What are the rules for valid identifiers (e.g. functions, vars, etc) in Bash?

Question

What are the syntax rules for identifiers, especially function and variable names, in Bash?

I wrote a Bash script and tested it on various versions of Bash on Ubuntu

Answer 1

The question was about "the rules", which has been answered two different ways, each correct in some sense, depending on what you want to call "the rules". Just to flesh out @rici's point that you can shove about any character in a function name, I wrote a small bash script to try to check every possible (0-255) character as a function name, as well as as the second character of a function name:

#!/bin/bash
ASCII=( nul soh stx etx eot enq ack bel bs tab nl vt np cr so si dle \
            dc1 dc2 dc3 dc4 nak syn etb can em sub esc fs gs rs us sp )

for((i=33; i < 127; ++i)); do
    printf -v Hex "%x" $i

    printf -v Chr "\x$Hex"
    ASCII[$i]="$Chr"
done
ASCII[127]=del
for((i=128; i < 256; ++i)); do
    ASCII[$i]=$(printf "0X%x" $i)
done

# ASCII table is now defined

function Test(){
    Illegal=""
    for((i=1; i <= 255; ++i)); do
        Name="$(printf \\$(printf '%03o' $i))"
        eval "function $1$Name(){ return 0; }; $1$Name ;" 2>/dev/null
        if [[ $? -ne 0 ]]; then
            Illegal+=" ${ASCII[$i]}"
            #        echo Illegal: "${ASCII[$i]}"
        fi
    done
    printf "Illegal: %s\n" "$Illegal"
}
echo "$BASH_VERSION"
Test
Test "x"

# can we really do funky crap like this?
function [}{(){
   echo "Let me take you to, funkytown!"
}
[}{    # why yes, we can!
# though editor auto-indent modes may punish us

I actually skip NUL (0x00), as that's the one character bash may object to finding in the input stream. The output from this script was:

4.4.0(1)-release
Illegal:  soh tab nl sp ! " # $ % & ' ( ) * 0 1 2 3 4 5 6 7 8 9 ; < > \ ` { | } ~ del
Illegal:  soh " $ & ' ( ) ; < > [ \ ` | del
Let me take you to, funkytown!

Note that bash happily lets me name my function "[}{". Probably my code is not quite rigorous enough to provide the exact rules for legality-in-practice, but it should give a flavor of what manner of abuse is possible. I wish I could mark this answer "For mature audiences only."