Basic command classification of LInux system operation explanation

1. The concept of command

• Execution process of command

  When the system executes an external command for the first time The hash cache table is empty. The system will first search for the command from the PTAH path. After finding it, the path will be added to the Hasa cache. When the command is executed again, it will be executed directly from the hash path. If it exists, it will be executed directly. If it does not exist, it will be executed directly. The search will continue from the path under PATH, and the Hash table can improve the calling rate of the command.

• Priority of command

  alias -------------------------- ----------------Alias ​​
  builtin-------------------------------- -Internal command
  ˆ ˆhash-------------------------Cache table
  ˆ ˆˆˆˆhash-------------------------- ----Executable program or script (external command)

• Internal command and external command

  The internal command is the shell’s own
  External commands are installed by default when installing the system, and have corresponding paths in the file system

• Check whether the command is an internal command or an external commandtype [commnd ]

  [root@centos6 ~]# type cat                  #判断cat命令，外部命令显示文件路径
  cat is /bin/cat
  [root@centos6 ~]# type cd                   #判断cd命令
  cd is a shell builtin

  ---

  2. Command alias

  The named alias is only valid in the current process
  If you want it to be permanently valid, it must be defined in the configuration file
  Only for the current user: ~/.bashrc
  Valid for all users: /etc/bashrc

• View all aliases in the processalias

  [root@centos6 ~]#alias
  alias cp='cp -i'
  alias l.='ls -d .* --color=auto'
  alias ll='ls -l --color=auto'
  alias ls='ls --color=auto'
  ......

• Define aliasalias NAME="VALUE"

  [root@centos6 ~]#alias aubin=cat
  [root@centos6 ~]#aubin test
  hello world

• Delete alias

  [root@centos6 ~]#unalias aubin
  [root@centos6 ~]#aubin test
  -bash: aubin: command not found

• Define the alias that is permanently effective for the current user

  [root@centos6 ~]#vim .bashrc 
  # .bashrc
  # User specific aliases and functions
  alias rm='rm -i'
  alias cp='cp -i'
  alias mv='mv -i'
  alias aubin=cat                                # 

• Definition The alias that is effective for the specified user

  [root@centos6 ~]#cd ~ li
  [root@centos6 li]#vim .bashrc                 #编辑用户目录下的.bashrc

• DefinitionAll usersEffective aliases

  [root@centos6 ~]#vim /etc/bashrc
  alias aubin=cat                                # <hr><h1>3. Internal commands</h1>

The shell program finds the executable program or code corresponding to the typed command. After analysis by the shell, it is submitted to the kernel to allocate resources and run it.

• View all internal commands

  [root@centos6 ~]#help

  [root@centos6 ~]#enable
  enable .
  enable :
  enable [
  enable alias
  enable bg
  enable bind
  ......

• Disable and enable internal commandsenable

  [root@centos6 li]#enable -n cd                 #禁用内部命令
  [root@centos6 li]#enable cd                    #启用内部命令

• Disable internal commandsInvalid

  [root@centos6 li]#enable -n pwd
  [root@centos6 li]#enable -n                    #查看禁用的内部命令或如下图用help
  enable -n pwd

  You can also check the disabled commands byhelp【*# before the command ##Represents that the command has been used】
  Disables the internal command
  enable -n pwd can still be used

  [root@centos6 li]#pwd
  /home/li

  Use

  which to view the command The executable file

  [root@centos6 li]#which pwd
  /bin/pwd

  When the internal command is disabled, continue to search the Hash table and

  \(PATH. Until the /bin/pwd# is found in \)PATH according to the bash priority. ##'s executable file will run it.

View disabled internal commands

• [root@centos6 li]#enable -n
  enable -n cd
  enable -n pwd

  Or use the

  help

  command as shown above

4.HASH cache table

---

is used to display and clear the hash table. When executing the command, the system will first query the hash table.

View the command cache
• hash

  <pre class="brush:php;toolbar:false">[root@centos6 ~]# hash   hits  command    3    /usr/bin/cal    1    /usr/bin/yum</pre> <pre class="brush:php;toolbar:false">[root@centos6 ~]# 查看详细的Hash表 [root@centos6 ~]#hash -l builtin hash -p /bin/dd dd builtin hash -p /usr/bin/yum yum</pre>

Add content to the Hash table
• hash - p path command

  <pre class="brush:php;toolbar:false">[root@centos6 ~]#将cat定义一个别名存在hash表 [root@centos6 ~]#hash -p /bin/cat aubin   [root@centos6 ~]#aubin test hello world</pre>

Print the path of the command in the Hash table
• hash -t command

  <pre class="brush:php;toolbar:false">[root@centos6 ~]#hash -t aubin /bin/cat</pre>

Delete Specify the command in the Hash table
• hash -d command

  <pre class="brush:php;toolbar:false">[root@centos6 ~]#hash -d aubin</pre>

Delete all commands in the Hash table
• hash -r

  <pre class="brush:php;toolbar:false">[root@centos6 ~]# hash -r</pre>

View the path of the command
• which

  <pre class="brush:php;toolbar:false">[root@centos6 ~]# which cat            #查看命令的路径，以第一个路径为准 /bin/cat [root@centos6 ~]# which -a cat         #查看命令所有路径，一个命令可能有多个路径 /bin/cat /usr/local/bin/cat</pre>

  5. External command

  ---

  The external command is an executable file. When executed When issuing an external command, the system will execute the corresponding executable file in the file directory.

List the path of the command

• [root@centos6 /]#which echo                    #列出命令的路径
  /bin/echo

  [root@centos6 /]#which cp                      #which列出文件路径会显示别名
  alias cp='cp -i'
      /bin/cp
  [root@centos6 /]#which --skip-alias cp         #列出文件路径而不显示别名
  /bin/cp

List all the paths of the command. When multiple bash have the same command, the command has Multiple paths.

• [root@centos6 /]#which -a echo              
  /bin/echo

List the paths to commands and help manuals

• [root@centos6 /]#whereis echo
  echo: /bin/echo /usr/share/man/man1/echo.1.gz /usr/share/man/man1p/echo.1p.gz

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