上一节，讲过了，执行hadoop namenode -format后
实际上是执行
/root/hadoop-2.7.0-bin/bin/hdfs namenode -format下面就来分析这个脚本
---
bin=`which $0`
bin=`dirname ${bin}`
bin=`cd "$bin" > /dev/null; pwd`打印
bin=/root/hadoop-2.7.0-bin/bin---
DEFAULT_LIBEXEC_DIR="$bin"/../libexec打印’
DEFAULT_LIBEXEC_DIR=/root/hadoop-2.7.0-bin/bin/../libexec---
cygwin=false
case "$(uname)" in
CYGWIN*) cygwin=true;;
esac这个不会执行，过滤
---
接下来执行一个脚本
HADOOP_LIBEXEC_DIR=${HADOOP_LIBEXEC_DIR:-$DEFAULT_LIBEXEC_DIR}
. $HADOOP_LIBEXEC_DIR/hdfs-config.sh实际上执行的是
/root/hadoop-2.7.0-bin/libexec/hdfs-config.sh这个脚本其实是调用另外一个脚本，调用的哪个脚本？读者可以自己去探索一下：）
---回到hdfs脚本
function print_usage(){
echo "Usage: hdfs [--config confdir] [--loglevel loglevel] COMMAND"
echo " where COMMAND is one of:"
echo " dfs run a filesystem command on the file systems supported in Hadoop."
echo " classpath prints the classpath"
echo " namenode -format format the DFS filesystem"
echo " secondarynamenode run the DFS secondary namenode"
echo " namenode run the DFS namenode"
echo " journalnode run the DFS journalnode"
echo " zkfc run the ZK Failover Controller daemon"
echo " datanode run a DFS datanode"
echo " dfsadmin run a DFS admin client"
echo " haadmin run a DFS HA admin client"
echo " fsck run a DFS filesystem checking utility"
echo " balancer run a cluster balancing utility"
echo " jmxget get JMX exported values from NameNode or DataNode."
echo " mover run a utility to move block replicas across"
echo " storage types"
echo " oiv apply the offline fsimage viewer to an fsimage"
echo " oiv_legacy apply the offline fsimage viewer to an legacy fsimage"
echo " oev apply the offline edits viewer to an edits file"
echo " fetchdt fetch a delegation token from the NameNode"
echo " getconf get config values from configuration"
echo " groups get the groups which users belong to"
echo " snapshotDiff diff two snapshots of a directory or diff the"
echo " current directory contents with a snapshot"
echo " lsSnapshottableDir list all snapshottable dirs owned by the current user"
echo " Use -help to see options"
echo " portmap run a portmap service"
echo " nfs3 run an NFS version 3 gateway"
echo " cacheadmin configure the HDFS cache"
echo " crypto configure HDFS encryption zones"
echo " storagepolicies list/get/set block storage policies"
echo " version print the version"
echo ""
echo "Most commands print help when invoked w/o parameters."
# There are also debug commands, but they don't show up in this listing.
}
if [ $# = 0 ]; then
print_usage
exit
fi这个太简单，就是一个函数而已，告诉用途
---
接下来到了最关键的时刻了，就是执行命令
if [ "$COMMAND" = "namenode" ] ; then
CLASS='org.apache.hadoop.hdfs.server.namenode.NameNode'
HADOOP_OPTS="$HADOOP_OPTS $HADOOP_NAMENODE_OPTS"其中
HADOOP_OPTS= -Djava.net.preferIPv4Stack=true -Dhadoop.log.dir=/root/hadoop-2.7.0-bin/logs -Dhadoop.log.file=hadoop.log -Dhadoop.home.dir=/root/hadoop-2.7.0-bin -Dhadoop.id.str=root -Dhadoop.root.logger=INFO,console -Djava.library.path=/root/hadoop-2.7.0-bin/lib/native -Dhadoop.policy.file=hadoop-policy.xml -Djava.net.preferIPv4Stack=true -Dhadoop.security.logger=INFO,RFAS -Dhdfs.audit.logger=INFO,NullAppender---
剩下的一段是cgwin,忽略
---
export CLASSPATH=$CLASSPATH
HADOOP_OPTS="$HADOOP_OPTS -Dhadoop.security.logger=${HADOOP_SECURITY_LOGGER:-INFO,NullAppender}"赋值语句不多说
---
接下来的一个if-else语句，实际上执行的是最后一个分支
else
# run it
exec "$JAVA" -Dproc_$COMMAND $JAVA_HEAP_MAX $HADOOP_OPTS $CLASS "$@"
fi庐山真面目要出来了，打印执行语句
/usr/java/jdk1.8.0_45/bin/java -Dproc_namenode -Xmx1000m -Djava.net.preferIPv4Stack=true -Dhadoop.log.dir=/root/hadoop-2.7.0-bin/logs -Dhadoop.log.file=hadoop.log -Dhadoop.home.dir=/root/hadoop-2.7.0-bin -Dhadoop.id.str=root -Dhadoop.root.logger=INFO,console -Djava.library.path=/root/hadoop-2.7.0-bin/lib/native -Dhadoop.policy.file=hadoop-policy.xml -Djava.net.preferIPv4Stack=true -Dhadoop.security.logger=INFO,RFAS -Dhdfs.audit.logger=INFO,NullAppender -Dhadoop.security.logger=INFO,NullAppender org.apache.hadoop.hdfs.server.namenode.NameNode -format哟，不错喔。
终于揭开了庐山真面目。
下一节，我们开始分析NameNode的源码。
来源：oschina
链接：https://my.oschina.net/u/1382024/blog/423681