TWRP automatically does not mount system partition. It has to be mounted manually via its UI (under Mount option) or via command line. The fact that
ls /system showed you nothing gives us the evidence that the partition indeed is not mounted. It can further be confirmed using
If you must insist on mounting system partition using command line, you must get the block device ID for that partition first. There are myriad ways to do that and some are listed here by Izzy. I think the easiest may be the command
blkid as mentioned here by Ryan. In my device it outputs:
~ # blkid
/dev/block/mmcblk0p42: UUID="57f8f4bc-abf4-655f-bf67-946fc0f9f25b" TYPE="ext4"
/dev/block/mmcblk0p41: LABEL="system" UUID="da594c53-9beb-f85c-85c5-cedf76546f7a" TYPE="ext4"
/dev/block/mmcblk0p38: UUID="57f8f4bc-abf4-655f-bf67-946fc0f9f25b" TYPE="ext4"
/dev/block/mmcblk0p26: UUID="57f8f4bc-abf4-655f-bf67-946fc0f9f25b" TYPE="ext4"
/dev/block/mmcblk0p21: LABEL="FSG" UUID="e4a4f807-109f-5459-8138-e744bc88c397" TYPE="ext4"
/dev/block/mmcblk0p1: LABEL="NONHLOS" UUID="75d3199b-b965-b45f-9298-59c724d8a58d" TYPE="ext4"
Once you have the block device ID and the filesystem type (ext4 default) you issue the command:
mount -t ext4 BLOCK_DEV_ID MOUNT_POINT
mount -t ext4 /dev/block/mmcblkop41 /system # this is as an example only
You can now pull the files.