CentOS-7 Systemd Unit File to Grow the Root LVM Disk on AWS EC2 Instances with an ext4 File System.

grow-root.service

# To install:
#     sudo cat grow-root.service > /etc/systemd/system/grow-root.service
#     sudo systemctl daemon-reload
#     sudo systemctl enable -f grow-root.service
#
# Start (auto disables after first successful expansion):
#     sudo systemctl start grow-root.service
#
# Debugging:
#     sudo systemctl status grow-root.service
#     journalctl -fn 50 -u grow-root.service
#
# To uninstall:
#     sudo systemctl disable -f grow-root.service

[Unit]
DefaultDependencies=no
Description=Grow root file system
Requires=local-fs.target
After=local-fs.target

[Service]
Type=oneshot
Environment="BIN_FS_RESIZER=/sbin/resize2fs"
Environment="DISK_DEVICE=/dev/xvda"
Environment="LVM_LV_ROOT=lv_root"
Environment="LVM_VG_ROOT=vg_root"
Environment="PARTITION_NUMBER=3"
Environment="PARTITION_TYPE=p"
Environment="PARTITION_TYPE_CODE=8e"
ExecStart=-/bin/bash -c \
  "if (( $(/bin/lsblk -lp --output NAME,SIZE | /bin/grep \"^${DISK_DEVICE} \" | /usr/bin/grep -o ' [0-9]*' || printf -- '0') > $(( $(/bin/lsblk -lp --output NAME,SIZE | /bin/grep \"^${DISK_DEVICE}1\" | /usr/bin/grep -o ' [0-9]*' || printf -- '0') + $(/bin/lsblk -lp --output NAME,SIZE | /bin/grep -m 1 \"^/dev/mapper/${LVM_VG_ROOT}-${LVM_LV_ROOT}\" | /usr/bin/grep -o ' [0-9]*' || printf -- '0') )) )); then \
    printf -- 'Space free on root volume\\n'; \
    if ! /bin/lsblk -lp --output NAME,SIZE | /bin/grep -q \"^${DISK_DEVICE}${PARTITION_NUMBER}\"; then \
      printf -- 'Growing root volume...\\n'; \
      printf -- 'n\\n%%s\\n%%s\\n\\n\\nt\\n%%s\\n%%s\\nw\\n' \
        \"${PARTITION_TYPE}\" \
        \"${PARTITION_NUMBER}\" \
        \"${PARTITION_NUMBER}\" \
        \"${PARTITION_TYPE_CODE}\" \
        | /sbin/fdisk ${DISK_DEVICE}; \
      /sbin/partprobe -s; \
      /sbin/pvcreate ${DISK_DEVICE}${PARTITION_NUMBER}; \
      /sbin/vgextend ${LVM_VG_ROOT} ${DISK_DEVICE}${PARTITION_NUMBER}; \
      /sbin/lvextend /dev/${LVM_VG_ROOT}/${LVM_LV_ROOT} ${DISK_DEVICE}${PARTITION_NUMBER}; \
      ${BIN_FS_RESIZER} /dev/${LVM_VG_ROOT}/${LVM_LV_ROOT}; \
      /bin/systemctl disable -f grow-root.service; \
    fi; \
  fi"

[Install]
WantedBy=local-fs.target