How to get available/free sdcard space in android with adb command

Use df, as with any Linux distro. Not all Android devices may have it, but it usually seems to be available.

df: Disk space details.
adb shell df
du: Disk space used by the specified files and subdirectories.
adb shell du
df -h: Disk space details in KB/MB format.
adb shell df -h
du -h: Disk space used by the specified files and subdirectories in KB/MB format.
adb shell du -h
For specific folder desk space details.
adb shell df /system
For specific folder and sub-directories desk space details
adb shell du /system

df -h to find disk usage and to find big files:

du -a <the_dir> | sort -n -r | head -n 20

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