SELECT (blocks_available * 100 / blocks) AS percent_disk_space_available,
  round((blocks_available * blocks_size * 10e-10),2) AS gigs_disk_space_available,
  round((blocks * blocks_size * 10e-10),2) AS gigs_total_disk_space,
  (SELECT round(SUM(blocks * blocks_size) * 10e-10, 2) FROM mounts WHERE
-- exclude mounts with no space
blocks > 0
AND blocks_size > 0

-- exclude external storage
AND path NOT LIKE '/media%' AND path NOT LIKE '/mnt%'
-- exclude device drivers
AND path NOT LIKE '/dev%' 

-- exclude kernel-related mounts
AND path NOT LIKE '/proc%'
AND path NOT LIKE '/sys%'

-- exclude process files
AND path NOT LIKE '/run%'
AND path NOT LIKE '/var/run%'

-- exclude boot files
AND path NOT LIKE '/boot%' 

-- exclude snap packages
AND path NOT LIKE '/snap%' AND path NOT LIKE '/var/snap%'

-- exclude virtualized mounts, would double-count bare metal storage
AND path NOT LIKE '/var/lib/docker%'
AND path NOT LIKE '/var/lib/containers%'

AND type IN (
'ext4', 
'ext3', 
'ext2', 
'xfs', 
'btrfs', 
'ntfs', 
'vfat',
'fuseblk', --seen on NTFS and exFAT volumes mounted via FUSE
'zfs' --also valid storage
)
AND (
device LIKE '/dev/sd%' 
OR device LIKE '/dev/hd%' 
OR device LIKE '/dev/vd%' 
OR device LIKE '/dev/nvme%' 
OR device LIKE '/dev/mapper%'
OR device LIKE '/dev/md%'
OR device LIKE '/dev/dm-%'
)) AS gigs_all_disk_space
FROM mounts WHERE path = '/' LIMIT 1;

df -P / | awk 'NR==2 { blocks=$2; blocks_available=$4; blocks_size=$2/$3 } END { printf "percent_disk_space_available=%.2f\n", (blocks_available * 100 / blocks); printf "gigs_disk_space_available=%.2f\n", (blocks_available * blocks_size * 10e-10); printf "gigs_total_disk_space=%.2f\n", (blocks * blocks_size * 10e-10) }'

PowerShell commands are currently work in progress, contributions welcome.

Bash commands are currently work in progress, contributions welcome.