A common idiom in Unix is to count the lines of output in a file or pipe with wc -l:

$ wc -l example.txt
43
$ ps -e | wc -l
97

Sometimes you want to count the number of lines of output from a grep call, however. You might do it this way:

$ ps -ef | grep apache | wc -l
6

But grep has built-in counting of its own, with the -c option:

$ ps -ef | grep -c apache
6

The above is more a matter of good style than efficiency, but another tool with a built-in counting option that could save you time is the oft-used uniq. The below example shows a use of uniq to filter a sorted list into unique rows:

$ ps -eo user= | sort | uniq
105
daemon
lp
mysql
nagios
postfix
root
snmp
tom
www-data

If it would be useful to know in this case how many processes were being run by each of these users, you can include the -c option for uniq:

$ ps -eo user= | sort | uniq -c
    1 105
    1 daemon
    1 lp
    1 mysql
    1 nagios
    2 postfix
    78 root
    1 snmp
    7 tom
    5 www-data

You could even sort this output itself to show the users running the most processes first with sort -rn:

$ ps -eo user= | sort | uniq -c | sort -rn
    78 root
    8 tom
    5 www-data
    2 postfix
    1 snmp
    1 nagios
    1 mysql
    1 lp
    1 daemon
    1 105

Incidentally, if you’re not counting results and really do just want a list of unique users, you can leave out the uniq and just add the -u flag to sort:

$ ps -eo user= | sort -u
105
daemon
lp
mysql
nagios
postfix
root
snmp
tom
www-data

The above means I actually find myself using uniq with no options quite seldom.