How to move /var directory to another partition

Your /var directory has filled up and you are left with with no free disk space available. This is a typical scenario which can be easily fixed by mounting your /var directory on different partition. Let's get started by attaching new storage, partitioning and creating a desired file system. The exact steps may vary and are not part of this config article. Once ready obtain partition UUID of your new var partition eg. /dev/sdc1:

# blkid | grep sdc1
/dev/sdc1: UUID="1de46881-1f49-440e-89dd-6c32592491a7" TYPE="ext4" PARTUUID="652a2fee-01"

Create a new mount point and mount your new partition:

# mkdir /mnt/newvar
# mount /dev/sdc1 /mnt/newvar

Confirm that it is mounted. Note, your output will be different:

# df -h /mnt/newvar
Filesystem      Size  Used Avail Use% Mounted on
/dev/sdc1       1.8T  1.6T  279G  85% /mnt/newvar

Copy current /var data to the new location:

# rsync -aqxP /var/* /mnt/newvar

Unmount new partition:

# umount /mnt/newvar/  /mnt/var/

Edit your /etc/fstab to include new partition and choosing a relevant file-system:

UUID=1de46881-1f49-440e-89dd-6c32592491a7 /var        ext4    defaults        0       2

Reboot your system and you are done. Confirm that everything is working correctly and optionally remove old var directory by booting to some Live Linux system etc.