[Comprehensive explanation of Linux commands] 191. Linux must-know commands: alias and unalias

clear

Clear any information on the current screen terminal

Additional information

clearCommand is used to clear any information on the current screen terminal.

grammar

clear

Example

Simply enter clearthe command to clear any information currently on the terminal.

alias

Define or display an alias.

overview

alias [-p] [name[=value] ...]

The main purpose

• Simplify long commands.
• Define one or more aliases.
• Modify the value of one or more defined aliases.
• Displays one or more defined aliases.
• Shows all defined aliases.

Options

• -p: Display all defined aliases.
• name(Optional): Specify the alias to (define, modify, display).
• value(optional): The value of the alias.

return value

aliasReturn trueunless the alias you want to display is undefined.

example

Show all defined aliases:

alias
alias -p

Display defined aliases (assuming the following aliases exist in the current environment):

alias ls
alias ls grep

Define or modify the value of an alias:

alias ls='ls --color=auto'
alias ls='ls --color=never' grep='grep --color=never'

Knowledge points

The command alias set directly in the shell will become invalid after the terminal is closed or the system is restarted. How can it be permanently valid?

Open it with an editor ~/.bashrc, add alias settings to the file, such as: alias rm='rm -i', save and execute source ~/.bashrc, so that the alias of the command can be permanently saved.

Because the files in the current user directory are modified ~/.bashrc, this method is only useful to the current user. If you want it to be valid for all users, /etc/bashrcjust modify the file.

Please note that the following content may differ from the system you are actually using:

• Under CentOS7, this file is /etc/bash.bashrc.

• In addition, under CentOS7, if you look closely at ~/.bashrcthe file, you will find this piece of code:

  if [ -f ~/.bash_aliases ]; then
    . ~/.bash_aliases
  fi
  

  The meaning of this code is to load the file if it exists .bash_aliases, so you can also create a new file in the user root directory to store the command alias settings separately.

Wrong usage

• The alias to be displayed is undefined.

• When you define (modify) the value of an alias, serious problems may occur because the value string contains spaces but is not enclosed in single quotes.

  Example:

  # 为方便演示，删除全部别名
  unalias -a
  # 没有用单引号扩起
  alias rm=rm -rf
  # 执行命令后报错 bash: alias: -rf: not found
  # 这时使用alias查看rm的别名时返回 alias rm='rm'
  # 更具有迷惑性的例子
  # 为方便演示，删除全部别名
  unalias -a
  # 仍然没有用单引号括起
  alias ls=ls --color=never
  # 执行命令后看起来没有报错
  
  # 使用alias查看全部别名会发现运行结果如下：
  # alias --color=never
  # alias ls='ls'
  # alias处理时将它们看成了两组
  

Q&A

Q: What should I do if I want to display one or more aliases, but I don’t know if any of them are undefined?

A: Just execute it normally. aliasThe execution of the remaining parameters will not end just because there is an undefined alias.

Q: If I define it this way alias cd='ls' ls='cd', what will be the consequences?

A: Running cdwill still switch directories, and running lswill still list the contents of the folder; do not define it this way.

Notice

• Please note when executing the script:
  • sourceIf a bash script executed using a command aliasor unaliascommand is executed, it may have an impact on the alias settings of the terminal environment; the alias settings of the terminal environment may also change the running results.
  • Bash scripts called through shmethods or directly run scripts that the current user has execution permissions are not affected by the alias of the terminal environment.
• To delete an alias, please see unaliasthe command.
• It is recommended that you do not set dangerous options for aliases of commands such as mv, cp, , etc. , such as .rm-falias rm='rm -f'

you cry

Delete aliasan alias set by

overview

unalias [-a] name [name ...]

The main purpose

• Delete one or more aliases.
• Delete all defined aliases.

Options

• -a: Delete all defined aliases.

parameter

• name:Specifies one or more defined aliases to be deleted.

return value

unaliasReturn trueunless the alias you want to delete is undefined.

example

Delete all defined aliases:

unalias -a

Delete defined aliases (assuming the following aliases exist in the current environment):

unalias vi
unalias ls grep

Wrong usage

• The alias to be deleted is undefined.
• -aNo arguments are passed when options are not used name.

Notice

• Please note when executing the script:
  • sourceIf a bash script executed using a command aliasor unaliascommand is executed, it may have an impact on the alias settings of the terminal environment; the alias settings of the terminal environment may also change the running results.
  • Bash scripts called through shmethods or directly run scripts that the current user has execution permissions are not affected by the alias of the terminal environment.

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