Using “@” as a bash function name

EDIT 2: While @ is unproblematic in vanilla bash, it is used as a pattern grouping operator when the extglob shell option is set, a simple echo @() can hang your shell under certain conditions. All the more reason to not use @ as an identifier`.

EDIT 1: To clarify, it is not allowed per the definition of an identifier. I'm not recommending it. I'm just saying that for backwards compatibility, it is possible in bash to use @ as an identifier.

@() { echo "$1 world"; }
@ hello
alias @="echo hello"
@ world

@ is used as a special parameter ($@) and for arrays (${arr[@]}) but nowhere else. Bash won't stop you using it for identifiers like aliases and functions but not variable names.

Some people alias @ to sudo, so they can execute commands as @ apt-get install.

The main reason why I would not use @ as a shell identifier is that I'm so used to the semantics of @ in Makefiles which silences commands without any side effects.

BTW: Works the same in zsh.

From man bash

   name   A word consisting only of alphanumeric characters and underscores, 
          and beginning with an  alphabetic character or an underscore.  Also 
          referred to as an identifier.

Shell functions are declared as follows:
          name () compound-command [redirection]
          function name [()] compound-command [redirection]

The naming of functions is quite similar to the allowed characters for alias:
From man bash:

The characters /, $, `, and = and any of the shell metacharacters or quoting characters listed above may not appear in an alias name.

metacharacter
One of the following: | & ; ( ) < > space tab

So, except for: / $ ` = | & ; ( ) < > space tab

any other character should be valid for alias and function names.

However, the character @ is also used for @(pattern-list) when extglob is active. By default extglob is active in interactive shells.

So, this should fail:

$ @(){ echo "hello"; }
bash: syntax error near unexpected token `}'

However, this works:

$  bash -c '@(){ echo "hello"; }; @'
hello

And this should work as well:

$ shopt -u extglob                 ### keep this as an independent line.
$ @(){ echo "hello"; }             
$ @
hello

It is also possible to do this:

$ f(){ echo "yes"; }
$ alias @=f
$ @
yes

An alias is flexible in what symbols it accepts.