I see some linux page says how to install the key:

 wget -O - | sudo apt-key add -

What is "-O -" in wget?

It seems related with "add -".

How these works?

Thank you~

1

2 Answers

I used ** to indicate it is important. from man wget

-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 -.) ** Use of -O is not intended to mean simply "use the name file instead of the one in the URL;" rather, it is analogous to shell redirection: wget -O file is intended to work like wget -O - > file; file will be truncated immediately, and all downloaded content will be written there.

and from man apt-key

add filename Add a new key to the list of trusted keys. The key is read from filename, or ** standard input if filename is -.**

this explains your command.

The option -O - prints the downloaded file to standard output (instead of a ordinary file) and the - option in apt-key reads from standard input. The command is equivalent to the two commands:

wget
sudo apt-key add ros.key

When you chain both commands you don't have to bother with saving a file and usually the command is shorter.

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