echo $ {1 +“ $ @”}是什么意思

Question

From /usr/local/bin/erl

ROOTDIR=/usr/local/lib/erlang
BINDIR=$ROOTDIR/erts-5.9.1/bin
EMU=beam
PROGNAME=`echo $0 | sed 's/.*\///'`
export EMU
export ROOTDIR
export BINDIR
export PROGNAME
exec $BINDIR/erlexec ${1+"$@"}

I know "$@" meams arguments. But {1+"$@"} means what?

Answer 1

From IEEE Std 1003.1 (POSIX 2013), Shell command language :

${parameter:+[word]}

Use Alternative Value. If parameter is unset or null, null shall be substituted; otherwise, the expansion of word (or an empty string if word is omitted) shall be substituted.

Ie, ${1+"$@"} expands to the value of "$@" , the command line arguments, except when $1 is not set, ie there are no command line arguments, in which case the expression expands to nothing. A simpler script that shows how this works is

echo '"' ${1+"$@"} '"'

If you store this in a file test.sh and run it, you get:

/tmp$ sh test.sh
" "
/tmp$ sh test.sh 1
" 1 "
/tmp$ sh test.sh 1 2
" 1 2 "

(The spaces at the begin and end come from echo .)

Answer 2

larsmans provides an explanation of the semantics, but doesn't clarify why ${1+"$@"} is different than simply "${@}" . In a properly behaving shell, "${@}" expands to nothing. That is: foo "$@" should call foo with no arguments if "$@" is empty. In an incorrectly behaving shell, foo "$@" will be invoked with one argument (the empty string). Many historical shells expanded "$@" to the empty string rather than to nothing, but ${1+"$@"} correctly expands to nothing.