I prefer the following so I can still use top interactively without having to look up the pids each time I run it:
top -p `pgrep process-name | tr "\n" "," | sed 's/,$//'`
Of course if the processes change you'll have to re-run the command.
Explanation:
pgrep process-name
returns a list of process ids which are separated by newlines
tr "\n" ","
translates these newlines into commas, because top wants a comma-separated list of process ids
sed
is a stream editor, and sed 's/,$//'
is used here to remove the trailing comma
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