How to parse $QUERY_STRING from a bash CGI script?

Try this:

saveIFS=$IFS
IFS='=&'
parm=($QUERY_STRING)
IFS=$saveIFS

Now you have this:

parm[0]=a
parm[1]=123
parm[2]=b
parm[3]=456
parm[4]=c
parm[5]=ok

In Bash 4, which has associative arrays, you can do this (using the array created above):

declare -A array
for ((i=0; i<${#parm[@]}; i+=2))
do
    array[${parm[i]}]=${parm[i+1]}
done

which will give you this:

array[a]=123
array[b]=456
array[c]=ok

Edit:

To use indirection in Bash 2 and later (using the parm array created above):

for ((i=0; i<${#parm[@]}; i+=2))
do
    declare var_${parm[i]}=${parm[i+1]}
done

Then you will have:

var_a=123
var_b=456
var_c=ok

You can access these directly:

echo $var_a

or indirectly:

for p in a b c
do
    name="var$p"
    echo ${!name}
done

If possible, it's better to avoid indirection since it can make code messy and be a source of bugs.

Please don't use the evil eval junk.

Here's how you can reliably parse the string and get an associative array:

declare -A param   
while IFS='=' read -r -d '&' key value && [[ -n "$key" ]]; do
    param["$key"]=$value
done <<<"${QUERY_STRING}&"

If you don't like the key check, you could do this instead:

declare -A param   
while IFS='=' read -r -d '&' key value; do
    param["$key"]=$value
done <<<"${QUERY_STRING:+"${QUERY_STRING}&"}"

Listing all the keys and values from the array:

for key in "${!param[@]}"; do
    echo "$key: ${param[$key]}"
done

you can break $QUERY down using IFS. For example, setting it to &

$ QUERY="a=123&b=456&c=ok"
$ echo $QUERY
a=123&b=456&c=ok
$ IFS="&"
$ set -- $QUERY
$ echo $1
a=123
$ echo $2
b=456
$ echo $3
c=ok

$ array=($@)

$ for i in "${array[@]}"; do IFS="=" ; set -- $i; echo $1 $2; done
a 123
b 456
c ok

And you can save to a hash/dictionary in Bash 4+

$ declare -A hash
$ for i in "${array[@]}"; do IFS="=" ; set -- $i; hash[$1]=$2; done
$ echo ${hash["b"]}
456