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
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.
The
--
tellscat
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 asPIDFILE="--version"
. On my machine, they give the following results:POSIX.1-2017
POSIX also specifies it at: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap12.html#tag_12_02
See also: https://unix.stackexchange.com/questions/11376/what-does-double-dash-mean-also-known-as-bare-double-dash