Add Display Free Disk Space as a unix til

unix/display-free-disk-space.md

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+# Display Free Disk Space
+
+The `df` utility is a handy way to display the free disk space available on
+on a specific file system or all mounted file systems.
+
+Use `df` with the `-h` flag to display the disk space usage and availability
+in a human-readable format.
+
+Here is the output from a linode box of mine:
+
+```bash
+df -h
+Filesystem      Size  Used Avail Use% Mounted on
+/dev/xvda        20G  3.8G   16G  20% /
+none            4.0K     0  4.0K   0% /sys/fs/cgroup
+devtmpfs        994M  4.0K  994M   1% /dev
+none            200M  196K  199M   1% /run
+none            5.0M     0  5.0M   0% /run/lock
+none            996M     0  996M   0% /run/shm
+none            100M     0  100M   0% /run/user
+```
+
+h/t Josh Davey