Re: The correspondence of the command-name and its key-sequence.

[<tomas@tuxteam.de>]

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On Mon, Jan 11, 2021 at 09:20:46AM +0100, Omar Polo wrote:
> 
> Hongyi Zhao <hongyi.zhao@gmail.com> writes:
> 
> > On Sat, Jan 9, 2021 at 6:00 PM Omar Polo <op@omarpolo.com> wrote:
> > [...]
> >
> > Why the \ has two forms in the ascii document as shown below:
> >
> > $ man ascii | grep -F '\\'
> >        034   28    1C    FS  (file separator)        134   92    5C    \  '\\'
> >
> > As you can see, the document gives two forms of it:
> >
> >  \  '\\'
> 
> I guess, but it's only a guess, that's because \ is used for escaping
> chars.

The man page quoted above confirms your guess. Let me quote some relevant
lines (elisions marked with [...]):

man ASCII:

 | DESCRIPTION
 |   ASCII  is  the  American  Standard  Code for Information Interchange.
 |   It is a 7-bit code.  Many 8-bit codes (e.g., ISO 8859-1) contain ASCII
 |   as their lower half.  The international counterpart of
 |   ASCII is known as ISO 646-IRV.
 | 
 |   The following table contains the 128 ASCII characters.
 | 
 |   C program '\X' escapes are noted.
 | 
 |   Oct   Dec   Hex   Char                        Oct   Dec   Hex   Char
 |   ────────────────────────────────────────────────────────────────────────
 |   000   0     00    NUL '\0' (null character)   100   64    40    @
 |   001   1     01    SOH (start of heading)      101   65    41    A
 |   002   2     02    STX (start of text)         102   66    42    B
 | [...]
 |   032   26    1A    SUB (substitute)            132   90    5A    Z
 |   033   27    1B    ESC (escape)                133   91    5B    [
 |   034   28    1C    FS  (file separator)        134   92    5C    \  '\\'
 |   035   29    1D    GS  (group separator)       135   93    5D    ]

The relevant bit is the phrase above "C program '\X' escapes are
noted". Background is that, for C character and string literal
notation, an "escape convention" is used: whenever you want to
write a non-printable character, you use a so-called "escape
character" followed by something else. For example, for a newline
character, you say '\n', the escape being the '\'.

Now what do you do when trying to represent the escape character
itself? You just double it, like so: '\\'.

For lots of background, history, and the different conventions
in use, the Wikipedia page [1] is, as often, a great resource.

Cheers

[1] https://en.wikipedia.org/wiki/Escape_character

 - t

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