Reverse the files

tac command is reverse of cat. It writes each file to stdout, last line first.

If you want to reverse the whole file in vim you can do it by :%!tac 

Find number of lines selected in visual mode

Command: 𝐠 𝐂𝐓𝐑𝐋-𝐆.

					*g_CTRL-G* *word-count* *byte-count*
g CTRL-G		Prints the current position of the cursor in five
			ways: Column, Line, Word, Character and Byte.  If the
			number of Characters and Bytes is the same then the
			Character position is omitted.
			If there are characters in the line that take more
			than one position on the screen (<Tab> or special
			character), both the "real" column and the screen
			column are shown, separated with a dash.
			See also 'ruler' option.  {not in Vi}

Sample: Col 1 of 14; Line 1 of 2091; Word 1 of 5806; Byte 1 of 83448

							*v_g_CTRL-G*
{Visual}g CTRL-G	Similar to "g CTRL-G", but Word, Character, Line, and
			Byte counts for the visually selected region are
			displayed.
			In Blockwise mode, Column count is also shown.  (For
			{Visual} see |Visual-mode|.)
			{not in VI}

Sample: Selected 13 of 2091 Lines; 35 of 5806 Words; 290 of 83448 Bytes

Insert a file or the result from running an external program into the vim opened file

Suppose you have a file open in the vim and you want to insert output of some shell command into the file instead of closing the file or leaving the current terminal :read can be utilized to directly append the output of shell command into the file.

The :read command can insert a file or the result from running an external program into the current buffer. In case of the shell command it has to be prefixed with the !

Examples:

:read !openssl rand -hex 32 :read /etc/passwd