如果，你的Ubuntu系统每次开机之后屏幕都特别亮，那么别怀疑，你不是第一个发现这个问题的人，本文简要介绍如何解决这个问题。
主要参考：
英语不好的看这边。本解决方法只针对12.04, 12.10, 13.10, 14.04, 14.10。
方法一：设置 /sys/class/backlight/acpi_video0/brightness或者/sys/class/backlight/intel_backlight/brightness
这种方法请参考：http://www.linuxdiyf.com/linux/26063.html
方法二：在终端下，执行如下脚本（直接复制粘贴并运行，或者建立一个sh脚本再运行都OK）
# Install xbacklight in case it is not already installed (small).
sudo apt-get install xbacklight
# You should test xbacklight with command-line to see if it works. For example:
# xbacklight = 100 ; sleep 2 ; xbacklight = 30
# Try to figure out a suitable value for your hardware and lighting conditions.
# This creates a small script running xbacklight.
# You can change the =30 into another value if you wish.
# The "|| true" ensures that if xbacklight fails for any reason, X can still start.
sudo bash -c '{
echo "#!/bin/bash"
echo "xbacklight =30 || true"
} >> /etc/lightdm/display-setup-script.sh '
# This makes that script executable
sudo chmod a+rx /etc/lightdm/display-setup-script.sh
# This instructs lightdm to run the script when starting X.
# Specifically, it adds a line display-setup-script in a lightdm configuration file/, but only if there is not one already.
if grep -ri ^display-setup-script /etc/lightdm/
then
echo "There may be already a display-setup-script. It may already do what you need. Else please adjust manually" ;
else
if [[ -d /etc/lightdm/lightdm.conf.d ]]
then
# Ubuntu 13.10 and above have lightdm.conf.d. 14.04 *only* have lightdm.conf.d
DESTCONFFILE=/etc/lightdm/lightdm.conf.d/20-default-brightness.conf
else
# Ubuntu 12.04, 12.10, 14.10 do not have lightdm.conf.d, we change main configuration file
DESTCONFFILE=/etc/lightdm/lightdm.conf
fi
echo "Writing into $DESTCONFFILE"
sudo bash -c "{ echo '[SeatDefaults]' ; echo display-setup-script=/etc/lightdm/display-setup-script.sh ; } >>$DESTCONFFILE" ;
fi
在我自己的 Ubuntu 14.04 上面，第一种方法无效，第二种方法有效。祝你好运！