Retrieving results from variations of system('top') returns errors

Question

Directly on the Ubuntu terminal, I can extract the results from top regarding a specific PID with:

top -p <pid> -n1 | awk '/<pid>/'

Where <pid> should be substituted with a PID number.

When I am inside gVim 8.1, I try doing the same with:

:call system("top -p <pid> -n1 | awk '\/<pid>\/'")

I use the backslashes to make the forward-slashes be interpreted correctly as the / character. However, to my surprise, I get the error:

TERM environment variable not set

I found that to be odd, since things like :call system("ps") work perfectly.

If I try the same in terminal Vim 8.1, I get:

top: failed tty get^@

But again all sorts of variations of :call system("ps") work well.

Why would that be the case and how could I make it work such that I retrieve the results of system("top -p <pid> -n1 | awk '\/<pid>\/'") inside a variable like:

let myvariable = system("top -p <pid> -n1 | awk '\/<pid>\/'")

Answer 1

top is a interactive program, so I'm not surprised it has problems when called in a non interactive shell without a terminal. Loosely speaking this is like

$ top -p 7783 -n1 </dev/null | awk "/7783/"
top: failed tty get

To use top like you want, add the option -b for "batch mode".

From the man page:

-b Batch-mode operation
Starts top in Batch mode, which could be useful for sending output from top to other programs or to a file. In this mode, top will not accept input and runs until the iterations limit you've set with the `-n' command-line option or until killed.