Switch distribution version in Debian

Use testing version of Debian as my primary working environment. But some softwares are available only in sid version, and bugs are fixed in sid firstly. So switching between testing and sid is routine. A simple script as follow is used to simplify/accelerate the operation.

#!/bin/sh

prog=$(basename $0)

usage()
{
        echo "Usage: $prog <testing|sid>"
}

current_dist()
{
        if grep sid /etc/apt/sources.list > /dev/null; then
                echo sid
        else
                echo testing
        fi
}

if [ $# -ne 1 ]; then
        usage
        exit -1
fi

tdist=$1
if [ $tdist != "sid" -a $tdist != "testing" ]; then
        usage
        exit -1
fi

cdist=$(current_dist)
if [ "$tdist" != "$cdist" ]; then
        cp /etc/apt/sources.list.$tdist /etc/apt/sources.list
        mv /var/lib/apt/lists /var/lib/apt/lists.$cdist
        if [ -d /var/lib/apt/lists.$tdist ]; then
                mv /var/lib/apt/lists.$tdist /var/lib/apt/lists
        fi
fi

apt-get update