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up vote 8 down vote favorite

I found this link on how to open tgz in one step.

gzip -dc target.tar.gz | tar xf -

What does the '-' mean?

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Also, most versions of tar now support gzip and bzip natively. For a gzip-ed tar file tar xfz target.tar.gz and for bzip tar xfj target.tar.bz2 – Deleted Account Mar 16 '11 at 23:04

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up vote 17 down vote accepted

It's a common convention to use - in a filename argument to mean stdin or stdout; tar follows the same convention. From the man page:

   -f, --file [HOSTNAME:]F
          use archive file or device F (default "-", meaning stdin/stdout)
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This is only a convention; you need to read the manpage for the particular command to see what - means to it. Stdin/stdout is by far the most common meaning, but there are others. For instance, to some versions of env it means "start with an empty environment". – cjm Mar 16 '11 at 23:05
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Another convention is to use '-' to indicate the end of flags, and the beginning of file names. That behavior is built-in to the getopt(3) on RedHat 4, at least. I can't find a Red Hat man page that uses '-' to indicate end-of-arguments, they all seem to use '--' to do that. The awk man page has '--' in that context, so maybe only old "heirloom" Unix or Solaris commands use '-' as a arguments - file names separator.

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-- signals the end of options, so everything that comes afterwards is an operand. - is not an option, so it and everything that comes afterwards is an operand. This is standard behavior. For many commands, such as tar, the operand - means standard input or output. On many shells, set - -a is equivalent to set -- -a, and on some old Bourne shells set - -a was the only accepted form, but this an exception with the set built-in. – Gilles Mar 16 '11 at 22:37
Solaris programs mainly follow the POSIX standard convention of -- to separate options from arguments. – alanc Jul 30 '11 at 7:35

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