docker exec using the Docker Engine API with /var/run/docker.sock and curl

docker-exec.sh

#!/bin/sh

# Why? In a containerised environment, you may need to run commands on other Docker containers
# within the Docker bridge network. For slim images such as Alpine-based distributions, you do
# not want the docker executable installed. Instead, you can simply pass the docker socket during
# runtime, and interact with it using cURL. The following functions allow you to easily interact
# with other containers, as you would using the native docker exec command.

# Example of mounting the docker socket:
#
#  sudo chmod o+w /var/run/docker.sock
#  docker run --name my-container -v /var/run/docker.sock:/var/run/docker.sock my-container
#

# Requirements:
#
#  python
#  sed
#  curl
#

# This will be used over your existing docker executable
docker() {
  # See https://docs.docker.com/engine/api/#api-version-matrix to use the correct version
  curl -fs --unix-socket /var/run/docker.sock "http:/v1.40$2" -X$1 \
    -H "Content-Type: application/json" \
    -d "$3"
}

# The exec function is already taken by the global namespace
execute() {
  CONTAINER=$1
  shift
  ARGS=$@
  COMMAND=$(echo $ARGS | sed 's/\ /", "/g')
  EXEC_RESPONSE=$(docker POST "/containers/$CONTAINER/exec" "{\"AttachStdout\": true, \"Tty\": true, \"Cmd\": [\"$COMMAND\"]}")
  if [[ $? -ne 0 ]]; then
    echo "Unable to complete docker exec with command: $ARGS"
    return 1
  fi
  EXEC_ID=$(echo $EXEC_RESPONSE | python -c 'import json, sys; print(json.loads(sys.stdin.read())["Id"]);')
  docker POST "/exec/$EXEC_ID/start" '{"Detach": false, "Tty": true}'
  EXEC_JSON_RESPONSE=$(docker GET "/exec/$EXEC_ID/json")
  if [[ $? -ne 0 ]]; then
    echo "Unable to read response from docker exec with command: $ARGS"
    return 1
  fi
  EXIT_CODE=$(echo $EXEC_JSON_RESPONSE | python -c 'import json, sys; print(json.loads(sys.stdin.read())["ExitCode"]);')
  return $EXIT_CODE
}

# Working example using the methods above:
#
#   execute my-container ls -lah
#

# Equivalent docker exec command:
#
#   docker exec my-container ls -lah
#