How know space disk left WITHOUT root

Question

I'd like to know if there is a way to get free space disk of a unix (debian) system without root (sudo) and if possible in only one command ?

Answer 1

Depends on what "free space" are you trying to get.

df provides information about free space for mounted filesystems and it doesn't require root:

$ df -h
Filesystem                           Size  Used Avail Use% Mounted on
devtmpfs                             717M     0  717M   0% /dev
tmpfs                                737M     0  737M   0% /dev/shm
tmpfs                                295M  1,3M  294M   1% /run
/dev/mapper/fedora-root               17G   11G  5,4G  67% /
tmpfs                                737M  9,5M  727M   2% /tmp
/dev/sda1                            976M  285M  625M  32% /boot

If you want to know how much free/unallocated space is on each disk (space not occupied by partitions), you need to use tools like parted or fdisk and you need root privileges for that. lsblk can help, but it doesn't print free space:

$ lsblk /dev/sdb
NAME   MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
sdb      8:16   0    1G  0 disk 
├─sdb1   8:17   0  100M  0 part 
├─sdb2   8:18   0  512B  0 part 
└─sdb3   8:19   0  100M  0 part 

But you can assume from its output that sdb here has approximately 800 MiB of free space for a new partition.

You can get better picture with UDisks, which also has information about partition layout, GNOME Disks uses this data to display free space on disks:

UDisks doesn't have a simple way to display this. You'd need to use its DBus API and calculate the free space yourself.