What does dash “-” mean in Unix command line? [duplicate]

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Given this diff command:

./a.out < 1.in | diff - 1.out

What does - mean after the word diff?
Thanks for the help!

As the link mentions, many commands use - to mean either stdin or stdout, as appropriate. Eg, wget -q -O- http://example.com dumps the fetched HTML data to stdout (the -q is for quiet mode so progress info isn't mixed with the HTML). — PM 2Ring, Jan 19 at 9:35

Alejandro · Accepted Answer · 2015-01-19 03:38:20Z

up vote 2 down vote accepted

Traditionally, - means stdin (standard input). As you're redirecting, the output of the first command is the input of the second.

answered Jan 19 at 3:38

Alejandro
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marked as duplicate by Michael Homer, cuonglm, Ramesh, jasonwryan, Kyle Jones Jan 19 at 4:33