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daniels
Asked: 2010-02-21 03:38:41 +0800 CST

What does "--" (double dash) mean in this shell command?

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I have this shell command: 

kill `cat -- $PIDFILE`

What the double -- does here? Why not use just 

kill `cat $PIDFILE`
bash

2 Answers

  1. Best Answer

    The -- tells cat not to try to parse what comes after it as command line options.

    As an example, think of what would happen in the two cases if the variable $PIDFILE was defined as PIDFILE="--version". On my machine, they give the following results:

    $ cat $PIDFILE
cat (GNU coreutils) 6.10
Copyright (C) 2008 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Written by Torbjorn Granlund and Richard M. Stallman.

$ cat -- $PIDFILE
cat: --version: No such file or directory
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  2. POSIX.1-2017

    POSIX also specifies it at: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap12.html#tag_12_02

    12.2 Utility Syntax Guidelines

    Guideline 10:

    The first -- argument that is not an option-argument should be accepted as a delimiter indicating the end of options. Any following arguments should be treated as operands, even if they begin with the '-' character.

    See also: https://unix.stackexchange.com/questions/11376/what-does-double-dash-mean-also-known-as-bare-double-dash

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