xargs expr ...+

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http://www.unix.com – I would like to use expr on stdin, but I know it doesn't work. I have seen this syntax: Code: ls -l|wc -l | xargs expr -3 + and it seems to do what I need, but could someone explain what the final + is for and why it doesn't work if I run: Code: ls -l|wc -l | xargs expr +3 + Thanks in advance! (HowTos)