Automatically change the /etc/hosts file depending on the network you are currently in

change-hosts-file.sh

#!/bin/bash
# Change the hosts file /etc/hosts - must run as root.
# To be used with a cron job or systemd service to automatically
# adjust the hosts file depending on the network the computer is in.

printUsage() {
    cat <<EOF
usage: $PROGNAME (-a | -n NETWORK) DOMAIN_NAME IP_ADDRESS
       $PROGNAME -r DOMAIN_NAME
options:
  -a
      add an entry to the hosts file
  -r
      remove an entry from the hosts file
  -n NETWORK
      switch the entry in the hosts file depending on the current network the
      computer is in. NETWORK must be the IPv4 broadcast address of the
      network as printed in 'ip addr'. use -N to see the current networks.
  -N
      print networks to which this computer is currently connected
  -h
      print this help message
EOF
}

readonly PROGNAME=$(basename "$0")

# $1: error message
exitWithError() {
    echo "$1" >&2
    exit 1
}

# $*: command line arguments = "$@"
parseCommandLine() {
    declare commandOptionCounter=0
    while getopts 'harn:N' OPTION; do
        case $OPTION in
            h)
                printUsage
                exit 0
                ;;
            a)  declare -rg ADD_ENTRY=1
                ((commandOptionCounter++))
                ;;
            r)  declare -rg REMOVE_ENTRY=1
                ((commandOptionCounter++))
                ;;
            n)  declare -rg NETWORK=$OPTARG
                ((commandOptionCounter++))
                ;;
            N)  printNetworks
                exit
                ;;
            *)  printUsage >&2
                # prints usage after the default error message (invalid option or missing option argument).
                # default error messages are disabled if the first character of optstring is ":".
                exit 1
                ;;
        esac
    done
    shift $((OPTIND-1))

    (( commandOptionCounter == 1 )) \
        || exitWithError "error: you must provide exactly one command option (-a, -r, or -n)"

    if (( $# != 2 )); then
        printUsage
        exit 1
    fi

    declare -gr DOMAIN_NAME=$1
    declare -gr IP_ADDRESS=$2

    [[ -n $ADD_ENTRY && -z $IP_ADDRESS ]] \
        && exitWithError "error: you must specify an IP address when using -a"

    return 0
}

createBackupOnFirstRun() {
    [[ -e /etc/hosts.orig ]] \
        || cp /etc/hosts /etc/hosts.orig
}

printNetworks() {
    ip addr | awk '$1=="inet"{print $4}'
    # ifconfig | awk '$1=="inet"{print $6}'
}

isInNetwork() {
    printNetworks | grep --quiet "$NETWORK"
}

addEntry() {
    if grep --quiet "$DOMAIN_NAME" /etc/hosts; then
        echo "info: hosts file already contains entries for '$DOMAIN_NAME'"
    else
        echo "debug: adding entry to hosts file"
        echo "$IP_ADDRESS    $DOMAIN_NAME" >> /etc/hosts
    fi
}

removeEntry() {
    declare tmp
    tmp=$(mktemp)
    echo "debug: removing entry from hosts file"
    grep -vF "$DOMAIN_NAME" /etc/hosts > "$tmp" && cp "$tmp" /etc/hosts
}

main() {
    parseCommandLine "$@"

    createBackupOnFirstRun

    if [[ -n $NETWORK ]]; then
        if isInNetwork; then
            echo "debug: is in network"
            addEntry
        else
            echo "debug: is not in network"
            removeEntry
        fi
    elif [[ -n $REMOVE_ENTRY ]]; then
        removeEntry
    elif [[ -n $ADD_ENTRY ]]; then
        addEntry
    else
        printUsage
    fi
}

main "$@"

Example installation using a cronjob:

$ sudo crontab -e
*/3 * * * * /root/bin/change-hosts-file  -n 192.168.156.255  example.com 192.168.156.201
^D