You can use
ps -C to only display process information for a particular command name.
ps -C opera
You can then use other
ps options to extract just the data you are looking for. In particular,
--no-headers to suppress the column headers, and
-o pmem to show the percentage of memory used by the process.
ps -C opera --no-headers -o pmem
That will give you a bunch of memory-usage percentages, one per line.
There are numerous methods for summing data like that, one of the methods I use frequently is to pipe it into xargs to convert it into one line with elements delimited by spaces, then into sed to convert spaces to
+ symbols, and then into
bc to perform the calculation. Your method of piping into
paste -sd+ works as well or arguably better than
| xargs | sed.
Putting that all together, you get:
ps -C opera --no-headers -o pmem | xargs | sed -e 's/ /+/g' | bc
ps -C opera --no-headers -o pmem | paste -sd+ | bc
In other words, you can use
ps -C instead of multiple greps if you just want data about one particular running program.
NOTE: You can use multiple
-C options on the same command line if you want info about more than one program at a time. e.g.
ps -C iceweasel -C chromium -C opera