POSIX-compliant function to prepend a path to an environment variable

pushv.sh

#!/bin/sh
#
# Usage: pushv VAR PATH
#
# Prepends PATH to the VAR variable. VAR should be a variable name, the
# variable itself should contain a colon-separated list of paths. The function
# sets VAR if previously unset and marks it for exporting. This function does
# nothing if PATH does not exists or if it is already present in VAR.
#
# EXAMPLE
#
# $ echo $PATH
# /usr/bin:/bin
# $ push PATH /usr/local/bin
# $ echo $PATH
# /usr/local/bin:/usr/bin:/bin
# $ push PATH /usr/local/bin
# $ echo $PATH
# /usr/local/bin:/usr/bin:/bin
#
# It is definitely a ugly hack, but what in the shell scripting world isn't :)

pushv() {
    # Don't bother if PATH does not exist
    if [ -d "$2" ]; then
        # Appends the content of VAR to the argument array. Ugly, but
        # in POSIX there is no local, and this keeps the namespace clean.
        eval set -- "$1 $2 \$$1"
               
        # If the variable is empty, just set it
        if [ -z "$3" ]; then
            eval "export $1=$2"
        else
            case ":$3:" in
                # If PATH already exists in VAR do nothing
                *":$2:"*) ;;
                *) eval "export $1=$2:$3" ;;
            esac
        fi
    fi
}