How to set GRUB timeout to 0 on Ubuntu 18.04

In /boot/grub/grub.cfg file there is a condition, almost at the end of the file, that sets the timeout to 10 if the timeout is set to 0. In other words, if you set the timeout to 0 in your /etc/default/grub and then update grub, the condition above reset it to 10 seconds.

if [ "${timeout}" = 0 ]; then
     set timeout=10
fi

However, /boot/grub/grub.cfg is a read-only file and I cannot remove that condition. I made some tests with different values of the timeout in /etc/default/grub. I tried with 1ms (0.001), 0.1s and 1s and I found out that values below 1 (like 0.1 and 0.001) work in the same way and almost like timeout set to 0.

In my case, the problem was that my system didn't support "recordfail" which caused a separate block to get added to the grub.cfg which defaults to a timeout of 30 seconds. The relevant code in /etc/grub.d/00_header:

if [ "$recordfail_broken" = 1 ]; then
  cat << EOF
if lsefi; then
  set timeout=${GRUB_RECORDFAIL_TIMEOUT:-30}
  if [ x\$feature_timeout_style = xy ] ; then
    set timeout_style=menu
  fi
fi
EOF

The fix is simply to add a value for GRUB_RECORDFAIL_TIMEOUT in /etc/default/grub and run update-grub again. For example:

GRUB_CMDLINE_LINUX_DEFAULT=""
GRUB_CMDLINE_LINUX=""

# Adjusted timeout for system which doesn't support recordfail
GRUB_RECORDFAIL_TIMEOUT=2

# Uncomment to enable BadRAM filtering, modify to suit your needs
# This works with Linux (no patch required) and with any kernel that obtains
# the memory map information from GRUB (GNU Mach, kernel of FreeBSD ...)
#GRUB_BADRAM="0x01234567,0xfefefefe,0x89abcdef,0xefefefef"

Like the other answers say, uncomment GRUB_HIDDEN_TIMEOUT and run update-grub. Then comment out the

if [ "${timeout}" = 0 ]; then
  set timeout=10
fi

section in /boot/grub/grub.cfg. In vim you can just override the read-only property with an exclamation point :x!. Or you can run

sudo chmod +w /boot/grub/grub.cfg
sudo vim /boot/grub/grub.cfg
sudo chmod -w /boot/grub/grub.cfg

to temporarily have write permission while editing the file.

You can set GRUB_TIMEOUT to 0.

The part overwriting timeout value is written in ajust_timeout function in the top of /etc/grub.d/30_os-prober.

ajust_timeout () {
...
if [ "\${timeout}" = 0]; then
  set timeout=10
fi
...
}

So, you can set the value by editing the file and comment out if-block.

Uncomment GRUB_HIDDEN_TIMEOUT=0 and run update-grub again.

You can set GRUB_TIMEOUT to -1.

Ex:GRUB_TIMEOUT="-1"

This behaviour is experienced in both Ubuntu 18.04 and 20.04 and even though some of the other answers has solutions that work as expected, either files that has on the header an "do not edit this file" notice such as the /boot/grub/grub.cfg are manually edited, or the changes are not persistent after system updates or regenerating grub configuration files (at least weren't for me). Here's a step by step of what I've been doing to change such behaviour in a persistent fashion.

Step 1 - Modify the template file /etc/grub.d/30_os-prober

Grub uses some template files located at /etc/grub.d in order to generate the /boot/grub/grub.cfg. Edit the file with your editor of choice...

# vim /etc/grub.d/30_os-prober

... and either comment or remove the lines below that override the timeout when it's set to zero.

if [ "\${timeout}" = 0 ]; then
  set timeout=10
fi

Step 2 - Modify the file /etc/default/grub

Grub uses this file to set configuration values in order to generate the /boot/grub/grub.cfg. Edit the file with your editor of choice...

# vim /etc/default/grub

... and set the timeout for zero (use the second option if the GRUB_TIMEOUT_STYLE is set to hidden)

GRUB_TIMEOUT=0

GRUB_HIDDEN_TIMEOUT=0

Step 3 - Generate the file /boot/grub/grub.cfg

# update-grub2

... and after the next machine reboot the expected behaviour is having a timeout of zero on grub menu.

We can simply add the line

set timeout=0

to /etc/grub.d/40_custom, make the file executable with

chmod +x /etc/grub.d/40_custom

and run

sudo update-grub

to generate the new /boot/grub/grub.cfg-file. The set timeout-command we added is the last set timeout-command in /boot/grub/grub.cfg now, that is what counts.

The solution is to add

GRUB_RECORDFAIL_TIMEOUT=0

to /etc/default/grub.

All other solutions will be reset on grub updates.