Actually @Graeme's answer - and your question - are just referencing side-effects of how the shell handles "$@positional $*parameters". These are assigned by the shell to its arguments upon invocation and at any time later with the built-in set utility. They can be called upon at any time with either "$*" which splits each positional with the first character in "$IFS" or "$@" which quotes each positional and splits them with all of "$IFS."
man set
NAME
set — set or unset options and positional parameters
SYNOPSIS
set [−abCefhmnuvx] [−o option] [argument...]
set [+abCefhmnuvx] [+o option] [argument...]
set −− [argument...]
set −o
set +o
If you already have the values that you're feeding the shell, you don't need to -- three times. Shell parameters are setable - always, at any time, not just at invocation (excepting $0 and -i):
su - mikeserv -c 'set -- "$*" ; echo "$*" ;
set -- 4 5 6 ; echo "$*"' -- -- 7 8 9
7 8 9
4 5 6
And all of that shell quoting can be confusing. This simplifies things a little:
( set -- 4 5 6
su - mikeserv 4<<-\CMD /dev/fd/4 "$@"
echo $0 "$*"
set -- "$*"
echo "$*"
set -- 7 8 9
echo "$*"
CMD
)
/dev/fd/4 4 5 6
4 5 6
7 8 9
The parent shell's arguments are set to 4, 5, and 6 and are then passed to the subshell invoked by su via the positional parameter "$@array".
Note how I ( subshell ) the above command - I do that because I don't want to mess around with my current shell environment - because I can inadvertently change something I'd rather not if I did with set.
ABOUT REDIRECTION:
First of all, your Unix system works with files - file permissions, file contents, file attributes. In one way or another, every data object you use can (and, at least in my opinion, should) be addressed as a file. Redirection points to a file - that's all. A <<HERE-DOCUMENT will describe a file in-line then redirect it. Either shell expansions are interpreted or they are not.
The asker notes in the comments below that when he attempts to use this method as the root user he is delivered a permissions error. When I responded I suggested he chown or chgrp the /dev/fd/${num} special file, but this is probably not the best method. The reason he encounters this issue is root is granted read permissions but not execute permissions. You can easily handle this by just avoiding an exec call. Instead of invoking the /dev/fd/${num} file directly on the command line do:
su -c '. /dev/fd/'${num} ${num}<<SCRIPT
Using two heredocs can help with escaping. Here is what happens in every case:
NO SET IN <<HEREDOC
sh 3<<\CMD /dev/fd/3
( echo 'without set "$@" or \$@ in here-doc' ; echo
set -- '1 "2" 3' 4 "5 6"
su - mikeserv 4<<-UNQUOTED 5<<-\PREQUOTED /dev/fd/4
echo UNQUOTED; echo $0 "$*"
printf "%s\\t\\t%s\\t\\t%s\\t\\t%s\\n" $(printf "'%s' " "$@") \\
$@ '$@' "$@" '"$@"' "'$@'" \$@ '\$@' "\$@" '"\$@"'
. /dev/fd/5
UNQUOTED
echo PREQUOTED ; echo $0 "$*"
printf "%s\t\t%s\t\t%s\t\t%s\n" $(printf "'%s' " "$@") \
$@ '$@' "$@" '"$@"' \$@ '\$@' "\$@" '"\$@"'
PREQUOTED
)
CMD
OUTPUT
without set "$@" or \$@ in here-doc
UNQUOTED
/dev/fd/3 1 2 3 4 5 6
1 "2" 3 4 5 6 1
2 3 4 5
6 1 "2" 3 4 5 6 1 2 3 4 5 6 "1 "2" 3 4 5 6"
'1 2 3 4 5 6' $@ "$@"
PREQUOTED
/dev/fd/5
'' $@ "$@" $@
\$@ $@ "\$@"
SET "$@"IN <<HEREDOC
sh 3<<\CMD /dev/fd/3
( echo 'set "$@" and \$@ in here-doc' ; echo
set -- '1 "2" 3' 4 "5 6"
su - mikeserv 4<<-UNQUOTED 5<<-\PREQUOTED /dev/fd/4
set -- "$@" "\$@"
echo UNQUOTED; echo $0 "$*"
printf "%s\\t\\t%s\\t\\t%s\\t\\t%s\\n" $(printf "'%s' " "$@") \\
$@ '$@' "$@" '"$@"' "'$@'" \$@ '\$@' "\$@" '"\$@"'
. /dev/fd/5
UNQUOTED
set -- "$@" "\$@"
echo PREQUOTED ; echo $0 "$*"
printf "%s\t\t%s\t\t%s\t\t%s\n" $(printf "'%s' " "$@") \
$@ '$@' "$@" '"$@"' \$@ '\$@' "\$@" '"\$@"'
PREQUOTED
)
CMD
OUTPUT
set "$@" and \$@ in here-doc
UNQUOTED
/dev/fd/3 1 2 3 4 5 6
1 "2" 3 4 5 6 1
2 3 4 5
6 1 "2" 3 4 5 6 1 2 3 4 5 6 "1 "2" 3 4 5 6"
'1 2 3 4 5 6' 1 2 3 4 5 6 $@ 1 2 3 4 5 6
"$@"
PREQUOTED
/dev/fd/5 1 2 3 4 5 6 $@
'1 2 3 4
5 6' '$@' 1 2 3 4 5 6
$@ $@ 1 2 3 4 5 6 $@
"$@" $@ \$@ $@
"\$@"
SET "$@"AND MORE IN <<HEREDOC
sh 3<<\CMD /dev/fd/3
( echo 'set "$@" and \$@ AND additional parameters in here-doc' ; echo
set -- '1 "2" 3' 4 "5 6"
su - mikeserv 4<<-UNQUOTED 5<<-\PREQUOTED /dev/fd/4
set -- "$@" "\$@" '7 "8" 9' 10 "11 12"
echo UNQUOTED; echo $0 "$*"
printf "%s\\t\\t%s\\t\\t%s\\t\\t%s\\n" $(printf "'%s' " "$@") \\
$@ '$@' "$@" '"$@"' "'$@'" \$@ '\$@' "\$@" '"\$@"'
. /dev/fd/5
UNQUOTED
set -- "$@" "\$@" '13 "14" 15' 16 "17 18"
echo PREQUOTED ; echo $0 "$*"
printf "%s\t\t%s\t\t%s\t\t%s\n" $(printf "'%s' " "$@") \
$@ '$@' "$@" '"$@"' \$@ '\$@' "\$@" '"\$@"'
PREQUOTED
)
CMD
OUTPUT
set "$@" and \$@ AND additional parameters in here-doc
UNQUOTED
/dev/fd/3 1 2 3 4 5 6
1 "2" 3 4 5 6 1
2 3 4 5
6 1 "2" 3 4 5 6 1 2 3 4 5 6 "1 "2" 3 4 5 6"
'1 2 3 4 5 6' 1 2 3 4 5 6 7 "8" 9 10
11 12 $@ 1 2 3 4 5 6 7 "8" 9
10 11 12 "$@"
PREQUOTED
/dev/fd/5 1 2 3 4 5 6 7 "8" 9 10 11 12 $@ 13 "14" 15 16 17 18
'1 2 3 4
5 6' '7 "8"
9' '10' '11 12'
'$@' '13 "14" 15'
'16' '17 18' 1 2 3 4 5 6
7 "8" 9 10 11 12 $@
13 "14" 15 16 17 18 $@
1 2 3 4 5 6 7 "8" 9 10 11 12
$@ 13 "14" 15 16 17 18
"$@" $@ \$@ $@
"\$@"
bash 4.2.45on both source and destination accounts.