今天看了一下启动文件“hadoop-daemon.sh”，仔细看了一下大概知道它的作用，使用“hadoop-daemon.sh“脚本启动和停止hadoop后台程序。它可以做到在A机器上启动”namenode“，B机器启动”secondarynamenode“ C机器启动”datanode“， ”tasktracker“，具体启动如下 ：
./hadoop-daemon.sh start namenode
./hadoop-daemon.sh start secondarynamenode
./hadoop-daemon.sh start jobtracker
./hadoop-daemon.sh start datanode
./hadoop-daemon.sh start tasktracker
如果要停止可以运行如下命令
./hadoop-daemon.sh stop namenode
./hadoop-daemon.sh stop secondarynamenode
./hadoop-daemon.sh stop jobtracker
./hadoop-daemon.sh stop datanode
./hadoop-daemon.sh stop tasktracker
下面是是代码分析，把主要部分都写了注释，代码不多，比较容易看懂，但有一处没看懂
# Runs a Hadoop command as a daemon.
#
# Environment Variables
#
# HADOOP_CONF_DIR Alternate conf dir. Default is ${HADOOP_HOME}/conf.
# HADOOP_LOG_DIR Where log files are stored. PWD by default.
# HADOOP_MASTER host:path where hadoop code should be rsync'd from
# HADOOP_PID_DIR The pid files are stored. /tmp by default.
# HADOOP_IDENT_STRING A string representing this instance of hadoop. $USER by default
# HADOOP_NICENESS The scheduling priority for daemons. Defaults to 0.
##
usage="Usage: hadoop-daemon.sh [--config <conf-dir>] [--hosts hostlistfile] (start|stop) <hadoop-command> <args...>"
# 第一个参数是下面的“startStop”是"start" or "end"
# 第二个参数是“hadoop-command”
# hadoop-command：就是namenode|datanode|secondarynamenode|jobtracker|tasktracke
# if no args specified, show usage
if [ $# -le 1 ]; then
echo $usage
exit 1
fi
bin=`dirname "$0"`
bin=`cd "$bin"; pwd`
. "$bin"/hadoop-config.sh
# get arguments
startStop=$1
shift
command=$1
shift
# 日记文件默认一共5个，后缀是log,log.1,log.2,log.3,log4,log5,
# 每次写日记的文件名都是.log, 但是上一次的.log.num变成.log.num+1,
# .log.5被.log.4覆盖后结束
hadoop_rotate_log ()
{
log=$1;
num=5;
if [ -n "$2" ]; then
num=$2
fi
if [ -f "$log" ]; then # rotate logs
while [ $num -gt 1 ]; do
prev=`expr $num - 1`
[ -f "$log.$prev" ] && mv "$log.$prev" "$log.$num"
num=$prev
done
mv "$log" "$log.$num";
fi
}
if [ -f "${HADOOP_CONF_DIR}/hadoop-env.sh" ]; then
. "${HADOOP_CONF_DIR}/hadoop-env.sh"
fi
# get log directory
if [ "$HADOOP_LOG_DIR" = "" ]; then
export HADOOP_LOG_DIR="$HADOOP_HOME/logs"
fi
mkdir -p "$HADOOP_LOG_DIR"
if [ "$HADOOP_PID_DIR" = "" ]; then
HADOOP_PID_DIR=/tmp
fi
if [ "$HADOOP_IDENT_STRING" = "" ]; then
export HADOOP_IDENT_STRING="$USER"
fi
# some variables
export HADOOP_LOGFILE=hadoop-$HADOOP_IDENT_STRING-$command-$HOSTNAME.log
export HADOOP_ROOT_LOGGER="INFO,DRFA"
log=$HADOOP_LOG_DIR/hadoop-$HADOOP_IDENT_STRING-$command-$HOSTNAME.out
pid=$HADOOP_PID_DIR/hadoop-$HADOOP_IDENT_STRING-$command.pid
# Set default scheduling priority
if [ "$HADOOP_NICENESS" = "" ]; then
export HADOOP_NICENESS=0
fi
case $startStop in
(start)
mkdir -p "$HADOOP_PID_DIR"
# 如果已经启动就提示先关闭，然后程序退出
if [ -f $pid ]; then
# 假如$command是“namenode”，就先判断它是否启动，由于$pid是存放
# “namenode”运行的进程号，所以可以通过kill -0 `cat $pid`判断
if kill -0 `cat $pid` > /dev/null 2>&1; then
echo $command running as process `cat $pid`. Stop it first.
exit 1
fi
fi
# 不知道干啥？？？
if [ "$HADOOP_MASTER" != "" ]; then
echo rsync from $HADOOP_MASTER
rsync -a -e ssh --delete --exclude=.svn --exclude='logs/*' --exclude='contrib/hod/logs/*' $HADOOP_MASTER/ "$HADOOP_HOME"
fi
# 记录日志
hadoop_rotate_log $log
echo starting $command, logging to $log
cd "$HADOOP_HOME"
# nice指令可以改变程序执行的优先权等级
# 下面这段代码是核心代码，怎么样启动hadoop
nohup nice -n $HADOOP_NICENESS "$HADOOP_HOME"/bin/hadoop --config $HADOOP_CONF_DIR $command "$@" > "$log" 2>&1 < /dev/null &
# 将刚刚启动的进程号写入“$pid”文件
echo $! > $pid
sleep 1; head "$log"
;;
(stop)
if [ -f $pid ]; then
if kill -0 `cat $pid` > /dev/null 2>&1; then
echo stopping $command
# 通过“$pid”文件内容，也就是进程号来关闭
kill `cat $pid`
else
echo no $command to stop
fi
else
echo no $command to stop
fi
;;
(*)
echo $usage
exit 1
;;
esac