alias

Define or display aliases.

Synopsis

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

Main Purpose

• Simplify long commands.
• Define, modify, or display one or more aliases.

Options

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

Return Value

alias returns true unless an alias you try to display is undefined.

Examples

# Display all defined aliases
alias
alias -p

# Display specific defined aliases (assuming they exist in the current environment)
alias ls
alias ls grep

# Define or modify an alias
alias ls='ls --color=auto'
alias ls='ls --color=never' grep='grep --color=never'

Knowledge Points

Aliases set directly in the shell will be lost after the terminal is closed or the system is restarted. How can they be made permanent?

Open ~/.bashrc with an editor and add your alias settings, for example: alias rm='rm -i'. After saving, execute source ~/.bashrc to apply the changes. This will permanently save the command aliases.

Since this modifies the ~/.bashrc file in the current user's directory, it only affects the current user. To make it effective for all users, modify the /etc/bashrc file instead.

Note: The following may vary depending on your system:

On CentOS 7, this file is /etc/bash.bashrc. Additionally, if you examine ~/.bashrc on CentOS 7, you might find a block of code like this:

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

This code means that if .bash_aliases exists, it will be loaded. You can create this file in your home directory to store your alias settings separately.

Incorrect Usage

• The alias to be displayed is undefined.
• When defining or modifying an alias, failing to wrap the value in single quotes if it contains spaces can lead to serious issues:

# Clear all aliases for demonstration
unalias -a
# Not wrapped in single quotes
alias rm=rm -rf
# Execution results in an error: bash: alias: -rf: not found
# Checking with 'alias' returns: alias rm='rm'

# A more confusing example
# Clear all aliases for demonstration
unalias -a
# Still not wrapped in single quotes
alias ls=ls --color=never
# Execution appears to succeed without error

# Checking with 'alias' reveals:
# alias --color=never
# alias ls='ls'
# The 'alias' command treated these as two separate groups.

Q&A

Q: What happens if I try to display multiple aliases and some are undefined?

A: Run it as usual; alias will continue to process the remaining arguments even if one is undefined.

Q: What if I define alias cd='ls' ls='cd'?

A: Running cd will still change the directory, and ls will still list files. Do not define aliases this way.

Notes

1. When executing scripts:
   • If a bash script executed with the source command runs alias or unalias, it may affect the terminal environment's alias settings; conversely, terminal aliases can affect the script's results.
   • Bash scripts called via sh or executed directly (with execution permissions) are not affected by terminal environment aliases.
2. To remove an alias, see the unalias command.
3. It is recommended NOT to use dangerous options like -f in aliases for commands like mv, cp, or rm (e.g., alias rm='rm -f').
4. Be aware of potential conflicts between aliases and other commands.
5. This is a bash built-in command; for more help, use the help command.

Other References

• alias(1p) - Linux manual page