GRUB 2 Menu Not Shown When Booting Linux

Problem:

The GRUB 2 menu is not shown when booting into a Linux installation.

Cause:

By default, the GRUB 2 menu is hidden on many standard Linux installations.

Solution:

Depending on the system and the GRUB configuration, it may be possible to press ESC or hold the Shift key during startup to have the menu displayed. Otherwsie, you can edit the GRUB configuration options as detailed below.

Instructions:

1. Open a terminal.
   
   
2. Open the /etc/default/grub file in a text editor using root privileges or sudo. The command will vary depending on what editors are installed. For example, using gedit in Ubuntu:
   sudo gedit /etc/default/grub
   
   
3. The file will open and show the current options. The first few lines of the file from an Ubuntu install are shown here:
   # If you change this file, run 'update-grub' afterwards to update
   # /boot/grub/grub.cfg.
   # For full documentation of the options in this file, see:
   # info -f grub -n 'Simple configuration'
   GRUB_DEFAULT=0
   GRUB_HIDDEN_TIMEOUT=0
   GRUB_HIDDEN_TIMEOUT_QUIET=true
   GRUB_TIMEOUT=10
   GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian`
   GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
   GRUB_CMDLINE_LINUX=""
   
   
4. Comment out the GRUB_HIDDEN_TIMEOUT option by placing the # character at the beginning of the line:
   #GRUB_HIDDEN_TIMEOUT=0
   
   The menu will be displayed for the timeout period specified by the GRUB_TIMEOUT option. This option must be a positive integer value (default is 10 seconds).
   GRUB_TIMEOUT=10
   
   If you would like the menu to remain on the screen until a selection is made you can set the value to -1:
   GRUB_TIMEOUT=-1
   
   
5. Save the file and exit the editor.
   
   
6. From the terminal, run the update-grub command to update the GRUB files. This command must be run with root privileges or sudo.
   sudo update-grub
   
   
7. The GRUB menu will now be displayed as configured when booting the Linux installation.

The following link provides more details on the GRUB 2 options configured in this article as well as others:
https://help.ubuntu.com/community/Grub2/Setup#Configuring_GRUB_2