When working with Linux networking, especially in shell scripting, you'll often need to programmatically determine the default gateway IP address. Here are the most reliable methods:

# Method 1: Using ip route (modern recommended approach)
ip route show default | awk '/default/ {print $3}'

# Method 2: Using netstat (legacy systems)
netstat -rn | awk '/^0.0.0.0|^default/ {print $2}'

# Method 3: Using route command
route -n | awk '/UG/ {print $2}'

The ip route command has become the standard for modern Linux distributions, replacing older utilities like ifconfig and route. It provides consistent output across different Linux flavors and handles IPv6 seamlessly.

# Example output from ip route:
default via 192.168.1.1 dev eth0 proto dhcp metric 100 
192.168.1.0/24 dev eth0 proto kernel scope link src 192.168.1.100 metric 100

For robust scripting, you should implement error handling and interface-specific queries. Here's an example function:

#!/bin/bash

get_gateway() {
    local interface=${1:-eth0}
    local gateway
    
    # Try ip route first
    gateway=$(ip route show default dev "$interface" 2>/dev/null | awk '/default/ {print $3}')
    
    # Fallback to netstat if ip route fails
    if [ -z "$gateway" ]; then
        gateway=$(netstat -rn | awk -v iface="$interface" '$0 ~ iface && /^0.0.0.0|^default/ {print $2}')
    fi
    
    echo "${gateway:-UNKNOWN}"
}

# Usage example:
router_ip=$(get_gateway eth0)
echo "Gateway IP for eth0: $router_ip"

In systems with several interfaces, you might need to process routing information differently:

# Get all gateways with their interfaces
ip -4 route show | awk '/default/ {print "Interface:", $5, "Gateway:", $3}'

# Sample output:
# Interface: eth0 Gateway: 192.168.1.1
# Interface: wlan0 Gateway: 10.0.0.1

When your script doesn't return expected results, consider these scenarios:

1. No default route configured (common in servers)
2. Multiple routing tables (check with ip rule list)
3. VPN connections altering routing

For VPN-specific cases:

# Find the original physical interface gateway
ip route show table main | awk '/default/ {print $3}'

When working with Linux networking configurations, especially in shell scripting or network troubleshooting, knowing your default gateway IP is fundamental. Here are the most reliable command-line methods:

# Method 1: Using ip route (modern recommended approach)
ip route show default | grep eth0 | awk '{print $3}'

# Method 2: Using route command (legacy systems)
route -n | grep 'UG[ \t]' | grep eth0 | awk '{print $2}'

# Method 3: Using netstat (deprecated but still works on some systems)
netstat -rn | grep 'UG' | grep eth0 | awk '{print $2}'

The ip route command is currently the most robust solution as it handles both IPv4 and IPv6 routes and works across most modern Linux distributions.

For scripting purposes, you'll typically want the clean output version that only returns the IP address:

GATEWAY_IP=$(ip route show default | grep eth0 | awk '{print $3}')
echo "Gateway IP: $GATEWAY_IP"

If your system has multiple network interfaces (eth0, eth1, wlan0, etc.), you'll need to be specific:

# Get gateway for specific interface
get_gateway_for_interface() {
    local iface=$1
    ip route show default | grep "$iface" | awk '{print $3}'
}

# Usage:
eth0_gateway=$(get_gateway_for_interface eth0)
echo "eth0 Gateway: $eth0_gateway"

For systems without the iproute2 package (though rare these days), you can use these alternatives:

# Using nmcli (NetworkManager systems)
nmcli device show eth0 | grep IP4.GATEWAY | awk '{print $2}'

# Using ifconfig + route (very old systems)
ifconfig eth0 && route -n | grep UG | awk '{print $2}'

When implementing this in production scripts, always include error checking:

#!/bin/bash

# Function to safely get gateway
get_gateway_ip() {
    local iface=${1:-eth0}
    local gw_ip
    
    if ! gw_ip=$(ip route show default | grep "$iface" | awk '{print $3}' 2>/dev/null); then
        echo "ERROR: Failed to determine gateway for $iface" >&2
        return 1
    fi
    
    if [ -z "$gw_ip" ]; then
        echo "ERROR: No gateway configured for $iface" >&2
        return 2
    fi
    
    echo "$gw_ip"
    return 0
}

# Main script usage
if gateway=$(get_gateway_ip eth0); then
    echo "Successfully retrieved gateway: $gateway"
    # Continue with your script logic
else
    exit 1
fi

If you're not getting expected results:

• Verify interface name with ip link show or ifconfig -a
• Check if interface is up with ip link set eth0 up
• Confirm routing table with ip route list
• Test connectivity with ping -c 4 $(get_gateway_ip eth0)

Remember that in some network configurations (like bridges or VLANs), the gateway might be associated with a different interface than you expect.