How can I do traffic shaping in Linux by IP?

The traffic shaping layer of the kernel is, basically, a packet scheduler attached to your network card. So one traffic shaping policy applies to one network card.

What you can do, in your case, is to create a list of IP and bandwidth attached, and then, for each IP, you create:

• One traffic shaping rule identified by a classid
• One netfilter rule that will mark packets to a specific mark value
• One Filter that will bind that packets marks to the classid, thus applying the traffic control rule to the specified packets.

The example given by @Zoredache works, but I personnally prefer to use Netfilter capability instead of TC to filter packets, and HTB instead of CBQ for the shapping algorithm. So you can try something like this (requires Bash 4 for associative arrays):

#! /bin/bash
NETCARD=eth0
MAXBANDWIDTH=100000

# reinit
tc qdisc del dev $NETCARD root handle 1
tc qdisc add dev $NETCARD root handle 1: htb default 9999

# create the default class
tc class add dev $NETCARD parent 1:0 classid 1:9999 htb rate $(( $MAXBANDWIDTH ))kbit ceil $(( $MAXBANDWIDTH ))kbit burst 5k prio 9999

# control bandwidth per IP
declare -A ipctrl
# define list of IP and bandwidth (in kilo bits per seconds) below
ipctrl[192.168.1.1]="256"
ipctrl[192.168.1.2]="128"
ipctrl[192.168.1.3]="512"
ipctrl[192.168.1.4]="32"

mark=0
for ip in "${!ipctrl[@]}"
do
    mark=$(( mark + 1 ))
    bandwidth=${ipctrl[$ip]}

    # traffic shaping rule
    tc class add dev $NETCARD parent 1:0 classid 1:$mark htb rate $(( $bandwidth ))kbit ceil $(( $bandwidth ))kbit burst 5k prio $mark

    # netfilter packet marking rule
    iptables -t mangle -A INPUT -i $NETCARD -s $ip -j CONNMARK --set-mark $mark

    # filter that bind the two
    tc filter add dev $NETCARD parent 1:0 protocol ip prio $mark handle $mark fw flowid 1:$mark

    echo "IP $ip is attached to mark $mark and limited to $bandwidth kbps"
done

#propagate netfilter marks on connections
iptables -t mangle -A POSTROUTING -j CONNMARK --restore-mark

#!/bin/bash
set -x

DEV=eth0
export DEV

tc qdisc del dev $DEV root
tc qdisc del dev $DEV root
tc qdisc add dev $DEV root handle 1: cbq avpkt 1000 bandwidth 100mbit

# setup a class to limit to 1500 kilobits/s
tc class add dev $DEV parent 1: classid 1:1 cbq rate 1500kbit \
   allot 1500 prio 5 bounded isolated

# add traffic from 10.2.1.37 to that class
tc filter add dev $DEV parent 1: protocol ip prio 16 u32 \
   match ip src 10.2.1.37 flowid 1:1