mjhjmtu

Per the POSIX Shell Command Language Page:

The <backslash> shall retain its special meaning as an escape character (see Escape Character (Backslash)) only when followed by one of the following characters when considered special:

$ ` " <newline>

This would seem to imply that escaping these five characters with a backslash would not have the effect of escaping them and having them be treated literally if they are not "special".

Am I interpreting this correctly, and if so, are there cases where escaping one of these five special characters with a would not have the intended effect of escaping it?

@MichaelHomer explains it very well. Let's try a few practical cases with PS1='$ ':

$ echo "$ at start"
$ at start
$ echo "at end $"
at end $
$ echo "$before"

$ echo "after$"
after$

So $ is only "special" before a word, making it a parameter substitution. What happens if we put a backslash before all of them?

$ echo "$ at start"
$ at start
$ echo "at end $"
at end $
$ echo "$before"
$before
$ echo "after$"
after$

Only the "special" line changes - the dollar sign is now considered literal. It was always considered literal. What happens with other characters?

$ echo " at start"
 at start
$ echo "at end "
> ^C
$ echo "before"
before
$ echo "after"
> ^C

So backslash is just another literal character before a non-special character. (^C is where I had to cancel the command line because the quote character had been escaped.)

Lets try to simplify.

The text, as written is:

The <backslash> shall retain its special meaning as an escape character (see Escape Character (Backslash)) only when followed by one of the following characters when considered special:

We can convert some parts of it:

1. The <backslash> ==> The B



2. 
   

   ... shall retain its special meaning as an escape character
   
   ... __________/------------------------__________________/
   
   ==> still is -------------------------------- an escape character
   
   ==> still is an escape

   
   
   

   As it was in un-quoted strings.

   
   



3. ... only




4. ... when followed by one of the following characters
   ==> A B before [$`"NL]




5. ... when considered special

Then, 1,2,and 4 read as: B still is an escape before [$`"NL]

• 
  

  The controversial part is: when considered special

  
  
  

  What is being considered special ?

  
  
  

  I believe that it is >> "the characters" << . As written:

  
  
  

  
  

  one of the following characters when considered special:

  
  

  
  
  

  Q: For example: when is a " special (inside double quotes)?

  
  
  

  A: Always (not considering nested structures like echo "$(sed 's/["]//' file)").

  
  
  

  So, a backslash will always quote a ": echo "a"b"b"" (again: no nested structures).

  
  


• 
  

  Then, 1,2,3,4 and 5 should read as:

  
  
  

  B still is an escape only before special [$`"NL]

  
  

Then, the answer to your question:

are there cases where fails to escape $, ```, ", or <newline>?

Is: YES, when the characters [$`"NL] are not special.

They (the characters) may be not special inside nested structures (for example) yet inside quotes.