Solved sh shell variables by reference

[freethread]

I'm not an expert about programming sh script and I run in trouble when I try to understand what happens in a big shell script (like portmaster). What I wish to do is writing functions with writable variables in parameters. I make an example:

#! /bin/sh

varResult=0

# Add two numbers and put result in a variable (by reference)
# Params:
#    $1    first number
#    $2    second number
#    $3    [ref] result
addFunc ()
{
    local num1 num2 result
    if [ "$#" -ne "3" ]; then
        echo "wrong params"
        return 1
    fi
    num1=$1
    num2=$2
    result=$3
    $result=$(( num1 + num2 ))
    return 0
}

# Main
addFunc "6" "4" "varResult"
echo "6 + 4 = $varResult"

I hope this example explain what I was say. Executing it give the result:

/root/bin/test.sh: varResult=10: not found
6 + 4 = 0

 

[SirDice]

    result=$3
    $result=$(( num1 + num2 ))

This doesn't do what you think it does, you need to eval(1) it.

    result=$3
    eval "$result=$(( num1 + num2 ))"

dice@molly:~/temp % ./test.sh
6 + 4 = 10

 

[Erichans]

There doesn't seem to be a FreeBSD man page with a description for eval.
Have a look at POSIX' eval; from the EXAMPLES:

foo=10 x=foo
y='$'$x
echo $y
$foo
eval y='$'$x
echo $y
10

I've found Sh - the POSIX Shell very useful.

 

[freethread]

Many thanks. I not take in account eval

 

[SirDice]

There doesn't seem to be a FreeBSD man page with a description for eval.

It's a shell builtin. Granted, the explanation in sh(1) is rather terse too. It doesn't provide much clues on how to use it or what you can do with it.

     eval string ...
             Concatenate all the arguments with spaces.  Then re-parse and
             execute the command.

 

[NapoleonWils0n]

my take using getopts for checking the arguments
and bc for calculations

#!/bin/sh

#===============================================================================
# number
# add two numbers
#===============================================================================

# dependencies:
# bc

#===============================================================================
# script usage
#===============================================================================

usage () {
# if argument passed to function echo it
[ -z "${1}" ] || echo "! ${1}"
# display help
echo "\
add two numbers
$(basename "$0") -f 1 -s 2"
exit 2
}


#===============================================================================
# error messages
#===============================================================================

INVALID_OPT_ERR='Invalid option:'
REQ_ARG_ERR='requires an argument'
WRONG_ARGS_ERR='wrong number of arguments passed to script'


#===============================================================================
# check the number of arguments passed to the script
#===============================================================================

[ $# -gt 0 ] || usage "${WRONG_ARGS_ERR}"


#===============================================================================
# getopts check the options passed to the script
#===============================================================================

while getopts ':f:s:h' opt
do
  case ${opt} in
     f) first="${OPTARG}";;
     s) second="${OPTARG}";;
     h) usage;;
     \?) usage "${INVALID_OPT_ERR} ${OPTARG}" 1>&2;;
     :) usage "${INVALID_OPT_ERR} ${OPTARG} ${REQ_ARG_ERR}" 1>&2;;
  esac
done
shift $((OPTIND-1))


#===============================================================================
# function
#===============================================================================

numberadd () {
printf "%s %s %s\n" "${first}" "+" "${second}" | bc
}


#===============================================================================
# run function
#===============================================================================

numberadd

run the script

./number -f 20 -s 23.5

output

43.5

help

./number -h

add two numbers
number -f 1 -s 2

run without any arguments

./number

! wrong number of arguments passed to script
add two numbers
number -f 1 -s 2

forgetting to pass an argument

./number -f 1 -s

! Invalid option: s requires an argument
add two numbers
number -f 1 -s 2

invalid option

./number -f 1 -x 3

! Invalid option: x
add two numbers
number -f 1 -s 2