idioms for returning multiple values in shell scripting

Question

Are there any idioms for returning multiple values from a bash function within a script?

http://tldp.org/LDP/abs/html/assortedtips.html describes how to echo multipl

Answer 1

In order version of Bash which doesn't support nameref (introduced in Bash 4.3-alpha) I may define helper function in which the return value is assigned to the given variable. It's sort of like using eval to do the same kind of variable assignment.

Example 1

##  Add two complex numbers and returns it.
##  re: real part, im: imaginary part.
##
##  Helper function named by the 5th positional parameter
##  have to have been defined before the function is called.
complexAdd()
{
    local re1="$1" im1="$2" re2="$3" im2="$4" fnName="$5" sumRe sumIm

    sumRe=$(($re1 + $re2))
    sumIm=$(($im1 + $im2))

    ##  Call the function and return 2 values.
    "$fnName" "$sumRe" "$sumIm"
}

main()
{
    local fooRe='101' fooIm='37' barRe='55' barIm='123' bazRe bazIm quxRe quxIm

    ##  Define the function to receive mutiple return values
    ##  before calling complexAdd().
    retValAssign() { bazRe="$1"; bazIm="$2"; }
    ##  Call comlexAdd() for the first time.
    complexAdd "$fooRe" "$fooIm" "$barRe" "$barIm" 'retValAssign'

    ##  Redefine the function to receive mutiple return values.
    retValAssign() { quxRe="$1"; quxIm="$2"; }
    ##  Call comlexAdd() for the second time.
    complexAdd "$barRe" "$barIm" "$bazRe" "$bazIm" 'retValAssign'

    echo "foo = $fooRe + $fooIm i"
    echo "bar = $barRe + $barIm i"
    echo "baz = foo + bar = $bazRe + $bazIm i"
    echo "qux = bar + baz = $quxRe + $quxIm i"
}

main

Example 2

##  Add two complex numbers and returns it.
##  re: real part, im: imaginary part.
##
##  Helper functions
##      getRetRe(), getRetIm(), setRetRe() and setRetIm()
##  have to have been defined before the function is called.
complexAdd()
{
    local re1="$1" im1="$2" re2="$3" im2="$4"

    setRetRe "$re1"
    setRetRe $(($(getRetRe) + $re2))

    setRetIm $(($im1 + $im2))
}

main()
{
    local fooRe='101' fooIm='37' barRe='55' barIm='123' bazRe bazIm quxRe quxIm

    ##  Define getter and setter functions before calling complexAdd().
    getRetRe() { echo "$bazRe"; }
    getRetIm() { echo "$bazIm"; }
    setRetRe() { bazRe="$1"; }
    setRetIm() { bazIm="$1"; }
    ##  Call comlexAdd() for the first time.
    complexAdd "$fooRe" "$fooIm" "$barRe" "$barIm"

    ##  Redefine getter and setter functions.
    getRetRe() { echo "$quxRe"; }
    getRetIm() { echo "$quxIm"; }
    setRetRe() { quxRe="$1"; }
    setRetIm() { quxIm="$1"; }
    ##  Call comlexAdd() for the second time.
    complexAdd "$barRe" "$barIm" "$bazRe" "$bazIm"

    echo "foo = $fooRe + $fooIm i"
    echo "bar = $barRe + $barIm i"
    echo "baz = foo + bar = $bazRe + $bazIm i"
    echo "qux = bar + baz = $quxRe + $quxIm i"
}

main