mysqld_safe 是一个 shell 脚本，通常用来启动 MySQL 服务进程，在这篇文章中，我们看下该脚本具体做了什么。
简介
该 shell 脚本，详细交代了 MySQL 的启动流程：查找 MySQL 相关目录；解析配置文件；调用 mysqld 程序来启动实例。常见的配置项有：
--help
查看帮助。
--no-defaults
不要读任何配置项文件。如果给出，必须首选该选项。
--defaults-extra-file=path
除了通用选项文件所读取的选项文件名。如果给出，必须首选该选项。
--defaults-file=path
读取的代替通用选项文件的选项文件名。如果给出，必须首选该选项。
--basedir=path
指定 MySQL 安装目录的路径。
--datadir=path
数据目录的路径。
--syslog, --skip-syslog
是否使用syslog记录mysqld_safe的日志。如果是skip-syslog，会写到文件中，默认是hostname.err。
--log-error
指定错误日志的文件路径，需要同时设置--skip-syslog 。
接下来看看 mysqld_safe 是如何执行的。
执行流程简介
如下是其简单的处理流程。
1. 检查系统和选项
强制设置四个选项，并检查其有效性，包括了默认配置、basedir、datadir、plugin-dir 。
2. 检查MyISAM表
默认是注释掉的，也可以在启动前检查 MyISAM 表。
3. 监控服务
在 MySQL 服务器异常宕机时，mysqld_safe 默认会自动重启服务，而正常关闭则不会重启服务，那么它是如何判断 MySQL 是否是正常关闭的呢？能否控制不自动重启呢？
如果 MySQL 服务正常关闭，默认会删除 PID 文件，mysqld_safe 会通过判断是否存在该文件来决定是否要重新拉起 MySQL 服务；当然，如果想即使 MySQL 服务宕机也不启动文件，也可以新建一个名为 PID 文件名+ .shutdown 的文件即可，例如 /tmp/mysql.pid.shutdown 。
在此，还有些异常处理：
- 如果在 MySQL 服务在一秒以内重启，则说明有异常，此时会通过
sleep 1休眠一秒。 - 在发现 MySQL 宕后，默认会检查是否有 mysqld 进程 hanging ，如果有，则直接通过
kill -9强制关闭。
4. 记录日志
将 mysqld 的错误消息发送到数据目录中的 host_name.err 文件，或者写入到 syslog 中。
源码解析
该脚本实际上最终会通过 eval_log_error() 函数启动 MySQL 服务，并将输出日志记录到文件或者 syslog 中，这个函数应该是 mysqld_safe 脚本的核心处理函数。
#!/bin/sh
#----- 首先是一些变量参数的定义
KILL_MYSQLD=1; # 试图kill多余的mysqld_safe程序，1表示需要kill
MYSQLD= # mysqld二进制可执行文件的目录名称
niceness=0 # 进程的调度优先级
mysqld_ld_preload= # 通常指定libjemalloc.so.1所在路径
mysqld_ld_library_path= # 库目录
#----- 下面的变量用于设置日志，默认不打开错误日志，不使用syslog
logging=init # 日志记录状态，init(输出到终端)，可以查看log_generic()
want_syslog=0 # 是否要使用syslog
syslog_tag= # 如果使用，则需要增加的标示
user='mysql' # 启动时使用的用户，也就是指定--user的选项值
pid_file= # pid文件的路径
err_log= # 错误日志的路径
timestamp_format=UTC # 使用UTC时间格式
#----- 定义的syslog中标志位，在后面需要写入日志到syslog中时使用
syslog_tag_mysqld=mysqld
syslog_tag_mysqld_safe=mysqld_safe
syslog_facility=daemon
trap '' 1 2 3 15 # 不允许程序在终端上被kill，包括挂起、中断、退出、系统终止
trap '' 13 # 也包括SIGPIPE，可以通过kill -l查看数字对应的信号量
#----- 设置默认权限为770，可以查看umask(2)，为了兼容所以处理的时候比较复杂
umask 007 # fallback
UMASK="${UMASK-0640}"
fmode=`echo "$UMASK" | sed -e 's/[^0246]//g'`
octalp=`echo "$fmode"|cut -c1`
fmlen=`echo "$fmode"|wc -c|sed -e 's/ //g'`
if [ "x$octalp" != "x0" -o "x$UMASK" != "x$fmode" -o "x$fmlen" != "x5" ]
then
fmode=0640
echo "UMASK must be a 3-digit mode with an additional leading 0 to indicate octal." >&2
echo "The first digit will be corrected to 6, the others may be 0, 2, 4, or 6." >&2
fi
fmode=`echo "$fmode"|cut -c3-4`
fmode="6$fmode"
if [ "x$UMASK" != "x0$fmode" ]
then
echo "UMASK corrected from $UMASK to 0$fmode ..."
fi
#----- 使用的默认配置文件，可以在命令行中指定
defaults=
case "$1" in
--no-defaults|--defaults-file=*|--defaults-extra-file=*)
defaults="$1"; shift
;;
esac
#----- 使用--help选项时输出的使用帮助信息
usage () {
cat <<EOF
Usage: $0 [OPTIONS]
--no-defaults Don't read the system defaults file
... ...
EOF
exit 1
}
#----- 相当于which，检索$PATH中的路径，打印出命令的全路径
my_which () # 该函数只在my_which logger用到，也就是转换为/usr/bin/logger
{
save_ifs="${IFS-UNSET}" # 保存当前的分隔符，用于后面恢复IFS
IFS=: # 使用:来分割PATH中的路径
ret=0
for file # 等同于for file in $*
do
for dir in $PATH
do
if [ -f "$dir/$file" ]
then
echo "$dir/$file"
continue 2 # 跳出外层循环
fi
done
ret=1 #signal an error
break
done
if [ "$save_ifs" = UNSET ] # 重置IFS变量
then
unset IFS
else
IFS="$save_ifs"
fi
return $ret # Success
}
#----- 通用的日志输出，被log_error和log_notice函数调用
log_generic () {
priority="$1" # 日志级别，有daemon.error和daemon.notice两种类别
shift
msg="`eval $DATE` mysqld_safe $*" # 设置日志格式，时间+mysqld_safe+日志
echo "$msg"
case $logging in
init) ;; # 只输出到命令行，不记录到日志
file) echo "$msg" >> "$err_log" ;; # 输出到错误日志文件
# 用logger命令将日志记录到系统日志syslog
syslog) logger -t "$syslog_tag_mysqld_safe" -p "$priority" "$*" ;;
both) echo "$msg" >> "$err_log"; logger -t "$syslog_tag_mysqld_safe" -p "$priority" "$*" ;;
*)
echo "Internal program error (non-fatal):" \
" unknown logging method '$logging'" >&2
;;
esac
}
log_error () {
log_generic ${syslog_facility}.error "$@" >&2
}
log_notice () {
log_generic ${syslog_facility}.notice "$@"
}
#----- 记录日志的初始化操作，会根据日志类新记录一些初始化日志
eval_log_error () {
cmd="$1" # 最后的eval命令会解析$cmd中的值并执行命令
case $logging in
file) cmd="$cmd >> "`shell_quote_string "$err_log"`" 2>&1" ;;
syslog)
cmd="$cmd 2>&1 | logger -t '$syslog_tag_mysqld' -p daemon.error"
;;
*)
echo "Internal program error (non-fatal):" \
" unknown logging method '$logging'" >&2
;;
esac
#!!!可以如下命令查看真正执行的命令!!!
#echo "Running mysqld: [$cmd]"
eval "$cmd"
}
#----- 转义函数，用于在非"a-z"、"A-Z"、"0-9"、'/'、'_'、'.'、'='、'-'的特殊字符前加上转义字符"\"
shell_quote_string() {
echo "$1" | sed -e 's,\([^a-zA-Z0-9/_.=-]\),\\\1,g' # sed中的\1代表引用前面\(\)中匹配的值
}
#----- 该函数用于解析配置文件中的选项，并赋值给相应的变量，无法识别则转递给服务器
parse_arguments() {
pick_args=
if test "$1" = PICK-ARGS-FROM-ARGV
then
pick_args=1
shift
fi
for arg do # 取出参数值
val=`echo "$arg" | sed -e "s;--[^=]*=;;"`
optname=`echo "$arg" | sed -e 's/^\(--[^=]*\)=.*$/\1/'`
optname_subst=`echo "$optname" | sed 's/_/-/g'`
arg=`echo $arg | sed "s/^$optname/$optname_subst/"`
case "$arg" in # 将参数值传递给对应的变量
--basedir=*) MY_BASEDIR_VERSION="$val" ;;
--datadir=*) DATADIR="$val" ;;
--pid-file=*) pid_file="$val" ;;
--plugin-dir=*) PLUGIN_DIR="$val" ;;
--user=*) user="$val"; SET_USER=1 ;;
# 有些值已经在my.cnf配置文件的[mysqld_safe]组下设置，可被命令行覆盖
--log-error=*) err_log="$val" ;;
--port=*) mysql_tcp_port="$val" ;;
--socket=*) mysql_unix_port="$val" ;;
# 如下的配置项必须在配置文件的[mysqld_safe]组中设置
--core-file-size=*) core_file_size="$val" ;;
--ledir=*) ledir="$val" ;;
--malloc-lib=*) set_malloc_lib "$val" ;;
--mysqld=*)
if [ -z "$pick_args" ]; then
log_error "--mysqld option can only be used as command line option, found in config file"
exit 1
fi
MYSQLD="$val" ;;
--mysqld-version=*)
if [ -z "$pick_args" ]; then
log_error "--mysqld-version option can only be used as command line option, found in config file"
exit 1
fi
if test -n "$val"
then
MYSQLD="mysqld-$val"
PLUGIN_VARIANT="/$val"
else
MYSQLD="mysqld"
fi
;;
--nice=*) niceness="$val" ;;
--open-files-limit=*) open_files="$val" ;;
--open-files-limit=*) open_files="$val" ;;
--skip-kill-mysqld*) KILL_MYSQLD=0 ;;
--mysqld-safe-log-timestamps=*) timestamp_format="$val" ;;
--syslog) want_syslog=1 ;;
--skip-syslog) want_syslog=0 ;;
--syslog-tag=*) syslog_tag="$val" ;;
--timezone=*) TZ="$val"; export TZ; ;; # 生效了一下时区设置
--help) usage ;; # 调用了usage函数，输出帮助信息
*)
if test -n "$pick_args"
then
append_arg_to_args "$arg" # 将其他命令行参数值附加到$arg的后面
fi
;;
esac
done
}
# 添加到LD_PRELOAD，使用空格分割
add_mysqld_ld_preload() {
lib_to_add="$1"
log_notice "Adding '$lib_to_add' to LD_PRELOAD for mysqld"
case "$lib_to_add" in
*' '*)
# Must strip path from lib, and add it to LD_LIBRARY_PATH
lib_file=`basename "$lib_to_add"`
case "$lib_file" in
*' '*)
# The lib file itself has a space in its name, and can't
# be used in LD_PRELOAD
log_error "library name '$lib_to_add' contains spaces and can not be used with LD_PRELOAD"
exit 1
;;
esac
lib_path=`dirname "$lib_to_add"`
lib_to_add="$lib_file"
[ -n "$mysqld_ld_library_path" ] && mysqld_ld_library_path="$mysqld_ld_library_path:"
mysqld_ld_library_path="$mysqld_ld_library_path$lib_path"
;;
esac
# LD_PRELOAD is a space-separated
[ -n "$mysqld_ld_preload" ] && mysqld_ld_preload="$mysqld_ld_preload "
mysqld_ld_preload="${mysqld_ld_preload}$lib_to_add"
}
# Returns LD_PRELOAD (and LD_LIBRARY_PATH, if needed) text, quoted to be
# suitable for use in the eval that calls mysqld.
#
# All values in mysqld_ld_preload are prepended to LD_PRELOAD.
mysqld_ld_preload_text() {
text=
if [ -n "$mysqld_ld_preload" ]; then
new_text="$mysqld_ld_preload"
[ -n "$LD_PRELOAD" ] && new_text="$new_text $LD_PRELOAD"
text="${text}LD_PRELOAD="`shell_quote_string "$new_text"`' '
fi
if [ -n "$mysqld_ld_library_path" ]; then
new_text="$mysqld_ld_library_path"
[ -n "$LD_LIBRARY_PATH" ] && new_text="$new_text:$LD_LIBRARY_PATH"
text="${text}LD_LIBRARY_PATH="`shell_quote_string "$new_text"`' '
fi
echo "$text"
}
# set_malloc_lib LIB
# - If LIB is empty, do nothing and return
# - If LIB is 'tcmalloc', look for tcmalloc shared library in $malloc_dirs.
# tcmalloc is part of the Google perftools project.
# - If LIB is an absolute path, assume it is a malloc shared library
#
# Put LIB in mysqld_ld_preload, which will be added to LD_PRELOAD when
# running mysqld. See ld.so for details.
set_malloc_lib() {
# This list is kept intentionally simple.
malloc_dirs="/usr/lib /usr/lib64 /usr/lib/i386-linux-gnu /usr/lib/x86_64-linux-gnu"
malloc_lib="$1"
if [ "$malloc_lib" = tcmalloc ]; then
malloc_lib=
for libdir in $(echo $malloc_dirs); do
for flavor in _minimal '' _and_profiler _debug; do
tmp="$libdir/libtcmalloc$flavor.so"
#log_notice "DEBUG: Checking for malloc lib '$tmp'"
[ -r "$tmp" ] || continue
malloc_lib="$tmp"
break 2
done
done
if [ -z "$malloc_lib" ]; then
log_error "no shared library for --malloc-lib=tcmalloc found in $malloc_dirs"
exit 1
fi
fi
# Allow --malloc-lib='' to override other settings
[ -z "$malloc_lib" ] && return
case "$malloc_lib" in
/*)
if [ ! -r "$malloc_lib" ]; then
log_error "--malloc-lib can not be read and will not be used"
exit 1
fi
# Restrict to a the list in $malloc_dirs above
case "$(dirname "$malloc_lib")" in
/usr/lib) ;;
/usr/lib64) ;;
/usr/lib/i386-linux-gnu) ;;
/usr/lib/x86_64-linux-gnu) ;;
*)
log_error "--malloc-lib must be located in one of the directories: $malloc_dirs"
exit 1
;;
esac
;;
*)
log_error "--malloc-lib must be an absolute path or 'tcmalloc'; " \
"ignoring value '$malloc_lib'"
exit 1
;;
esac
add_mysqld_ld_preload "$malloc_lib"
}
#########################################################
# 正式工作开始了！！！
#########################################################
#########################################################
# 1. 查找base目录和mysqld所在目录
#########################################################
if echo '/opt/mysql-5.7/share' | grep '^/opt/mysql-5.7' > /dev/null
then
# 一口气用了三个替换，分别为：
# 第一步：将/opt/mysql-5.7/share去除开始的basedir，转换为/share
# 第二步：将/share开头的/转换为空，也就是share
# 第三步：在share开头加上./，结果即：./share
relpkgdata=`echo '/opt/mysql-5.7/share' | sed -e 's,^/opt/mysql-5.7,,' -e 's,^/,,' -e 's,^,./,'`
else
# pkgdatadir is not relative to prefix
relpkgdata='/opt/mysql-5.7/share'
fi
case "$0" in
/*)
MY_PWD='/opt/mysql-5.7'
;;
*)
MY_PWD=`dirname $0`
MY_PWD=`dirname $MY_PWD`
;;
esac
# 查找mysqld可执行文件，分别判断了libexec和sbin目录，找不到就使用bin目录
if test -n "$MY_BASEDIR_VERSION" -a -d "$MY_BASEDIR_VERSION"
then
# BASEDIR is already overridden on command line. Do not re-set.
# Use BASEDIR to discover le.
if test -x "$MY_BASEDIR_VERSION/libexec/mysqld"
then
ledir="$MY_BASEDIR_VERSION/libexec"
elif test -x "$MY_BASEDIR_VERSION/sbin/mysqld"
then
ledir="$MY_BASEDIR_VERSION/sbin"
else
ledir="$MY_BASEDIR_VERSION/bin"
fi
elif test -f "$relpkgdata"/english/errmsg.sys -a -x "$MY_PWD/bin/mysqld"
then
MY_BASEDIR_VERSION="$MY_PWD" # Where bin, share and data are
ledir="$MY_PWD/bin" # Where mysqld is
# Check for the directories we would expect from a source install
elif test -f "$relpkgdata"/english/errmsg.sys -a -x "$MY_PWD/libexec/mysqld"
then
MY_BASEDIR_VERSION="$MY_PWD" # Where libexec, share and var are
ledir="$MY_PWD/libexec" # Where mysqld is
elif test -f "$relpkgdata"/english/errmsg.sys -a -x "$MY_PWD/sbin/mysqld"
then
MY_BASEDIR_VERSION="$MY_PWD" # Where sbin, share and var are
ledir="$MY_PWD/sbin" # Where mysqld is
# Since we didn't find anything, used the compiled-in defaults
else
MY_BASEDIR_VERSION='/opt/mysql-5.7'
ledir='/opt/mysql-5.7/bin'
fi
#########################################################
# 2. 查找base目录和mysqld所在目录
#########################################################
# 可以从my_print_defaults脚本中获得默认的读取my.cnf顺序，如下
# Default options are read from the following files in the given order:
# /etc/mysql/my.cnf /etc/my.cnf ~/.my.cnf
# 2.1 查找下BASEDIR目录
if test -d $MY_BASEDIR_VERSION/data/mysql
then
DATADIR=$MY_BASEDIR_VERSION/data
# Next try where the source installs put it
elif test -d $MY_BASEDIR_VERSION/var/mysql
then
DATADIR=$MY_BASEDIR_VERSION/var
# Or just give up and use our compiled-in default
else
DATADIR=/opt/data
fi
if test -z "$MYSQL_HOME"
then
MYSQL_HOME=$MY_BASEDIR_VERSION
fi
export MYSQL_HOME
# 2.2 找到my_print_defaults所在目录，并打印[mysqld]和[mysqld_safe]中的配置项，然后与从命令行
# 传入的参数合并，优先级为
if test -x "$MY_BASEDIR_VERSION/bin/my_print_defaults"
then
print_defaults="$MY_BASEDIR_VERSION/bin/my_print_defaults"
elif test -x `dirname $0`/my_print_defaults
then
print_defaults="`dirname $0`/my_print_defaults"
elif test -x ./bin/my_print_defaults
then
print_defaults="./bin/my_print_defaults"
elif test -x /opt/mysql-5.7/bin/my_print_defaults
then
print_defaults="/opt/mysql-5.7/bin/my_print_defaults"
elif test -x /opt/mysql-5.7/bin/mysql_print_defaults
then
print_defaults="/opt/mysql-5.7/bin/mysql_print_defaults"
else
print_defaults="my_print_defaults"
fi
# 这个函数可以将一个指定的参数附加到$arg中(在此同时执行了转义操作)
append_arg_to_args () {
args="$args "`shell_quote_string "$1"`
}
args=
# 通过SET_USER变量判断来源，2：初始值；1：配置文件或者命令行中配置；0：没有配置
SET_USER=2
# 解析配置文件中的参数，只显示[mysqld]和[server]分组中的内容
parse_arguments `$print_defaults $defaults --loose-verbose mysqld server`
if test $SET_USER -eq 2
then
SET_USER=0
fi
# 如上，又读取[safe_mysqld]和[mysqld_safe]分组中配置项
parse_arguments `$print_defaults $defaults --loose-verbose mysqld_safe safe_mysqld`
# 用命令行输入选项，也就是$@，从而可以覆盖配置文件中的选项
parse_arguments PICK-ARGS-FROM-ARGV "$@"
# Sort out date command from $timestamp_format early so we'll start off
# with correct log messages.
case "$timestamp_format" in
UTC|utc) DATE="date -u +%Y-%m-%dT%H:%M:%S.%06NZ";;
SYSTEM|system) DATE="date +%Y-%m-%dT%H:%M:%S.%06N%:z";;
HYPHEN|hyphen) DATE="date +'%Y-%m-%d %H:%M:%S'";;
LEGACY|legacy) DATE="date +'%y%m%d %H:%M:%S'";;
*) DATE="date -u +%Y-%m-%dT%H:%M:%S.%06NZ";
log_error "unknown data format $timestamp_format, using UTC";;
esac
# 2.3 查找plugin目录
# Use user-supplied argument
if [ -n "${PLUGIN_DIR}" ]; then
plugin_dir="${PLUGIN_DIR}"
else
# Try to find plugin dir relative to basedir
for dir in lib64/mysql/plugin lib64/plugin lib/mysql/plugin lib/plugin
do
if [ -d "${MY_BASEDIR_VERSION}/${dir}" ]; then
plugin_dir="${MY_BASEDIR_VERSION}/${dir}"
break
fi
done
# Give up and use compiled-in default
if [ -z "${plugin_dir}" ]; then
plugin_dir='/opt/mysql-5.7/lib/plugin'
fi
fi
plugin_dir="${plugin_dir}${PLUGIN_VARIANT}"
# A pid file is created for the mysqld_safe process. This file protects the
# server instance resources during race conditions.
safe_pid="$DATADIR/mysqld_safe.pid"
if test -f $safe_pid
then
PID=`cat "$safe_pid"`
if kill -0 $PID > /dev/null 2> /dev/null
then
if ps wwwp $PID | grep -v mysqld_safe | grep -- $MYSQLD > /dev/null
then
log_error "A mysqld_safe process already exists"
exit 1
fi
fi
if [ ! -h "$safe_pid" ]; then
rm -f "$safe_pid"
fi
if test -f "$safe_pid"
then
log_error "Fatal error: Can't remove the mysqld_safe pid file"
exit 1
fi
fi
# Insert pid proerply into the pid file.
ps -e | grep [m]ysqld_safe | awk '{print $1}' | sed -n 1p > $safe_pid
# End of mysqld_safe pid(safe_pid) check.
#########################################################
# log日志方式设置，共stdin,file,syslog三种
#########################################################
# 如果是写入到syslog，则判断logger工具是否可用
if [ $want_syslog -eq 1 ]
then
my_which logger > /dev/null 2>&1
if [ $? -ne 0 ]
then
log_error "--syslog requested, but no 'logger' program found. Please ensure
that 'logger' is in your PATH, or do not specify the --syslog option to mysqld_safe."
if [ ! -h "$safe_pid" ]; then
rm -f "$safe_pid" # Clean Up of mysqld_safe.pid file.
fi
exit 1
fi
fi
if [ $want_syslog -eq 1 ]
then
if [ -n "$syslog_tag" ]
then
# Sanitize the syslog tag
syslog_tag=`echo "$syslog_tag" | sed -e 's/[^a-zA-Z0-9_-]/_/g'`
syslog_tag_mysqld_safe="${syslog_tag_mysqld_safe}-$syslog_tag"
syslog_tag_mysqld="${syslog_tag_mysqld}-$syslog_tag"
fi
log_notice "Logging to syslog."
logging=syslog
fi
# 如果是将日志记录到文件中
if [ -n "$err_log" -o $want_syslog -eq 0 ]
then
if [ -n "$err_log" ]
then
# 如果没有.err后缀，则为err_log添加一个；此时，mysqld和mysqld_safe会写入到
# 同一个日志文件中。如果是"--log-error=foo."则认为文件名是有效的
# 这里是识别文件名的字符总数量(包括.)，如果没有设置后缀或者以'/'结尾则返回0
if expr "$err_log" : '.*\.[^/]*$' > /dev/null
then
:
else
err_log="$err_log".err
fi
case "$err_log" in
/* ) ;;
* ) err_log="$DATADIR/$err_log" ;;
esac
else
# 默认设置的错误日志名称
err_log=$DATADIR/`hostname`.err
fi
# 同时会将该参数添加到mysqld启动的参数中
append_arg_to_args "--log-error=$err_log"
# 提示日志写入到的文件名称
log_notice "Logging to '$err_log'."
if [ $want_syslog -eq 1 ]
then
logging=both
else
logging=file # 正式把logging改成file使用错误日志来记录日志
fi
if [ ! -f "$err_log" ]; then # if error log already exists,
touch "$err_log" # we just append. otherwise,
chmod "$fmode" "$err_log" # fix the permissions here!
fi
fi
# 设置--user选项
USER_OPTION=""
if test -w / -o "$USER" = "root" # 根目录是否可写，或者当前用户为root
then
if test "$user" != "root" -o $SET_USER = 1
then
USER_OPTION="--user=$user"
fi
# 创建错误日志，并将日志授权给指定的用户
if [ $want_syslog -eq 0 -a ! -h "$err_log" ]; then
touch "$err_log"
chown $user "$err_log"
fi
# 这里它还对当前用户做了ulimit设置，包括可以打开的文件数量--open_files-limit选项
if test -n "$open_files"
then
ulimit -n $open_files
fi
fi
if test -n "$open_files"
then
append_arg_to_args "--open-files-limit=$open_files"
fi
safe_mysql_unix_port=${mysql_unix_port:-${MYSQL_UNIX_PORT:-/tmp/mysql.sock}}
# 确保 $safe_mysql_unix_port 目录是存在的
# Make sure that directory for $safe_mysql_unix_port exists
mysql_unix_port_dir=`dirname $safe_mysql_unix_port`
if [ ! -d $mysql_unix_port_dir ]
then
if [ ! -h $mysql_unix_port_dir ]; then
mkdir $mysql_unix_port_dir
chown $user $mysql_unix_port_dir
chmod 755 $mysql_unix_port_dir
fi
fi
# 如果用户没有制定mysqld程序的名称，这里就默认赋值为mysqld
# If the user doesn't specify a binary, we assume name "mysqld"
if test -z "$MYSQLD"
then
MYSQLD=mysqld
fi
# 下面几段分别是对 mysqld ， pid ， port文件选项的检查和设置，省略100个字
if test ! -x "$ledir/$MYSQLD"
then
log_error "The file $ledir/$MYSQLD
does not exist or is not executable. Please cd to the mysql installation
directory and restart this script from there as follows:
./bin/mysqld_safe&
See http://dev.mysql.com/doc/mysql/en/mysqld-safe.html for more information"
if [ ! -h "$safe_pid" ]; then
rm -f "$safe_pid" # Clean Up of mysqld_safe.pid file.
fi
exit 1
fi
if test -z "$pid_file"
then
pid_file="$DATADIR/`hostname`.pid"
else
case "$pid_file" in
/* ) ;;
* ) pid_file="$DATADIR/$pid_file" ;;
esac
fi
append_arg_to_args "--pid-file=$pid_file"
if test -n "$mysql_unix_port"
then
append_arg_to_args "--socket=$mysql_unix_port"
fi
if test -n "$mysql_tcp_port"
then
append_arg_to_args "--port=$mysql_tcp_port"
fi
# 接下来是关于优先级的设置
if test $niceness -eq 0
then
NOHUP_NICENESS="nohup"
else
NOHUP_NICENESS="nohup nice -$niceness"
fi
# Using nice with no args to get the niceness level is GNU-specific.
# This check could be extended for other operating systems (e.g.,
# BSD could use "nohup sh -c 'ps -o nice -p $$' | tail -1").
# But, it also seems that GNU nohup is the only one which messes
# with the priority, so this is okay.
# 将当前的默认优先级设置为0
if nohup nice > /dev/null 2>&1
then
# normal_niceness记载默认的调度优先级
normal_niceness=`nice`
# nohup_niceness记载使用nohup执行方式的调度优先级
nohup_niceness=`nohup nice 2>/dev/null`
numeric_nice_values=1
# 这个for是为了检查$normal_niceness $nohup_niceness两个变量值的合法性
for val in $normal_niceness $nohup_niceness
do
case "$val" in
-[0-9] | -[0-9][0-9] | -[0-9][0-9][0-9] | \
[0-9] | [0-9][0-9] | [0-9][0-9][0-9] )
;;
* )
numeric_nice_values=0 ;;
esac
done
# 这个判断结构很重要
# 它保证了使用nohup执行的mysqld程序在调度优先级上不会低于直接执行mysqld程序的方式
if test $numeric_nice_values -eq 1
then
nice_value_diff=`expr $nohup_niceness - $normal_niceness`
if test $? -eq 0 && test $nice_value_diff -gt 0 && \
nice --$nice_value_diff echo testing > /dev/null 2>&1
then
# nohup increases the priority (bad), and we are permitted
# to lower the priority with respect to the value the user
# might have been given
# 进入分支说明$nohup_niceness的值比$normal_niceness大，即nohup执行方式调度优先级比正常执行方式低
# 这是不希望看到的，所以下面就人为的提升了nohup的优先级（降低niceness的值）
niceness=`expr $niceness - $nice_value_diff`
NOHUP_NICENESS="nice -$niceness nohup"
fi
fi
else
# 下面是测试nohup在当前系统中是否可用，不可用的话就置空NOHUP_NICENESS
if nohup echo testing > /dev/null 2>&1
then
:
else
# nohup doesn't work on this system
NOHUP_NICENESS=""
fi
fi
# Try to set the core file size (even if we aren't root) because many systems
# don't specify a hard limit on core file size.
# 指定core file大小，即使非root用户也尝试设置
if test -n "$core_file_size"
then
ulimit -c $core_file_size
fi
# 如果已经存在一个pid文件，则检查是否有已经启动的mysqld进程，如果已经启动则退出
if test -f "$pid_file"
then
PID=`cat "$pid_file"`
# 检查进程是否存在
if kill -0 $PID > /dev/null 2> /dev/null
then
if ps wwwp $PID | grep -v mysqld_safe | grep -- $MYSQLD > /dev/null
then # The pid contains a mysqld process
log_error "A mysqld process already exists"
if [ ! -h "$safe_pid" ]; then
rm -f "$safe_pid" # Clean Up of mysqld_safe.pid file.
fi
exit 1
fi
fi
if [ ! -h "$pid_file" ]; then
rm -f "$pid_file"
fi
if test -f "$pid_file"
then
log_error "Fatal error: Can't remove the pid file:
$pid_file
Please remove it manually and start $0 again;
mysqld daemon not started"
if [ ! -h "$safe_pid" ]; then
rm -f "$safe_pid" # Clean Up of mysqld_safe.pid file.
fi
exit 1
fi
fi
#
# Uncomment the following lines if you want all tables to be automatically
# checked and repaired during startup. You should add sensible key_buffer
# and sort_buffer values to my.cnf to improve check performance or require
# less disk space.
# Alternatively, you can start mysqld with the "myisam-recover" option. See
# the manual for details.
#
# echo "Checking tables in $DATADIR"
# $MY_BASEDIR_VERSION/bin/myisamchk --silent --force --fast --medium-check $DATADIR/*/*.MYI
# $MY_BASEDIR_VERSION/bin/isamchk --silent --force $DATADIR/*/*.ISM
# Does this work on all systems?
#if type ulimit | grep "shell builtin" > /dev/null
#then
# ulimit -n 256 > /dev/null 2>&1 # Fix for BSD and FreeBSD systems
#fi
#########################################################
# 开始拼接执行语句
#########################################################
cmd="`mysqld_ld_preload_text`$NOHUP_NICENESS"
# 检查一下命令以及参数，如果需要则进行转义操作
for i in "$ledir/$MYSQLD" "$defaults" "--basedir=$MY_BASEDIR_VERSION" \
"--datadir=$DATADIR" "--plugin-dir=$plugin_dir" "$USER_OPTION"
do
cmd="$cmd "`shell_quote_string "$i"`
done
cmd="$cmd $args"
# Avoid 'nohup: ignoring input' warning
test -n "$NOHUP_NICENESS" && cmd="$cmd < /dev/null"
log_notice "Starting $MYSQLD daemon with databases from $DATADIR"
# variable to track the current number of "fast" (a.k.a. subsecond) restarts
fast_restart=0
# maximum number of restarts before trottling kicks in
max_fast_restarts=5
# flag whether a usable sleep command exists
have_sleep=1
#########################################################
# 后台循环，如果mysqld宕，会尝试自动拉起服务进程
#########################################################
while true
do
# 一些安全性检查，如果如下文件存在则删除
if [ ! -h "$safe_mysql_unix_port" ]; then
rm -f "$safe_mysql_unix_port"
fi
if [ ! -h "$pid_file" ]; then
rm -f "$pid_file"
fi
if [ ! -h "$pid_file.shutdown" ]; then
rm -f "$pid_file.shutdown"
fi
start_time=`date +%M%S`
# 传入$cmd参数的值，会使用eval命令启动mysqld
eval_log_error "$cmd"
if [ $want_syslog -eq 0 -a ! -f "$err_log" -a ! -h "$err_log" ]; then
touch "$err_log" # hypothetical: log was renamed but not
chown $user "$err_log" # flushed yet. we'd recreate it with
chmod "$fmode" "$err_log" # wrong owner next time we log, so set
fi # it up correctly while we can!
end_time=`date +%M%S`
# 如果正常关闭，也就是检查PID文件被正常删除，则退出分支，不尝试重启进程
if test ! -f "$pid_file" # This is removed if normal shutdown
then
break
fi
# 在进程宕之后，如果不希望重启则直接退出mysqld_safe
if test -f "$pid_file.shutdown" # created to signal that it must stop
then
log_notice "$pid_file.shutdown present. The server will not restart."
break
fi
# sanity check if time reading is sane and there's sleep
if test $end_time -gt 0 -a $have_sleep -gt 0
then
# 如果在一秒以内完成重启，则尝试睡眠1秒
if test $end_time -eq $start_time
then
fast_restart=`expr $fast_restart + 1`
if test $fast_restart -ge $max_fast_restarts
then
log_notice "The server is respawning too fast. Sleeping for 1 second."
sleep 1
sleep_state=$?
if test $sleep_state -gt 0
then
log_notice "The server is respawning too fast and no working sleep command. Turning off trottling."
have_sleep=0
fi
fast_restart=0
fi
else
fast_restart=0
fi
fi
# mysqld_safe已经启动的处理方法，保证只有一个mysqld_safe程序启动
if true && test $KILL_MYSQLD -eq 1
then
# 统计启动的mysqld进程的数目:A)正常为0；B)进程hang，为1；C)有历史进程hang，为多个
numofproces=`ps xaww | grep -v "grep" | grep "$ledir/$MYSQLD\>" | grep -c "pid-file=$pid_file"`
log_notice "Number of processes running now: $numofproces"
I=1
while test "$I" -le "$numofproces"
do
# 过滤启动的MySQL服务进程，通过sed -n '$p'每次只获取最后一行记录
PROC=`ps xaww | grep "$ledir/$MYSQLD\>" | grep -v "grep" | grep "pid-file=$pid_file" | sed -n '$p'`
# 上述过滤的第一列为进程ID，使用T变量来获取进程ID
for T in $PROC
do
break
done
# 因为已经hang了，直接kill -9强制删除
if kill -9 $T
then
log_error "$MYSQLD process hanging, pid $T - killed"
else
break
fi
# 每干掉一个mysqld就把I加一，如果有N个mysqld进程，则执行N次kill -9命令
I=`expr $I + 1`
done
fi
log_notice "mysqld restarted"
done
if [ ! -h "$pid_file.shutdown" ]; then
rm -f "$pid_file.shutdown"
fi
log_notice "mysqld from pid file $pid_file ended"
if [ ! -h "$safe_pid" ]; then
rm -f "$safe_pid" # Some Extra Safety. File is deleted
fi # once the mysqld process ends.
简单来说，mysqld_safe 会执行如下命令，其中参数会转换为绝对路径。
$ nohup mysqld ... < /dev/null >> localhost.err 2>&1