Re: Why no closing single quote is needed when a single quote is escaped?

[Eli Schwartz]

On 12/31/20 6:35 PM, Peng Yu wrote:
> > > x='\'
> > The difference between single quoting and double quoting is fully
> > explained in the bash manual page, under the section "QUOTING".
>
> The man page says the following. But it doesn't say why a closing
> single quote is not need to pair with the first quote. Why \' makes
> the closing single quote unnecessary.
>
>         Enclosing characters in single quotes preserves the literal
> value of each character within the  quotes.   A
>         single quote may not occur between single quotes, even when
> preceded by a backslash.
>
> If this sentence is not the relevant description in the manpage,
> please be specific which sentence is, as I don't find it. Thanks.

Those two sentences are indeed the relevant description. Due to the 
first sentence, there is no such thing as the syntax token "backslash" 
(i.e. "the escape token") inside single quotes (because it is merely an 
ASCII string character).

The second sentence makes the witty observation that since there is no 
such thing as the syntax token backslash inside of single quotes, you 
cannot use nonexistent things to escape a single quote, hence a single 
quote will always indicate "end of quoted string". You cannot tell bash 
that it is actually an ASCII string character.

Remember your original example:

x='\'

You asked:

> It doesn't make sense to me why the above command should work > without closing 
> the initial single quote.

But your question itself makes no sense.

$ echo "«$x»"
«\»

Note how the variable does not contain a single quote, so it could not 
have been escaped and turned into a string.

x='\'

x=''
   ^contents go here
    contents are one byte, being the unicode codepoint U+005C
    a.k.a. the backslash, a.k.a. \


Meanwhile, the double quotes did indeed first refuse to end (since \" 
got parsed as a string character, so <ENTER> prompted a multiline 
command with the line continuation PS2 "> ")

$ x="\"
> "
$ echo "«$x»"
«"
»

Note the double quote there, indicating it did not get used as a syntax 
end-of-string marker. Instead, the string contained the newline too, and 
ended on the second double quote.

--
Eli Schwartz
Arch Linux Bug Wrangler and Trusted User

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