Linux Usefull Commands

chmod u+x   runnable file # user + Execute
chmod o+x   runnable file # other + Execute


tar -xvf file.tar

# Show right version of java
file /etc/alternatives/java /etc/alternatives/javac



#Empty A File 
cat /dev/null > ImageStore.log

#See Overall Storage 
df

#see disk usage 
du -h  /tmp

#empty old kernel that caused /boot full 
sudo apt-get purge $( dpkg --list | grep -P -o "linux-image-\d\S+" | grep -v $(uname -r | grep -P -o ".+\d") )

or 
      Type uname -r. This will show you the kernel you're using currently, so you don't want to remove this.
      
      Run the following command: dpkg --list | grep linux-image. This will show the all the kernels that are installed.
      
      Note down the names of all the kernels which you want to remove.
      
      To remove the kernels, run: sudo apt-get purge linux-image-x.x.x.x-xyz (replace the kernel name with an appropriate one).
      
      Update the GRUB: sudo update-grub2