官方指定安装Docker的方法有两种，都很简单：

1. 通过源安装（例如，在Centos下使用yum安装）

   $ sudo yum install docker-engine

2. 使用脚本安装

   curl -fsSL https://get.docker.com/ | sh

本文主要分析脚本安装具体做了哪些事情。

• 首先设置 set -e，若后续命令执行失败（传回值不等于0），则立即退出shell。
• 定义和使用do_install()函数完成所有的安装，通过wrapped的方式防止在使用 curl | sh 拉取部分文件时导致脚本执行异常。

• do_install()完成的事情。

  • 通过uname -m查看机器类型，比如，x86_64、i686等。Docker目前仅支持64位平台，若不是则报错退出。
  • 通过command_exists()检查一些命令是否存在。其中，@表示传递给当前进程的命令行参数，置于双引号内，会展开为个别的参数。使用command -v的方式检查某个命令是否存在。

  command_exists() {
  command -v "$@" > /dev/null 2>&1
  }

  如果docker命令存在，通过docker -v | awk -F '[ ,]+' '{ print $3 }'可以得到Docker的版本号，例如， 1.10.1。然后使用semverParse()函数解析版本信息得到，major（主版本号），minor（次版本号），以及patch（补丁号）。如果版本号低于1.10.x，则进行告警，告知用户在安装前应该迁移备份镜像，并留给用户20秒的时间来选择放弃还是继续安装，可以使用 Ctrl+C 来停止脚本运行：

  You can find instructions for this here:
  https://github.com/docker/docker/wiki/Engine-v1.10.0-content-addressability-migration

  • 通过user="$(id -un 2>/dev/null || true)"得到当前的用户名。此脚本运行需要root权限，根据当前的用户名设置sh_c='sh -c'变量。
  • 初步的平台检查判断：
    
    • 首先检查是否存在lsb_release命令，如果存在可以获得平台信息，例如，Ubuntu。如果不存在则继续使用下面方法判断。
    • 判断-r /etc/lsb-release文件是是否可读，如果存在则通过$(. /etc/lsb-release && echo "$DISTRIB_ID")的方法读取环境变量得到平台信息。使用类似的方法，依次对不同的特定平台进行检测。最后通过，lsb_dist="$(echo "$lsb_dist" | tr '[:upper:]' '[:lower:]')"将平台标识统一转换为小写形式，便于后续的判断得到dist_version，例如，Ubuntu的dist_version为trusty。
    • 最后，根据不同的lsb_dist，执行对应的安装操作。每个平台的安装方法都有一些差异。
    • 如果平台检查失败，则提示用户查看https://docs.docker.com/engine/installation/，退出安装脚本。
  • 安装成功后，调用echo_docker_as_nonroot函数，告知用户如何使用非root权限操作Docker。
    

  If you would like to use Docker as a non-root user, you should now consider
  adding your user to the “docker” group with something like:
  sudo usermod -aG docker $your_user
  Remember that you will have to log out and back in for this to take effect!

下面是使用脚步升级Docker成功后的提示信息：

补充说明：如何卸载Docker？
以Centos为例，使用yum卸载Docker：

• List the package you have installed.

$ yum list installed | grep docker
docker-engine.x86_64   1.7.1-1.el7 @/docker-engine-1.7.1-1.el7.x86_64.rpm

• Remove the package.
  This command does not remove images, containers, volumes, or user-created configuration files on your host.

$ sudo yum -y remove docker-engine.x86_64

• To delete all images, containers, and volumes, run the following command:

$ rm -rf /var/lib/docker

• Locate and delete any user-created configuration files.

---

下面是安装脚本的所有源码：

#!/bin/sh
set -e
#
# This script is meant for quick & easy install via:
#   'curl -sSL https://get.docker.com/ | sh'
# or:
#   'wget -qO- https://get.docker.com/ | sh'
#
# For test builds (ie. release candidates):
#   'curl -fsSL https://test.docker.com/ | sh'
# or:
#   'wget -qO- https://test.docker.com/ | sh'
#
# For experimental builds:
#   'curl -fsSL https://experimental.docker.com/ | sh'
# or:
#   'wget -qO- https://experimental.docker.com/ | sh'
#
# Docker Maintainers:
#   To update this script on https://get.docker.com,
#   use hack/release.sh during a normal release,
#   or the following one-liner for script hotfixes:
#     s3cmd put --acl-public -P hack/install.sh s3://get.docker.com/index
#

url='https://get.docker.com/'

command_exists() {
    command -v "$@" > /dev/null 2>&1
}

echo_docker_as_nonroot() {
    if command_exists docker && [ -e /var/run/docker.sock ]; then
        (
            set -x
            $sh_c 'docker version'
        ) || true
    fi
    your_user=your-user
    [ "$user" != 'root' ] && your_user="$user"
    # intentionally mixed spaces and tabs here -- tabs are stripped by "<<-EOF", spaces are kept in the output
    cat <<-EOF

    If you would like to use Docker as a non-root user, you should now consider
    adding your user to the "docker" group with something like:

      sudo usermod -aG docker $your_user

    Remember that you will have to log out and back in for this to take effect!

    EOF
}

# Check if this is a forked Linux distro
check_forked() {

    # Check for lsb_release command existence, it usually exists in forked distros
    if command_exists lsb_release; then
        # Check if the `-u` option is supported
        set +e
        lsb_release -a -u > /dev/null 2>&1
        lsb_release_exit_code=$?
        set -e

        # Check if the command has exited successfully, it means we're in a forked distro
        if [ "$lsb_release_exit_code" = "0" ]; then
            # Print info about current distro
            cat <<-EOF
            You're using '$lsb_dist' version '$dist_version'.
            EOF

            # Get the upstream release info
            lsb_dist=$(lsb_release -a -u 2>&1 | tr '[:upper:]' '[:lower:]' | grep -E 'id' | cut -d ':' -f 2 | tr -d '[[:space:]]')
            dist_version=$(lsb_release -a -u 2>&1 | tr '[:upper:]' '[:lower:]' | grep -E 'codename' | cut -d ':' -f 2 | tr -d '[[:space:]]')

            # Print info about upstream distro
            cat <<-EOF
            Upstream release is '$lsb_dist' version '$dist_version'.
            EOF
        else
            if [ -r /etc/debian_version ] && [ "$lsb_dist" != "ubuntu" ]; then
                # We're Debian and don't even know it!
                lsb_dist=debian
                dist_version="$(cat /etc/debian_version | sed 's/\/.*//' | sed 's/\..*//')"
                case "$dist_version" in
                    8|'Kali Linux 2')
                        dist_version="jessie"
                    ;;
                    7)
                        dist_version="wheezy"
                    ;;
                esac
            fi
        fi
    fi
}

rpm_import_repository_key() {
    local key=$1; shift
    local tmpdir=$(mktemp -d)
    chmod 600 "$tmpdir"
    gpg --homedir "$tmpdir" --keyserver ha.pool.sks-keyservers.net --recv-keys "$key"
    gpg --homedir "$tmpdir" --export --armor "$key" > "$tmpdir"/repo.key
    rpm --import "$tmpdir"/repo.key
    rm -rf "$tmpdir"
}

semverParse() {
    major="${1%%.*}"
    minor="${1#$major.}"
    minor="${minor%%.*}"
    patch="${1#$major.$minor.}"
    patch="${patch%%[-.]*}"
}

do_install() {
    case "$(uname -m)" in
        *64)
            ;;
        *)
            cat >&2 <<-'EOF'
            Error: you are not using a 64bit platform.
            Docker currently only supports 64bit platforms.
            EOF
            exit 1
            ;;
    esac

    if command_exists docker; then
        version="$(docker -v | awk -F '[ ,]+' '{ print $3 }')"
        MAJOR_W=1
        MINOR_W=10

        semverParse $version

        shouldWarn=0
        if [ $major -lt $MAJOR_W ]; then
            shouldWarn=1
        fi

        if [ $major -le $MAJOR_W ] && [ $minor -lt $MINOR_W ]; then
            shouldWarn=1
        fi

        cat >&2 <<-'EOF'
            Warning: the "docker" command appears to already exist on this system.

            If you already have Docker installed, this script can cause trouble, which is
            why we're displaying this warning and provide the opportunity to cancel the
            installation.

            If you installed the current Docker package using this script and are using it
        EOF

        if [ $shouldWarn -eq 1 ]; then
            cat >&2 <<-'EOF'
            again to update Docker, we urge you to migrate your image store before upgrading
            to v1.10+.

            You can find instructions for this here:
            https://github.com/docker/docker/wiki/Engine-v1.10.0-content-addressability-migration
            EOF
        else
            cat >&2 <<-'EOF'
            again to update Docker, you can safely ignore this message.
            EOF
        fi

        cat >&2 <<-'EOF'

            You may press Ctrl+C now to abort this script.
        EOF
        ( set -x; sleep 20 )
    fi

    user="$(id -un 2>/dev/null || true)"

    sh_c='sh -c'
    if [ "$user" != 'root' ]; then
        if command_exists sudo; then
            sh_c='sudo -E sh -c'
        elif command_exists su; then
            sh_c='su -c'
        else
            cat >&2 <<-'EOF'
            Error: this installer needs the ability to run commands as root.
            We are unable to find either "sudo" or "su" available to make this happen.
            EOF
            exit 1
        fi
    fi

    curl=''
    if command_exists curl; then
        curl='curl -sSL'
    elif command_exists wget; then
        curl='wget -qO-'
    elif command_exists busybox && busybox --list-modules | grep -q wget; then
        curl='busybox wget -qO-'
    fi

    # check to see which repo they are trying to install from
    repo='main'
    if [ "https://test.docker.com/" = "$url" ]; then
        repo='testing'
    elif [ "https://experimental.docker.com/" = "$url" ]; then
        repo='experimental'
    fi

    # perform some very rudimentary platform detection
    lsb_dist=''
    dist_version=''
    if command_exists lsb_release; then
        lsb_dist="$(lsb_release -si)"
    fi
    if [ -z "$lsb_dist" ] && [ -r /etc/lsb-release ]; then
        lsb_dist="$(. /etc/lsb-release && echo "$DISTRIB_ID")"
    fi
    if [ -z "$lsb_dist" ] && [ -r /etc/debian_version ]; then
        lsb_dist='debian'
    fi
    if [ -z "$lsb_dist" ] && [ -r /etc/fedora-release ]; then
        lsb_dist='fedora'
    fi
    if [ -z "$lsb_dist" ] && [ -r /etc/oracle-release ]; then
        lsb_dist='oracleserver'
    fi
    if [ -z "$lsb_dist" ]; then
        if [ -r /etc/centos-release ] || [ -r /etc/redhat-release ]; then
            lsb_dist='centos'
        fi
    fi
    if [ -z "$lsb_dist" ] && [ -r /etc/os-release ]; then
        lsb_dist="$(. /etc/os-release && echo "$ID")"
    fi

    lsb_dist="$(echo "$lsb_dist" | tr '[:upper:]' '[:lower:]')"

    case "$lsb_dist" in

        ubuntu)
            if command_exists lsb_release; then
                dist_version="$(lsb_release --codename | cut -f2)"
            fi
            if [ -z "$dist_version" ] && [ -r /etc/lsb-release ]; then
                dist_version="$(. /etc/lsb-release && echo "$DISTRIB_CODENAME")"
            fi
        ;;

        debian)
            dist_version="$(cat /etc/debian_version | sed 's/\/.*//' | sed 's/\..*//')"
            case "$dist_version" in
                8)
                    dist_version="jessie"
                ;;
                7)
                    dist_version="wheezy"
                ;;
            esac
        ;;

        oracleserver)
            # need to switch lsb_dist to match yum repo URL
            lsb_dist="oraclelinux"
            dist_version="$(rpm -q --whatprovides redhat-release --queryformat "%{VERSION}\n" | sed 's/\/.*//' | sed 's/\..*//' | sed 's/Server*//')"
        ;;

        fedora|centos)
            dist_version="$(rpm -q --whatprovides redhat-release --queryformat "%{VERSION}\n" | sed 's/\/.*//' | sed 's/\..*//' | sed 's/Server*//')"
        ;;

        *)
            if command_exists lsb_release; then
                dist_version="$(lsb_release --codename | cut -f2)"
            fi
            if [ -z "$dist_version" ] && [ -r /etc/os-release ]; then
                dist_version="$(. /etc/os-release && echo "$VERSION_ID")"
            fi
        ;;


    esac

    # Check if this is a forked Linux distro
    check_forked

    # Run setup for each distro accordingly
    case "$lsb_dist" in
        amzn)
            (
            set -x
            $sh_c 'sleep 3; yum -y -q install docker'
            )
            echo_docker_as_nonroot
            exit 0
            ;;

        'opensuse project'|opensuse)
            echo 'Going to perform the following operations:'
            if [ "$repo" != 'main' ]; then
                echo '  * add repository obs://Virtualization:containers'
            fi
            echo '  * install Docker'
            $sh_c 'echo "Press CTRL-C to abort"; sleep 3'

            if [ "$repo" != 'main' ]; then
                # install experimental packages from OBS://Virtualization:containers
                (
                    set -x
                    zypper -n ar -f obs://Virtualization:containers Virtualization:containers
                    rpm_import_repository_key 55A0B34D49501BB7CA474F5AA193FBB572174FC2
                )
            fi
            (
                set -x
                zypper -n install docker
            )
            echo_docker_as_nonroot
            exit 0
            ;;
        'suse linux'|sle[sd])
            echo 'Going to perform the following operations:'
            if [ "$repo" != 'main' ]; then
                echo '  * add repository obs://Virtualization:containers'
                echo '  * install experimental Docker using packages NOT supported by SUSE'
            else
                echo '  * add the "Containers" module'
                echo '  * install Docker using packages supported by SUSE'
            fi
            $sh_c 'echo "Press CTRL-C to abort"; sleep 3'

            if [ "$repo" != 'main' ]; then
                # install experimental packages from OBS://Virtualization:containers
                echo >&2 'Warning: installing experimental packages from OBS, these packages are NOT supported by SUSE'
                (
                    set -x
                    zypper -n ar -f obs://Virtualization:containers/SLE_12 Virtualization:containers
                    rpm_import_repository_key 55A0B34D49501BB7CA474F5AA193FBB572174FC2
                )
            else
                # Add the containers module
                # Note well-1: the SLE machine must already be registered against SUSE Customer Center
                # Note well-2: the `-r ""` is required to workaround a known issue of SUSEConnect
                (
                    set -x
                    SUSEConnect -p sle-module-containers/12/x86_64 -r ""
                )
            fi
            (
                set -x
                zypper -n install docker
            )
            echo_docker_as_nonroot
            exit 0
            ;;

        ubuntu|debian)
            export DEBIAN_FRONTEND=noninteractive

            did_apt_get_update=
            apt_get_update() {
                if [ -z "$did_apt_get_update" ]; then
                    ( set -x; $sh_c 'sleep 3; apt-get update' )
                    did_apt_get_update=1
                fi
            }

            # aufs is preferred over devicemapper; try to ensure the driver is available.
            if ! grep -q aufs /proc/filesystems && ! $sh_c 'modprobe aufs'; then
                if uname -r | grep -q -- '-generic' && dpkg -l 'linux-image-*-generic' | grep -qE '^ii|^hi' 2>/dev/null; then
                    kern_extras="linux-image-extra-$(uname -r) linux-image-extra-virtual"

                    apt_get_update
                    ( set -x; $sh_c 'sleep 3; apt-get install -y -q '"$kern_extras" ) || true

                    if ! grep -q aufs /proc/filesystems && ! $sh_c 'modprobe aufs'; then
                        echo >&2 'Warning: tried to install '"$kern_extras"' (for AUFS)'
                        echo >&2 ' but we still have no AUFS.  Docker may not work. Proceeding anyways!'
                        ( set -x; sleep 10 )
                    fi
                else
                    echo >&2 'Warning: current kernel is not supported by the linux-image-extra-virtual'
                    echo >&2 ' package.  We have no AUFS support.  Consider installing the packages'
                    echo >&2 ' linux-image-virtual kernel and linux-image-extra-virtual for AUFS support.'
                    ( set -x; sleep 10 )
                fi
            fi

            # install apparmor utils if they're missing and apparmor is enabled in the kernel
            # otherwise Docker will fail to start
            if [ "$(cat /sys/module/apparmor/parameters/enabled 2>/dev/null)" = 'Y' ]; then
                if command -v apparmor_parser >/dev/null 2>&1; then
                    echo 'apparmor is enabled in the kernel and apparmor utils were already installed'
                else
                    echo 'apparmor is enabled in the kernel, but apparmor_parser missing'
                    apt_get_update
                    ( set -x; $sh_c 'sleep 3; apt-get install -y -q apparmor' )
                fi
            fi

            if [ ! -e /usr/lib/apt/methods/https ]; then
                apt_get_update
                ( set -x; $sh_c 'sleep 3; apt-get install -y -q apt-transport-https ca-certificates' )
            fi
            if [ -z "$curl" ]; then
                apt_get_update
                ( set -x; $sh_c 'sleep 3; apt-get install -y -q curl ca-certificates' )
                curl='curl -sSL'
            fi
            (
            set -x
            $sh_c "apt-key adv --keyserver hkp://p80.pool.sks-keyservers.net:80 --recv-keys 58118E89F3A912897C070ADBF76221572C52609D"
            $sh_c "mkdir -p /etc/apt/sources.list.d"
            $sh_c "echo deb [arch=$(dpkg --print-architecture)] https://apt.dockerproject.org/repo ${lsb_dist}-${dist_version} ${repo} > /etc/apt/sources.list.d/docker.list"
            $sh_c 'sleep 3; apt-get update; apt-get install -y -q docker-engine'
            )
            echo_docker_as_nonroot
            exit 0
            ;;

        fedora|centos|oraclelinux)
            $sh_c "cat >/etc/yum.repos.d/docker-${repo}.repo" <<-EOF
            [docker-${repo}-repo]
            name=Docker ${repo} Repository
            baseurl=https://yum.dockerproject.org/repo/${repo}/${lsb_dist}/${dist_version}
            enabled=1
            gpgcheck=1
            gpgkey=https://yum.dockerproject.org/gpg
            EOF
            if [ "$lsb_dist" = "fedora" ] && [ "$dist_version" -ge "22" ]; then
                (
                    set -x
                    $sh_c 'sleep 3; dnf -y -q install docker-engine'
                )
            else
                (
                    set -x
                    $sh_c 'sleep 3; yum -y -q install docker-engine'
                )
            fi
            echo_docker_as_nonroot
            exit 0
            ;;
        gentoo)
            if [ "$url" = "https://test.docker.com/" ]; then
                # intentionally mixed spaces and tabs here -- tabs are stripped by "<<-'EOF'", spaces are kept in the output
                cat >&2 <<-'EOF'

                  You appear to be trying to install the latest nightly build in Gentoo.'
                  The portage tree should contain the latest stable release of Docker, but'
                  if you want something more recent, you can always use the live ebuild'
                  provided in the "docker" overlay available via layman.  For more'
                  instructions, please see the following URL:'

                    https://github.com/tianon/docker-overlay#using-this-overlay'

                  After adding the "docker" overlay, you should be able to:'

                    emerge -av =app-emulation/docker-9999'

                EOF
                exit 1
            fi

            (
                set -x
                $sh_c 'sleep 3; emerge app-emulation/docker'
            )
            exit 0
            ;;
    esac

    # intentionally mixed spaces and tabs here -- tabs are stripped by "<<-'EOF'", spaces are kept in the output
    cat >&2 <<-'EOF'

      Either your platform is not easily detectable, is not supported by this
      installer script (yet - PRs welcome! [hack/install.sh]), or does not yet have
      a package for Docker.  Please visit the following URL for more detailed
      installation instructions:

        https://docs.docker.com/engine/installation/

    EOF
    exit 1
}

# wrapped up in a function so that we have some protection against only getting
# half the file during "curl | sh"
do_install