wget: what does this trailing dash represent/do?

Question

wget http://ipinfo.io/ip -qO -

I'm new to bash and can't seem to figure out what that trailing dash character does (or why it is required in this command).

Any help is much appreciated.

Answer 1

By convention (which not all programs follow), a dash in filename position refers to stdin or stdout, as appropriate. Since this is an argument to -O (output), it refers to stdout.

A more verbose way to write this (on Linux or other operating systems where /dev/stdout is usable by programs other than just the shell) would be:

wget http://ipinfo.io/ip --quiet --output-document=/dev/stdout

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As it happens, this behavior is defined by the POSIX Utility Syntax Guidelines. Specifically:

Guideline 5: One or more options without option-arguments, followed by at most one option that takes an option-argument, should be accepted when grouped behind one '-' delimiter.

...thus, -qO is treated identically to -q -O.

Guideline 13: For utilities that use operands to represent files to be opened for either reading or writing, the '-' operand should be used to mean only standard input (or standard output when it is clear from context that an output file is being specified) or a file named -.

...thus, the behavior regarding - is explicitly specified.

Answer 2

To print the output at the STDOUT which is usually your terminal.

See the manpage for wget which says :

-O file --output-document=file The documents will not be written to the appropriate files, but all will be concatenated together and written to file. If - is used as file, documents will be printed to standard output, disabling link conversion. (Use ./- to print to a file literally named -.)