Linux hash table Study Notes

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A, hash table summarizes

When a Linux system for the first time execute external commands, hash cache table is empty;
this time, the system will start looking at the PATH command, the path will be added to find the Hash cache;

When this command is executed again, the path will be executed directly from the hash table;
If there is direct execution, will continue to find if there is no path from the PATH;

Therefore, hash table can call the command rate of increase.

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Two, hash cache table position

Command executed on Linux systems, there is a execution priority, the location cache hash table is as follows:

alias -------------------------------------别名
  builtin------------------------------内部命令
    hash-------------------------缓存表
      $PATH---------------可执行程序或脚本(外部命令)

Linux on the difference between internal commands and external commands are interested can poke "Linux internal commands and external commands" understanding.

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Three, hash command Introduction

hashCommand can be used to display and configure the cache hash table;
when executing the command, the system will first check the cache hash table.

hash The main parameters command:

-l    显示哈希表所有的项目
-r    清除哈希表所有项
-d <名称>   删除哈希表其中的一项
-p <路径>   向哈希表中增加一项内容，添加后就可以使用了
-t  <命令>  显示hash表中命令的完整路径，如果没有就会报not found错误

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Four, hash command usage examples

:~> bash   # 进入一个全新的 shell
:~> hash
hash: hash table empty    # 由于是新建 shell，所以hash表为空，符合预期

:~> ls -l

:~> hash
hits    command
   1    /usr/bin/ls
:~> top

:~> hash
hits    command
   1    /usr/bin/ls
   1    /usr/bin/top

:~> uptime
 17:56:40 up 1138 days, 7 min,  1 user,  load average: 0.47, 0.36, 0.28

:~> hash   #  可以看到hash表在不断更新
hits    command
   1    /usr/bin/uptime
   1    /usr/bin/ls
   1    /usr/bin/top

:~> hash -l
builtin hash -p /usr/bin/uptime uptime
builtin hash -p /usr/bin/ls ls
builtin hash -p /usr/bin/top top

:~> hash -p /tmp/mydate  date   # 手动添加hash表

:~> hash -l
builtin hash -p /usr/bin/uptime uptime
builtin hash -p /tmp/mydate date   # 添加成功
builtin hash -p /usr/bin/ls ls
builtin hash -p /usr/bin/top top

:~> date
bash: /tmp/mydate: No such file or directory    # 由于手动添加的hash表指向执行文件不存在，所以报错，这说明新增的hash表确实在work

:~> hash -d  date   # 手动清楚刚加的hash表，使用 -r 则全部清除

:~> hash -l
builtin hash -p /usr/bin/uptime uptime
builtin hash -p /usr/bin/ls ls
builtin hash -p /usr/bin/top top

:~> date   # 恢复
Sun Jan 19 18:02:45 CST 2020