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Can I find free space of unmounted partition using system files like sys or proc?

I know how to find total space but have no idea about free space. Please suggest using system files only. For total space of unmounted partition I am using /proc/partitions file.

This is dependent on the filesystem type, but you could try to use fsck to find out how much free space there is left. Finding the free space requires a tool that understands the structure of the filesystem, and there usually aren't many of those in addition to fsck.

procfs should be used specifically for process related info. (as it is not stictly followed but still to be on safer side don't depend on it as there is chance of it getting deprecated).

so sysfs will give us system info.
this will give info of full disk size.

cat /sys/block/sda/size

replace sda with your partition name.

ls /sys/block/sda/

will give info about partitions available check for directory naming sda1, sda2 ...

same thing as sda can be used to check size of these partitions.

cat /sys/block/sda/sda1/size

Same note as above watch partition names. change sda to sdb,sdc, etc according to your requirement.

Hope this answers your question.

There is no general way for that, but there is a way which works in most cases.

The reason, why there is no general way for that, that also the kernel doesn't know it. To know it, first it would need to analyze the partition.

Without mounting the partition, the kernel knows only its existence, but no more. Essentially, it is a line of blocks with random data. The kernel knows, where is it and how to access, but doesn't know, what to do with it.

However, there are also user-space tools which can analyze the content of a filesystem, without mounting them. As all the filesystems have very different data structures, to handle them you need different tools.

In the case of ext2/3/4, it is

# dumpe2fs /dev/your_root|grep '^Free blocks'
dumpe2fs 1.42.13 (17-May-2015)
Free blocks: 5721580