Kubectl Rsync

krsync.sh

#!/bin/bash

#
# krsync - Kubernetes rsync wrapper
#
# This script enables rsync to work with Kubernetes pods by acting as a bridge
# between rsync and kubectl exec. It requires rsync to be installed in the pod
#
# It supports two syntaxes:
#
# 1. Simple pod name (uses default namespace):
#    krsync [rsync-options] source pod_name:destination
#
# 2. Pod with namespace:
#    krsync [rsync-options] source pod_name@namespace:destination
#
# Example usage:
#    ./krsync -avP . my-pod@my-namespace:/app/
#    ./krsync -avzhP --delete . my-pod:/app/
#
# The script operates in two phases:
# 1. Initial phase: Launches rsync with this script as the remote shell
# 2. RSH phase: Executes commands in the pod via kubectl exec
#
# Based on: https://serverfault.com/questions/741670/rsync-files-to-a-kubernetes-pod
#

set -euo pipefail  # Exit on error, undefined vars, and pipe failures

# Phase 1: Initial execution - launch rsync with this script as remote shell
if [[ -z "${KRSYNC_STARTED:-}" ]]; then
    export KRSYNC_STARTED=true

    # Validate that we have arguments
    if [[ $# -eq 0 ]]; then
        echo "Error: No arguments provided" >&2
        echo "Usage: $0 [rsync-options] source pod[@namespace]:destination" >&2
        exit 1
    fi

    # Launch rsync with this script as the remote shell handler
    # --blocking-io ensures proper I/O handling with kubectl exec
    exec rsync --blocking-io --rsh "$0" "$@"
fi

# Phase 2: RSH mode - we're being called by rsync as the remote shell
# At this point, rsync passes us the target and command to execute

# Initialize variables with safe defaults
namespace_flag=""
pod_name=""

# Validate we have at least one argument (the pod specification)
if [[ $# -eq 0 ]]; then
    echo "Error: No pod specification provided in RSH mode" >&2
    exit 1
fi

pod_name="$1"
shift

# Parse pod specification and extract namespace if provided
if [[ "$pod_name" == *"@"* ]]; then
    # Handle pod@namespace syntax: extract namespace and pod separately
    namespace_name="${pod_name#*@}"  # Everything after @
    pod_name="${pod_name%@*}"        # Everything before @

    # Validate extracted components
    if [[ -z "$namespace_name" ]]; then
        echo "Error: Empty namespace specified in pod@namespace syntax" >&2
        exit 1
    fi

    if [[ -z "$pod_name" ]]; then
        echo "Error: Empty pod name specified in pod@namespace syntax" >&2
        exit 1
    fi

    namespace_flag="--namespace=${namespace_name}"

elif [[ "$pod_name" == "-l" ]]; then
    # Handle legacy rsync -l flag format: rsync -l pod namespace ...
    # This happens when rsync interprets pod@namespace differently

    if [[ $# -lt 2 ]]; then
        echo "Error: Insufficient arguments for -l flag format" >&2
        exit 1
    fi

    pod_name="$1"
    shift
    namespace_name="$1"
    shift

    # Validate components
    if [[ -z "$namespace_name" ]]; then
        echo "Error: Empty namespace specified with -l flag" >&2
        exit 1
    fi

    if [[ -z "$pod_name" ]]; then
        echo "Error: Empty pod name specified with -l flag" >&2
        exit 1
    fi

    namespace_flag="--namespace=${namespace_name}"
fi

# Validate final pod name
if [[ -z "$pod_name" ]]; then
    echo "Error: No valid pod name could be determined" >&2
    exit 1
fi

# Execute the command in the specified pod using kubectl
# --stdin: Pass stdin to the container (required for rsync data transfer)
# --tty=false: Don't allocate a TTY (prevents interference with binary data)
# shellcheck disable=SC2086  # namespace_flag needs to expand as separate words
exec kubectl ${namespace_flag} exec --stdin --tty=false "$pod_name" -- "$@"