关于shell的EOF

之前在执行hbase shell的时候用过这个用法，现在温习一下。
Shell中通常将EOF与 << 结合使用，表示后续的输入作为子命令或子Shell的输入，直到遇到EOF为止，再返回到主调Shell。
可以把EOF替换成其他东西，只要前后字符串能对应起来就可以，意思是把内容当作标准输入传给程序。

其简单用法如下:

<<EOF        //开始
....
EOF            //结束

还可以自定义，比如自定义：

<<BBB        //开始
....
BBB              //结束

结合其他命令比如：

cat << CCC
> hello world!
> My name is Carson!
> CCC
#输出：
hello world!
My name is Carson!

执行mysql命令：

mysql -uroot -proot my_db <<EOF
> show tables;
> select count(*) from test;
> EOF

执行hbase shell：

exec $HBASE_HOME/bin/hbase shell <<EOF 
disable 't_abc'
drop 't_abc'
create 't_abc', 'info'
 
EOF

最后我们看一下flume启动命令里的帮助文档是如何显示的：

display_help() {
  cat <<EOF
Usage: $0 <command> [options]...

commands:
  help                      display this help text
  agent                     run a Flume agent
  avro-client               run an avro Flume client
  version                   show Flume version info

global options:
  --conf,-c <conf>          use configs in <conf> directory
  --classpath,-C <cp>       append to the classpath
  --dryrun,-d               do not actually start Flume, just print the command
  --plugins-path <dirs>     colon-separated list of plugins.d directories. See the
                            plugins.d section in the user guide for more details.
                            Default: \$FLUME_HOME/plugins.d
  -Dproperty=value          sets a Java system property value
  -Xproperty=value          sets a Java -X option

agent options:
  --name,-n <name>          the name of this agent (required)
  --conf-file,-f <file>     specify a config file (required if -z missing)
  --zkConnString,-z <str>   specify the ZooKeeper connection to use (required if -f missing)
  --zkBasePath,-p <path>    specify the base path in ZooKeeper for agent configs
  --no-reload-conf          do not reload config file if changed
  --help,-h                 display help text

avro-client options:
  --rpcProps,-P <file>   RPC client properties file with server connection params
  --host,-H <host>       hostname to which events will be sent
  --port,-p <port>       port of the avro source
  --dirname <dir>        directory to stream to avro source
  --filename,-F <file>   text file to stream to avro source (default: std input)
  --headerFile,-R <file> File containing event headers as key/value pairs on each new line
  --help,-h              display help text

  Either --rpcProps or both --host and --port must be specified.

Note that if <conf> directory is specified, then it is always included first
in the classpath.

EOF
}