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：http://askubuntu.com/questions/66751/how-do-i-set-default-display-brightness

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://blog.csdn.net/arthur_killer/article/details/44523559 或者http://www.linuxidc.com/Linux/2016-01/128043.htm

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 12.04,14.10 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