Hadoop学习笔记13：./bin/hadoop namenode -format之脚本分析
网上视频和书本都说：启动之前要格式化，本节就来讲讲解格式化HDFS命令背后的原理！
先来看看执行结果！
下面我们来研究原理！
=====================
首先需要研究下./bin/hadoop脚本怎么运行
THIS="$0"
while [ -h "$THIS" ]; do
ls=`ls -ld "$THIS"`
link=`expr "$ls" : '.*-> \(.*\)$'`
if expr "$link" : '.*/.*' > /dev/null; then
THIS="$link"
else
THIS=`dirname "$THIS"`/"$link"
fi
done
意思是判断./bin/hadoop是否是一个符号链接，
这里不是符号链接，所以不用太关注这段代码！
$0的值是 ./bin/hadoop
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# if no args specified, show usage if [ $# = 0 ]; then echo "Usage: hadoop COMMAND" echo "where COMMAND is one of:" echo " namenode -format format the DFS filesystem" echo " namenode run the DFS namenode" echo " datanode run a DFS datanode" echo " dfs run a DFS admin client" echo " fsck run a DFS filesystem checking utility" echo " jobtracker run the MapReduce job Tracker node" echo " tasktracker run a MapReduce task Tracker node" echo " job manipulate MapReduce jobs" echo " jar <jar> run a jar file" echo " or" echo " CLASSNAME run the class named CLASSNAME" echo "Most commands print help when invoked w/o parameters." exit 1 fi 接下来是判断./bin/hadoop后面的参数个数，如果是0，表明用户 还不太清楚这个命令的用法，则给出这个命令的用法！ 很简单！
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# get arguments COMMAND=$1 shift 这里是获取参数，如果执行./bin/hadoop namenode -format 则COMMAND的值就是namenode 然后执行完shift后，$1就变成-format
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# some directories THIS_DIR=`dirname "$THIS"` HADOOP_HOME=`cd "$THIS_DIR/.." ; pwd` 接下来是获取2个变量 值分别是 ./bin /usr/local/hadoop-0.1.0
---
# Allow alternate conf dir location.
HADOOP_CONF_DIR="${HADOOP_CONF_DIR:-$HADOOP_HOME/conf}"
接下来就是获取
HADOOP_CONF_DIR
值为/usr/local/hadoop-0.1.0/conf
=============
if [ -f "${HADOOP_CONF_DIR}/hadoop-env.sh" ]; then
source "${HADOOP_CONF_DIR}/hadoop-env.sh"
fi
导入hadoop-env.sh里的环境变量，
可见后续如果有需求的话，可以修改hadoop-env.sh这个文件就可以了
而不是修改hadoop文件，很方便！
备注：针对hadoop-env.sh这个文件，
我只添加了
export JAVA_HOME=/usr/lib/jvm/jdk1.7.0_21
===
# some Java parameters if [ "$JAVA_HOME" != "" ]; then #echo "run java in $JAVA_HOME" JAVA_HOME=$JAVA_HOME fi 给JAVA_HOME赋值! 结果自然就是 /usr/lib/jvm/jdk1.7.0_2
==========
if [ "$JAVA_HOME" = "" ]; then echo "Error: JAVA_HOME is not set." exit 1 fi 这个就是校验JAVA_HOME是否配置了
=======
JAVA=$JAVA_HOME/bin/java JAVA_HEAP_MAX=-Xmx1000m 设置2个变量，值分别如下： /usr/lib/jvm/jdk1.7.0_21/bin/java -Xmx1000m
---
# check envvars which might override default args if [ "$HADOOP_HEAPSIZE" != "" ]; then #echo "run with heapsize $HADOOP_HEAPSIZE" JAVA_HEAP_MAX="-Xmx""$HADOOP_HEAPSIZE""m" #echo $JAVA_HEAP_MAX fi 这里，通过echo得知 $HADOOP_HEAPSIZE为空，所以也不会执行！
===
# CLASSPATH initially contains $HADOOP_CONF_DIR
CLASSPATH="${HADOOP_CONF_DIR}"
CLASSPATH=${CLASSPATH}:$JAVA_HOME/lib/tools.jar
最终CLASSPATH的值是
/usr/local/hadoop-0.1.0/conf:/usr/lib/jvm/jdk1.7.0_21/lib/tools.jar
# for developers, add Hadoop classes to CLASSPATH
if [ -d "$HADOOP_HOME/build/classes" ]; then
CLASSPATH=${CLASSPATH}:$HADOOP_HOME/build/classes
fi
if [ -d "$HADOOP_HOME/build/webapps" ]; then
CLASSPATH=${CLASSPATH}:$HADOOP_HOME/build
fi
if [ -d "$HADOOP_HOME/build/test/classes" ]; then
CLASSPATH=${CLASSPATH}:$HADOOP_HOME/build/test/classes
fi
经过修饰，CLASSPATH最终的值是 /usr/local/hadoop-0.1.0/conf : /usr/lib/jvm/jdk1.7.0_21/lib/tools.jar : /usr/local/hadoop-0.1.0/build/classes : /usr/local/hadoop-0.1.0/build : /usr/local/hadoop-0.1.0/build/test/classes 这样就把所有的相关类包含进来了！！！！！！！！！！！！！
===
# so that filenames w/ spaces are handled correctly in loops below IFS= 这个简单！
---
# for releases, add hadoop jars & webapps to CLASSPATH
if [ -d "$HADOOP_HOME/webapps" ]; then
CLASSPATH=${CLASSPATH}:$HADOOP_HOME
fi
for f in $HADOOP_HOME/hadoop-*.jar; do
CLASSPATH=${CLASSPATH}:$f;
done
# add libs to CLASSPATH
for f in $HADOOP_HOME/lib/*.jar; do
CLASSPATH=${CLASSPATH}:$f;
done
for f in $HADOOP_HOME/lib/jetty-ext/*.jar; do
CLASSPATH=${CLASSPATH}:$f;
done
最终CLASSPATH的值是
/usr/local/hadoop-0.1.0/conf
:
/usr/lib/jvm/jdk1.7.0_21/lib/tools.jar
:
/usr/local/hadoop-0.1.0/build/classes
:
/usr/local/hadoop-0.1.0/build
:
/usr/local/hadoop-0.1.0/build/test/classes
:
/usr/local/hadoop-0.1.0
:
/usr/local/hadoop-0.1.0/hadoop-0.1.0-examples.jar
:
/usr/local/hadoop-0.1.0/hadoop-0.1.0.jar
:
/usr/local/hadoop-0.1.0/lib/commons-logging-api-1.0.4.jar
:
/usr/local/hadoop-0.1.0/lib/jetty-5.1.4.jar
:
/usr/local/hadoop-0.1.0/lib/junit-3.8.1.jar
:
/usr/local/hadoop-0.1.0/lib/lucene-core-1.9.1.jar
:
/usr/local/hadoop-0.1.0/lib/servlet-api.jar
:
/usr/local/hadoop-0.1.0/lib/jetty-ext/ant.jar
:
/usr/local/hadoop-0.1.0/lib/jetty-ext/commons-el.jar
:
/usr/local/hadoop-0.1.0/lib/jetty-ext/jasper-compiler.jar
:
/usr/local/hadoop-0.1.0/lib/jetty-ext/jasper-runtime.jar
:
/usr/local/hadoop-0.1.0/lib/jetty-ext/jsp-api.jar
===
# figure out which class to run
if [ "$COMMAND" = "namenode" ] ; then
CLASS='org.apache.hadoop.dfs.NameNode'
elif [ "$COMMAND" = "datanode" ] ; then
CLASS='org.apache.hadoop.dfs.DataNode'
elif [ "$COMMAND" = "dfs" ] ; then
CLASS=org.apache.hadoop.dfs.DFSShell
elif [ "$COMMAND" = "fsck" ] ; then
CLASS=org.apache.hadoop.dfs.DFSck
elif [ "$COMMAND" = "jobtracker" ] ; then
CLASS=org.apache.hadoop.mapred.JobTracker
elif [ "$COMMAND" = "tasktracker" ] ; then
CLASS=org.apache.hadoop.mapred.TaskTracker
elif [ "$COMMAND" = "job" ] ; then
CLASS=org.apache.hadoop.mapred.JobClient
elif [ "$COMMAND" = "jar" ] ; then
JAR="$1"
shift
CLASS=`"$0" org.apache.hadoop.util.PrintJarMainClass "$JAR"`
if [ $? != 0 ]; then
echo "Error: Could not find main class in jar file $JAR"
exit 1
fi
CLASSPATH=${CLASSPATH}:${JAR}
else
CLASS=$COMMAND
fi
这里显然执行./bin/hadoop namenode -format的话
CLASS就是
org.apache.hadoop.dfs.NameNode
===
# cygwin path translation if expr `uname` : 'CYGWIN*' > /dev/null; then CLASSPATH=`cygpath -p -w "$CLASSPATH"` fi 我的是在linux中运行的，所以不用管！
---
最后就是执行了！
# run it exec "$JAVA" $JAVA_HEAP_MAX $HADOOP_OPTS -classpath "$CLASSPATH" $CLASS "$@" 相关的值分别如下： /usr/lib/jvm/jdk1.7.0_21/bin/java -Xmx1000m -classpath /usr/local/hadoop-0.1.0/conf : /usr/lib/jvm/jdk1.7.0_21/lib/tools.jar : /usr/local/hadoop-0.1.0/build/classes : /usr/local/hadoop-0.1.0/build : /usr/local/hadoop-0.1.0/build/test/classes : /usr/local/hadoop-0.1.0 : /usr/local/hadoop-0.1.0/hadoop-0.1.0-examples.jar : /usr/local/hadoop-0.1.0/hadoop-0.1.0.jar : /usr/local/hadoop-0.1.0/lib/commons-logging-api-1.0.4.jar : /usr/local/hadoop-0.1.0/lib/jetty-5.1.4.jar : /usr/local/hadoop-0.1.0/lib/junit-3.8.1.jar : /usr/local/hadoop-0.1.0/lib/lucene-core-1.9.1.jar : /usr/local/hadoop-0.1.0/lib/servlet-api.jar : /usr/local/hadoop-0.1.0/lib/jetty-ext/ant.jar : /usr/local/hadoop-0.1.0/lib/jetty-ext/commons-el.jar : /usr/local/hadoop-0.1.0/lib/jetty-ext/jasper-compiler.jar : /usr/local/hadoop-0.1.0/lib/jetty-ext/jasper-runtime.jar : /usr/local/hadoop-0.1.0/lib/jetty-ext/jsp-api.jar org.apache.hadoop.dfs.NameNode -format
这样就运行起来了！
很简单！