blkid /dev/sda1 -s UUID -o value

cat << EOF > /etc/fstab
    UUID=$UEFI_UUID    /boot/efi     vfat     defaults,noatime     0 2
    UUID=$ROOT_UUID    /             btrfs    $BTRFS_OPTS,subvol=@ 0 1 
    UUID=$ROOT_UUID    /home         btrfs    $BTRFS_OPTS,subvol=@home 0 2 
    UUID=$ROOT_UUID    /.snapshots   btrfs    $BTRFS_OPTS,subvol=@snapshots 0 2 
    UUID=$ROOT_UUID    /var/log      btrfs    $BTRFS_OPTS,subvol=@var_log 0 2
    tmpfs              /tmp          tmpfs    defaults,nosuid,nodev     0 0 
EOF

cat << EOF > /filelocation/file.txt
       Content you
       want to add
       to the file
EOF