grub.cfg
  • Find the line where default variable is set; it should look something similar with:

    set default="..."
  • Replace the above line with next code:

    insmod datehook
    if [ "$HOUR" -ge "0" -a "$HOUR" -lt "6" ]; then set default="0"      #Slitaz time
    elif [ "$HOUR" -ge "6" -a "$HOUR" -lt "13" ]; then set default="4"   #Ubuntu time
    elif [ "$HOUR" -ge "13" -a "$HOUR" -lt "19" ]; then set default="5"  #Fedora time     
    else set default="6"                                                 #openSUSE time
  • Save the file and close it.

That's all! Restart your PC and check if it is working.

Note: to revert these settings, just run sudo update-grub in terminal. In fact, this command will automatically generate and replace /boot/grub/grub.cfg file using templates from /etc/grub.d and settings from /etc/default/grub. So it will be better to put the above code in one template inside /etc/grub.d directory.

Source of inspiration: Scripting a Simple Boot Time State Machine in GRUB2.

Other sources:


You have 2 possibilities. First, you can add to every OS a cron script that changes config by clock. However, you will not like what will happen if this script kicks in while OS is shutting down, so beware. A better idea would be to acquire a UEFI motherboard for your PC. There you can install Shellx86 which is a pre-bootloader scripting console. Creating script for what you want in Shellx64 is no harder than scripting in bash. I used it to boot different OS depending on if you have Internet connection.

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