linux - What's the magic of "-" (a dash) in command-line parameters?

Question

Examples:

• Create an ISO image and burn it directly to a CD.

  mkisofs -V Photos -r /home/vivek/photos | cdrecord -v dev=/dev/dvdrw -

• Change to the previous directory.

  cd -

• Listen on port 12345 and untar data sent to it.

  nc -l -p 12345 | tar xvzf -

What is the purpose of the dash and how do I use it?

Answer 1

If you mean the naked - at the end of the tar command, that's common on many commands that want to use a file.

It allows you to specify standard input or output rather than an actual file name.

That's the case for your first and third example. For example, the cdrecord command is taking standard input (the ISO image stream produced by mkisofs) and writing it directly to /dev/dvdrw.

With the cd command, every time you change directory, it stores the directory you came from. If you do cd with the special - "directory name", it uses that remembered directory instead of a real one. You can easily switch between two directories quite quickly by using that.

Other commands may treat - as a different special value.