摘要：其中從參數(shù)第個(gè)位置取字符，如為，則取字符，若條件為真，通過(guò)命令重置參數(shù)，添加前綴，即經(jīng)過(guò)處理后變?yōu)椤?/p>

Docker Hub中有很多好用的Docker鏡像，但鏡像到底如何工作、能做什么、怎么做值得我們研究，如下所示為MySQL官方鏡像的docker-entrypoint.sh腳本分析：

#!/bin/bash
set -eo pipefail
shopt -s nullglob

################################################################
# 若啟動(dòng)命令時(shí)附加了參數(shù)，則在參數(shù)前添加mysqld，如$0 -f test，則經(jīng)過(guò)此代碼處理后，
# $@參數(shù)變mysqld -f test。其中${1:0:1}從$1參數(shù)第0個(gè)位置取1字符，如$1為-f，則
# 取"-"字符，若條件為真，通過(guò)set命令重置$@參數(shù)，添加mysqld前綴，即經(jīng)過(guò)處理后$1變
# 為mysqld。
################################################################
# if command starts with an option, prepend mysqld
if [ "${1:0:1}" = "-" ]; then
    set -- mysqld "$@"
fi

# 解析參數(shù)，是否是獲取幫助信息參數(shù)，并設(shè)置wantHelp值
#####################################################
# skip setup if they want an option that stops mysqld
wantHelp=
for arg; do
    case "$arg" in
        -"?"|--help|--print-defaults|-V|--version)
            wantHelp=1
            break
            ;;
    esac
done

#############################
# 從文件中讀取變量值
#############################
# usage: file_env VAR [DEFAULT]
#    ie: file_env "XYZ_DB_PASSWORD" "example"
# (will allow for "$XYZ_DB_PASSWORD_FILE" to fill in the value of
#  "$XYZ_DB_PASSWORD" from a file, especially for Docker"s secrets feature)
file_env() {
    local var="$1"
    local fileVar="${var}_FILE"
    local def="${2:-}"
    if [ "${!var:-}" ] && [ "${!fileVar:-}" ]; then
        echo >&2 "error: both $var and $fileVar are set (but are exclusive)"
        exit 1
    fi
    local val="$def"
    if [ "${!var:-}" ]; then
        val="${!var}"
    elif [ "${!fileVar:-}" ]; then
        val="$(< "${!fileVar}")"
    fi
    export "$var"="$val"
    unset "$fileVar"
}

###########################################################################
# 運(yùn)行mysqld --help --verbose --help 2>&1 >/dev/null命令，
# 此命令會(huì)檢查配置文件，若配置文件沒(méi)問(wèn)題，則成功，不成功則輸出錯(cuò)誤信息，及if中添
# 加！取不成功。
###########################################################################
_check_config() {
        toRun=( "$@" --verbose --help )
        if ! errors="$("${toRun[@]}" 2>&1 >/dev/null)"; then
                cat >&2 <<-EOM
                        ERROR: mysqld failed while attempting to check config
                        command was: "${toRun[*]}"
                        $errors
                EOM
                exit 1
        fi
}

# 1. $1參數(shù)為mysqld 以及 wanthelp 參數(shù)為空 以及root用戶，執(zhí)行此代碼；
# 2. _check_config檢查配置文件是否正確
# 3. 獲取DATADIR目錄，執(zhí)行mysqld --verbose --help --log-bin-index=/tmp/tmp.4SyApJWeIo| 
#         awk "$1 == """datadir""" { print $2; exit }"
# 4. 創(chuàng)建并修改目錄權(quán)限
# 5. 執(zhí)行exec gosu mysql docker-entrypoint.sh "$@"，即重新以mysql用戶再次調(diào)用腳
#    本
# allow the container to be started with `--user`
if [ "$1" = "mysqld" -a -z "$wantHelp" -a "$(id -u)" = "0" ]; then
        _check_config "$@"
        DATADIR="$(_get_config "datadir" "$@")"
        mkdir -p "$DATADIR"
        chown -R mysql:mysql "$DATADIR"
        exec gosu mysql "$BASH_SOURCE" "$@"
fi

# 1. $1參數(shù)為mysqld 以及 wanthelp 參數(shù)為空，執(zhí)行此代碼，及exec gosu會(huì)執(zhí)行此代碼；
if [ "$1" = "mysqld" -a -z "$wantHelp" ]; then

# 2. 仍然檢查配置文件以及獲取datadir目錄
    # still need to check config, container may have started with --user
    _check_config "$@"
    # Get config
    DATADIR="$(_get_config "datadir" "$@")"

# 3. 若mysql數(shù)據(jù)庫(kù)未創(chuàng)建，則執(zhí)行本段邏輯
    if [ ! -d "$DATADIR/mysql" ]; then
# 4. 檢查是否設(shè)置變量，如root密碼、允許root密碼為空亦或者隨機(jī)密碼
        file_env "MYSQL_ROOT_PASSWORD"
        if [ -z "$MYSQL_ROOT_PASSWORD" -a -z "$MYSQL_ALLOW_EMPTY_PASSWORD" -a -z "$MYSQL_RANDOM_ROOT_PASSWORD" ]; then
            echo >&2 "error: database is uninitialized and password option is not specified "
            echo >&2 "  You need to specify one of MYSQL_ROOT_PASSWORD, MYSQL_ALLOW_EMPTY_PASSWORD and MYSQL_RANDOM_ROOT_PASSWORD"
            exit 1
        fi
# 5. 創(chuàng)建目錄
        mkdir -p "$DATADIR"

# 6. 執(zhí)行mysqld命令初始化數(shù)據(jù)庫(kù)
        echo "Initializing database"
        "$@" --initialize-insecure
        echo "Database initialized"

# 7. command -v mysql_ssl_rsa_setup檢查命令是否可執(zhí)行，以及是否存在
# server-key.pem文件，若不存在，則生成證書(shū)
        if command -v mysql_ssl_rsa_setup > /dev/null && [ ! -e "$DATADIR/server-key.pem" ]; then
            # https://github.com/mysql/mysql-server/blob/23032807537d8dd8ee4ec1c4d40f0633cd4e12f9/packaging/deb-in/extra/mysql-systemd-start#L81-L84
            echo "Initializing certificates"
            mysql_ssl_rsa_setup --datadir="$DATADIR"
            echo "Certificates initialized"
        fi

# 8. 獲取socket值并啟動(dòng)mysql
        SOCKET="$(_get_config "socket" "$@")"
        "$@" --skip-networking --socket="${SOCKET}" &
        pid="$!"

# 9. 設(shè)置mysql變量（列表形式），而后可以${mysql[@]}調(diào)用
        mysql=( mysql --protocol=socket -uroot -hlocalhost --socket="${SOCKET}" )

# 10. 運(yùn)行30次，驗(yàn)證mysql是否已經(jīng)啟動(dòng)完畢
        for i in {30..0}; do
            if echo "SELECT 1" | "${mysql[@]}" &> /dev/null; then
                break
            fi
            echo "MySQL init process in progress..."
            sleep 1
        done
# 11. 若i為0值，則表明mysql啟動(dòng)失敗
        if [ "$i" = 0 ]; then
            echo >&2 "MySQL init process failed."
            exit 1
        fi

# 11. 解決時(shí)區(qū)bug
        if [ -z "$MYSQL_INITDB_SKIP_TZINFO" ]; then
            # sed is for https://bugs.mysql.com/bug.php?id=20545
            mysql_tzinfo_to_sql /usr/share/zoneinfo | 
                         sed "s/Local time zone must be set--see zic manual page/FCTY/" | "${mysql[@]}" mysql
        fi

# 12. 生成root隨機(jī)密碼
        if [ ! -z "$MYSQL_RANDOM_ROOT_PASSWORD" ]; then
            export MYSQL_ROOT_PASSWORD="$(pwgen -1 32)"
            echo "GENERATED ROOT PASSWORD: $MYSQL_ROOT_PASSWORD"
        fi

# 13. 若MYSQL_ROOT_HOST不為空亦或者不為localhost，則創(chuàng)建root用戶
        rootCreate=
        # default root to listen for connections from anywhere
        file_env "MYSQL_ROOT_HOST" "%"
        if [ ! -z "$MYSQL_ROOT_HOST" -a "$MYSQL_ROOT_HOST" != "localhost" ]; then
            # no, we don"t care if read finds a terminating character in this heredoc
            # https://unix.stackexchange.com/questions/265149/why-is-set-o-errexit-breaking-this-read-heredoc-expression/265151#265151
            read -r -d "" rootCreate <<-EOSQL || true
                CREATE USER "root"@"${MYSQL_ROOT_HOST}" IDENTIFIED BY "${MYSQL_ROOT_PASSWORD}" ;
                GRANT ALL ON *.* TO "root"@"${MYSQL_ROOT_HOST}" WITH GRANT OPTION ;
            EOSQL
        fi

# 14. 為"root"@"localhost"重置root密碼
#     使用$rootCreate創(chuàng)建root
        "${mysql[@]}" <<-EOSQL
            -- What"s done in this file shouldn"t be replicated
            --  or products like mysql-fabric won"t work
            SET @@SESSION.SQL_LOG_BIN=0;
            SET PASSWORD FOR "root"@"localhost"=PASSWORD("${MYSQL_ROOT_PASSWORD}") ;
            GRANT ALL ON *.* TO "root"@"localhost" WITH GRANT OPTION ;
            ${rootCreate}
            DROP DATABASE IF EXISTS test ;
            FLUSH PRIVILEGES ;
        EOSQL

# 15. 已設(shè)置root密碼，故mysql需加上root密碼
        if [ ! -z "$MYSQL_ROOT_PASSWORD" ]; then
            mysql+=( -p"${MYSQL_ROOT_PASSWORD}" )
        fi

# 16. 若配置了MYSQL_DATABASE變量，則創(chuàng)建
        file_env "MYSQL_DATABASE"
        if [ "$MYSQL_DATABASE" ]; then
            echo "CREATE DATABASE IF NOT EXISTS `$MYSQL_DATABASE` ;" | "${mysql[@]}"
            mysql+=( "$MYSQL_DATABASE" )
        fi

# 17. 在數(shù)據(jù)庫(kù)內(nèi)創(chuàng)建用戶
        file_env "MYSQL_USER"
        file_env "MYSQL_PASSWORD"
        if [ "$MYSQL_USER" -a "$MYSQL_PASSWORD" ]; then
            echo "CREATE USER "$MYSQL_USER"@"%" IDENTIFIED BY "$MYSQL_PASSWORD" ;" | "${mysql[@]}"

            if [ "$MYSQL_DATABASE" ]; then
                echo "GRANT ALL ON `$MYSQL_DATABASE`.* TO "$MYSQL_USER"@"%" ;" | "${mysql[@]}"
            fi

            echo "FLUSH PRIVILEGES ;" | "${mysql[@]}"
        fi

# 18. 執(zhí)行/docker-entrypoint-initdb.d目錄下面的腳本，包含shell、sql
        echo
        for f in /docker-entrypoint-initdb.d/*; do
            case "$f" in
                *.sh)     echo "$0: running $f"; . "$f" ;;
                *.sql)    echo "$0: running $f"; "${mysql[@]}" < "$f"; echo ;;
                *.sql.gz) echo "$0: running $f"; gunzip -c "$f" | "${mysql[@]}"; echo ;;
                *)        echo "$0: ignoring $f" ;;
            esac
            echo
        done

# 19. 設(shè)置root密碼是否過(guò)期
        if [ ! -z "$MYSQL_ONETIME_PASSWORD" ]; then
            "${mysql[@]}" <<-EOSQL
                ALTER USER "root"@"%" PASSWORD EXPIRE;
            EOSQL
        fi

# 20. kill -s TERM "$pid" 殺掉mysql進(jìn)程，執(zhí)行成功則返回0，而！kill取反，即kill成
#     功后才執(zhí)行后面的!wait命令
        if ! kill -s TERM "$pid" || ! wait "$pid"; then
            echo >&2 "MySQL init process failed."
            exit 1
        fi

# 21. 初始化成功后，再次啟動(dòng)
        echo
        echo "MySQL init process done. Ready for start up."
        echo
    fi
fi

# 22. 正式啟動(dòng)數(shù)據(jù)庫(kù)
exec "$@"