POSIX-Compliant Way to Scope Variables to a Function in a Shell Script

Question

Is there a POSIX Compliant way to limit the scope of a variable to the function it is declared in? i.e.:

Testing()
{
    TEST=\"testing\"
}

Testing
echo \"T         


        

Answer 1

If you'd like to journey down to Hell with me, I've made a more elaborated implementation of the eval concept.

This one automatically keeps an account of your quasi-scoped variables, can be called with a more familiar syntax, and properly unsets (as opposed to merely nulling) variables when leaving nested scopes.

---

Usage

As you can see, you push_scope to enter a scope, _local to declare your quasi-local variables, and pop_scope to leave a scope. Use _unset to unset a variable, and pop_scope will re-unset it when you back out into that scope again.

your_func() {
    push_scope
    _local x="baby" y="you" z

    x="can"
    y="have"
    z="whatever"
    _unset z

    push_scope
    _local x="you"
    _local y="like"
    pop_scope

    pop_scope
}

---

Code

All of the gibberish variable name suffixes are to be extra-safe against name collisions.

# Simulate entering of a nested variable scope
# To be used in conjunction with push_scope(), pop_scope(), and _local()
push_scope() {
    SCOPENUM_CEDD88E463CF11E8A72A3F9E5F08767D=$(( $SCOPENUM_CEDD88E463CF11E8A72A3F9E5F08767D + 1 ))
}

# Store the present value of the specified variable(s), allowing use in a new scope.
# To be used in conjunction with push_scope(), pop_scope(), and _local()
#
# Parameters:
# $@ : string; name of variable to store the value of
scope_var() {
    for varname_FB94CFD263CF11E89500036F7F345232 in "${@}"; do
        eval "active_varnames_FB94CFD263CF11E89500036F7F345232=\"\${SCOPE${SCOPENUM_CEDD88E463CF11E8A72A3F9E5F08767D}_VARNAMES}\""

        # echo "Active varnames: ${active_varnames_FB94CFD263CF11E89500036F7F345232}"

        case " ${active_varnames_FB94CFD263CF11E89500036F7F345232} " in
            *" ${varname_FB94CFD263CF11E89500036F7F345232} "* )
                # This variable was already stored in a previous call
                # in the same scope. Do not store again.
                # echo "Push \${varname_FB94CFD263CF11E89500036F7F345232}, but already stored."
                :
                ;;

            * )
                if eval "[ -n \"\${${varname_FB94CFD263CF11E89500036F7F345232}+x}\" ]"; then
                    # Store the existing value from the previous scope.
                    # Only variables that were set (including set-but-empty) are stored
                    # echo "Pushing value of \$${varname_FB94CFD263CF11E89500036F7F345232}"
                    eval "SCOPE${SCOPENUM_CEDD88E463CF11E8A72A3F9E5F08767D}_VARVALUE_${varname_FB94CFD263CF11E89500036F7F345232}=\"\${${varname_FB94CFD263CF11E89500036F7F345232}}\""
                else
                    # Variable is unset. Do not store the value; an unstored
                    # value will be used to indicate its unset state. The
                    # variable name will still be registered.
                    # echo "Not pushing value of \$${varname_FB94CFD263CF11E89500036F7F345232}; was previously unset."
                    :
                fi

                # Add to list of variables managed in this scope.
                # List of variable names is space-delimited.
                eval "SCOPE${SCOPENUM_CEDD88E463CF11E8A72A3F9E5F08767D}_VARNAMES=\"\${SCOPE${SCOPENUM_CEDD88E463CF11E8A72A3F9E5F08767D}_VARNAMES}${varname_FB94CFD263CF11E89500036F7F345232} \""
                ;;
        esac

        unset active_varnames_FB94CFD263CF11E89500036F7F345232
    done

    unset varname_FB94CFD263CF11E89500036F7F345232
}

# Simulate declaration of a local variable
# To be used in conjunction with push_scope(), pop_scope(), and _local()
#
# This function is a convenience wrapper over scope_var().
#
# Can be called just like the local keyword.
# Example usage: _local foo="foofoofoo" bar="barbarbar" qux qaz=""
_local() {
    for varcouple_44D4987063D111E8A46923403DDBE0C7 in "${@}"; do
        # Example string: foo="barbarbar"
        varname_44D4987063D111E8A46923403DDBE0C7="${varcouple_44D4987063D111E8A46923403DDBE0C7%%=*}"
        varvalue_44D4987063D111E8A46923403DDBE0C7="${varcouple_44D4987063D111E8A46923403DDBE0C7#*=}"
        varvalue_44D4987063D111E8A46923403DDBE0C7="${varvalue_44D4987063D111E8A46923403DDBE0C7#${varcouple_44D4987063D111E8A46923403DDBE0C7}}"

        # Store the value for the previous scope.
        scope_var "${varname_44D4987063D111E8A46923403DDBE0C7}"

        # Set the value for this scope.
        eval "${varname_44D4987063D111E8A46923403DDBE0C7}=\"\${varvalue_44D4987063D111E8A46923403DDBE0C7}\""

        unset varname_44D4987063D111E8A46923403DDBE0C7
        unset varvalue_44D4987063D111E8A46923403DDBE0C7
        unset active_varnames_44D4987063D111E8A46923403DDBE0C7
    done

    unset varcouple_44D4987063D111E8A46923403DDBE0C7
}

# Simulate unsetting a local variable.
#
# This function is a convenience wrapper over scope_var().
# 
# Can be called just like the unset keyword.
# Example usage: _unset foo bar qux
_unset() {
    for varname_6E40DA2E63D211E88CE68BFA58FE2BCA in "${@}"; do
        scope_var "${varname_6E40DA2E63D211E88CE68BFA58FE2BCA}"
        unset "${varname_6E40DA2E63D211E88CE68BFA58FE2BCA}"
    done
}

# Simulate exiting out of a nested variable scope
# To be used in conjunction with push_scope(), pop_scope(), and _local()
pop_scope() {
    eval "varnames_2581E94263D011E88919B3D175643B87=\"\${SCOPE${SCOPENUM_CEDD88E463CF11E8A72A3F9E5F08767D}_VARNAMES}\""

    # Cannot iterate over $varnames by setting $IFS; $IFS does not work
    # properly on zsh. Workaround using string manipulation.
    while [ -n "${varnames_2581E94263D011E88919B3D175643B87}" ]; do
        # Strip enclosing spaces from $varnames.
        while true; do
            varnames_old_2581E94263D011E88919B3D175643B87="${varnames_2581E94263D011E88919B3D175643B87}"
            varnames_2581E94263D011E88919B3D175643B87="${varnames_2581E94263D011E88919B3D175643B87# }"
            varnames_2581E94263D011E88919B3D175643B87="${varnames_2581E94263D011E88919B3D175643B87% }"

            if [ "${varnames_2581E94263D011E88919B3D175643B87}" = "${varnames_2581E94263D011E88919B3D175643B87}" ]; then
                break
            fi
        done

        # Extract the variable name for the current iteration and delete it from the queue.
        varname_2581E94263D011E88919B3D175643B87="${varnames_2581E94263D011E88919B3D175643B87%% *}"
        varnames_2581E94263D011E88919B3D175643B87="${varnames_2581E94263D011E88919B3D175643B87#${varname_2581E94263D011E88919B3D175643B87}}"

        # echo "pop_scope() iteration on \$SCOPE${SCOPENUM_CEDD88E463CF11E8A72A3F9E5F08767D}_VARVALUE_${varname_2581E94263D011E88919B3D175643B87}"
        # echo "varname: ${varname_2581E94263D011E88919B3D175643B87}"

        if eval "[ -n \""\${SCOPE${SCOPENUM_CEDD88E463CF11E8A72A3F9E5F08767D}_VARVALUE_${varname_2581E94263D011E88919B3D175643B87}+x}"\" ]"; then
            # echo "Value found. Restoring value from previous scope."
            # echo eval "${varname_2581E94263D011E88919B3D175643B87}=\"\${SCOPE${SCOPENUM_CEDD88E463CF11E8A72A3F9E5F08767D}_VARVALUE_${varname_2581E94263D011E88919B3D175643B87}}\""
            eval "${varname_2581E94263D011E88919B3D175643B87}=\"\${SCOPE${SCOPENUM_CEDD88E463CF11E8A72A3F9E5F08767D}_VARVALUE_${varname_2581E94263D011E88919B3D175643B87}}\""
            unset "SCOPE${SCOPENUM_CEDD88E463CF11E8A72A3F9E5F08767D}_VARVALUE_${varname_2581E94263D011E88919B3D175643B87}"
        else
            # echo "Unsetting \$${varname_2581E94263D011E88919B3D175643B87}"
            unset "${varname_2581E94263D011E88919B3D175643B87}"
        fi

        # Variable cleanup.
        unset varnames_old_2581E94263D011E88919B3D175643B87
    done

    unset SCOPE${SCOPENUM_CEDD88E463CF11E8A72A3F9E5F08767D}_VARNAMES
    unset varname_2581E94263D011E88919B3D175643B87
    unset varnames_2581E94263D011E88919B3D175643B87

    SCOPENUM_CEDD88E463CF11E8A72A3F9E5F08767D=$(( $SCOPENUM_CEDD88E463CF11E8A72A3F9E5F08767D - 1 ))
}