In order to shrink a file system, the file system must be unmounted.
For example with a LVM Logical volume called testfilesystem in volume group testVG mount on /opt/testfilesystem ( /dev/testVG-testfilesystem or /dev/mapper/testVG-testfilesystem) that we want to make 162 GB in sise.
First unmount the filesystem:
umount /opt/testfilesystem
Then resize the filesystem:
resize2fs /dev/mapper/testVG-testfilesystem 162G
Check the file system integrety:
e2fsck -f /dev/mapper/testVG-testfilesystem
Now the logical volume can be shrunk to match the filesystem:
lvreduce -L 162G /dev/mapper/testVG-testfilesystem
Mount the file system again, assuming it is in /etc/fstab.
/opt/testfilesystem