Sample CNI plugin using Bash, for learning and debugging.

dummy.sh

#!/usr/bin/env bash

DEBUG=true
# LOGFILE=/tmp/seamless.log

# Outputs errors to stderr
errorlog () {
  >&2 echo $1
}

# Logs for debugging.
debuglog () {
  # Set DEBUG to anything to enable debug output.
  if [ -n "$DEBUG" ]; then
    # touch /tmp/seamless_a
    # Set logfile if you want to log to a flat file.
    if [ -n "$LOGFILE" ]; then
      # touch /tmp/seamless_b
      echo $1 >> $LOGFILE.$CNI_CONTAINERID
    else
      # Otherwise, log to stderr.
      >&2 echo $1
    fi
  fi
}

# Outputs an essentially dummy CNI result that's borderline acceptable by the spec.
# https://github.com/containernetworking/cni/blob/master/SPEC.md#result
cniresult () {
    cat << EOF
{
  "cniVersion": "0.4.0",
  "interfaces": [
      {
          "name": "dummy"
      }
  ],
  "ips": []
}
EOF
}

# Certain failures we want to exit on.
exit_on_error() {
    exit_code=$1
    last_command=${@:2}
    if [ $exit_code -ne 0 ]; then
        >&2 echo "dummy: \"${last_command}\" command failed with exit code ${exit_code}."
        cniresult
        exit $exit_code
    fi
}

# Overarching basic parameters.
containerifname=eth0

# --------------------------------------- REFERENCE: Common environment variables.
debuglog "CNI method: $CNI_COMMAND"
debuglog "CNI container id: $CNI_CONTAINERID"
debuglog "CNI netns: $CNI_NETNS"

cniresult