df command is dangerous for scripting - it gives incorrect results if you want to check for free space.

[Jim-JMCD]

In scripts, if you going to check for free space, before writing, you can't use the standard df $PWD or df $write_target as a check. According to the sequence below in WSL2, a script checking for free space will fail.

The script write until its filled up all the free space on the laptop/PC C:\ drive. This laptop only had WSL in its C:\ location.
If its a company server it could be a potential big problem for sysadmins.

$ df -h $PWD
Filesystem      Size  Used Avail Use% Mounted on
/dev/sdc       1007G  5.3G  951G   1% /

$ df -h /mnt/c
Filesystem      Size  Used Avail Use% Mounted on
C:\             931G  815G  116G  88% /mnt/c
$uname -a
Linux fred 5.15.153.1-microsoft-standard-WSL2 #1 SMP Fri Mar 29 23:14:13 UTC 2024 x86_64 x86_64 x86_64 GNU/Linux

• The first df tells me I have 951 GB of free space to play with in my WSL2 home directory.
• The second df tells me I have 116 GB of free space where WSL2 and everything else lives on the C:\ drive
• The reality is WSL has only 116 GB cause its VHD will dynamically grow until used up everything on the C:\ drive

This was reported earlier but the folks didn't realize the consequences #9757

Having WSL on a non-OS drive [D:] is interesting the drive size (1007G) is the same as previous but doesn't resemble any drive on both installations.

$ df -h $PWD
Filesystem      Size  Used Avail Use% Mounted on
/dev/sdc       1007G  2.5G  954G   1% /
$ df -h /mnt/d
Filesystem      Size  Used Avail Use% Mounted on
D:\             1.9T  1.6T  280G  85% /mnt/d
$ df -h /mnt/c
Filesystem      Size  Used Avail Use% Mounted on
C:\             112G   54G   59G  48% /mnt/c
$ echo $PWD
/home/jjm
bob@HP01:~$ df -h | grep /mnt/ | grep -v none
C:\             112G   54G   59G  48% /mnt/c
D:\             1.9T  1.6T  280G  85% /mnt/d
G:\             1.9T  1.7T  178G  91% /mnt/g
S:\             932G  700G  232G  76% /mnt/s
T:\             932G  900G   33G  97% /mnt/t
$ uname -a
Linux HP01 5.15.153.1-microsoft-standard-WSL2 #1 SMP Fri Mar 29 23:14:13 UTC 2024 x86_64 x86_64 x86_64 GNU/Linux