理解Hadoop脚本hadoop-2.5.0/bin/hadoop

tanggang1740

　　
　　1 #!/usr/bin/env bash    此处为什么不是  #!/bin/bash  ?  

考虑到程序的可移植性，env的作用就是为了找到正确的脚本解释器（这里就是bash），在不同的Linux系统上该解释器可能所处的位置不同
　　
    18  # This script runs the hadoop core commands.
    19
    20  bin=`which $0`                                  shell的默认赋值是字符串赋值， $0表示脚本本身的名字，which    locate the file
    21  bin=`dirname ${bin}`
    22  bin=`cd "$bin"; pwd`
    23
    24  DEFAULT_LIBEXEC_DIR="$bin"/../libexec
    25  HADOOP_LIBEXEC_DIR=${HADOOP_LIBEXEC_DIR:-$DEFAULT_LIBEXEC_DIR}
　　26 . $HADOOP_LIBEXEC_DIR/hadoop-config.sh
    27
    28  function print_usage(){
    29    echo "Usage: hadoop [--config confdir] COMMAND"
    30    echo "       where COMMAND is one of:"
    31    echo "  fs                   run a generic filesystem user client"
    32    echo "  version              print the version"
    33    echo "  jar             run a jar file"
    34    echo "  checknative [-a|-h]  check native hadoop and compression libraries availability"
    35    echo "  distcp   copy file or directories recursively"
    36    echo "  archive -archiveName NAME -p  *  create a hadoop archive"
    37    echo "  classpath            prints the class path needed to get the"
    38    echo "                       Hadoop jar and the required libraries"
    39    echo "  daemonlog            get/set the log level for each daemon"
    40    echo " or"
    41    echo "  CLASSNAME            run the class named CLASSNAME"
    42    echo ""
    43    echo "Most commands print help when invoked w/o parameters."
    44  }
    45
    46  if [ $# = 0 ]; then               $#,运行脚本时所带参数的个数， 注意 [ 后和 ]前要有一个空格， 判断=左右两个变量是否相等
    47    print_usage
    48    exit
    49  fi
    50
    51  COMMAND=$1                      $1, 执行的脚本的第一个参数，$2,$3.......以此类推
　　#case句法 ，每个匹配都用;;来表示结束

case ... in
...) do something here
;;
esac
　　#
    52  case $COMMAND in
    53    # usage flags
    54    --help|-help|-h)
    55      print_usage
    56      exit
    57      ;;
　　58
    59    #hdfs commands
    60    namenode|secondarynamenode|datanode|dfs|dfsadmin|fsck|balancer|fetchdt|oiv|dfsgroups|portmap|nfs3)
    61      echo "DEPRECATED: Use of this script to execute hdfs command is deprecated." 1>&2
    62      echo "Instead use the hdfs command for it." 1>&2
    63      echo "" 1>&2
    64      #try to locate hdfs and if present, delegate to it.
    65      shift
    66      if [ -f "${HADOOP_HDFS_HOME}"/bin/hdfs ]; then
    67        exec "${HADOOP_HDFS_HOME}"/bin/hdfs ${COMMAND/dfsgroups/groups}  "$@"
    68      elif [ -f "${HADOOP_PREFIX}"/bin/hdfs ]; then
    69        exec "${HADOOP_PREFIX}"/bin/hdfs ${COMMAND/dfsgroups/groups} "$@"
    70      else
    71        echo "HADOOP_HDFS_HOME not found!"
    72        exit 1
    73      fi
    74      ;;
    75
    76    #mapred commands for backwards compatibility
    77    pipes|job|queue|mrgroups|mradmin|jobtracker|tasktracker)
    78      echo "DEPRECATED: Use of this script to execute mapred command is deprecated." 1>&2
    79      echo "Instead use the mapred command for it." 1>&2
    80      echo "" 1>&2
    81      #try to locate mapred and if present, delegate to it.
    82      shift
    83      if [ -f "${HADOOP_MAPRED_HOME}"/bin/mapred ]; then
    84        exec "${HADOOP_MAPRED_HOME}"/bin/mapred ${COMMAND/mrgroups/groups} "$@"
    85      elif [ -f "${HADOOP_PREFIX}"/bin/mapred ]; then
    86        exec "${HADOOP_PREFIX}"/bin/mapred ${COMMAND/mrgroups/groups} "$@"
    87      else
    88        echo "HADOOP_MAPRED_HOME not found!"
    89        exit 1
　　90 fi
    91      ;;
    92
    93    classpath)
    94      echo $CLASSPATH
    95      exit
    96      ;;
    97
    98    #core commands
    99    *)
   100      # the core commands
   101      if [ "$COMMAND" = "fs" ] ; then
   102        class="org".apache.hadoop.fs.FsShell
   103      elif [ "$COMMAND" = "version" ] ; then
   104        class="org".apache.hadoop.util.VersionInfo
   105      elif [ "$COMMAND" = "jar" ] ; then
   106        class="org".apache.hadoop.util.RunJar
   107      elif [ "$COMMAND" = "checknative" ] ; then
   108        class="org".apache.hadoop.util.NativeLibraryChecker
   109      elif [ "$COMMAND" = "distcp" ] ; then
   110        class="org".apache.hadoop.tools.DistCp
   111        CLASSPATH=${CLASSPATH}:${TOOL_PATH}
   112      elif [ "$COMMAND" = "daemonlog" ] ; then
   113        class="org".apache.hadoop.log.LogLevel
   114      elif [ "$COMMAND" = "archive" ] ; then
   115        class="org".apache.hadoop.tools.HadoopArchives
   116        CLASSPATH=${CLASSPATH}:${TOOL_PATH}
   117      elif [[ "$COMMAND" = -*  ]] ; then
   118          # class and package names cannot begin with a -
   119          echo "Error: No command named \`$COMMAND' was found. Perhaps you meant \`hadoop ${COMMAND#-}'"
   120          exit 1
   121      else
　　122 CLASS=$COMMAND
   123      fi
   124      shift                                                      shift用来左移位置参数
   125
   126      # Always respect HADOOP_OPTS and HADOOP_CLIENT_OPTS
   127      HADOOP_OPTS="$HADOOP_OPTS $HADOOP_CLIENT_OPTS"
   128
   129      #make sure security appender is turned off
   130      HADOOP_OPTS="$HADOOP_OPTS -Dhadoop.security.logger=${HADOOP_SECURITY_LOGGER:-INFO,NullAppender}"
   131
   132      export CLASSPATH=$CLASSPATH
   133      exec "$JAVA" $JAVA_HEAP_MAX $HADOOP_OPTS $CLASS "$@"
   134      ;;
   135
   136  esac