Trailing newlines and the bash 'read' builtin

问题

In bash, this works:

echo -n $'a\nb\nc\n' | while read x; do echo = $x =; done

The while loops through three times

= a =
= b =
= c =

But imagine a text file that doesn't have the conventional trailing newline. I think that read should still work for all three lines, but it doesn't. I just get:

echo -n $'a\nb\nc' | while read x; do echo = $x =; done

= a =
= b =

The help read in bash doesn't really clarify.

Note: I don't need this resolved, and I can see some ways to fix it myself. I am curious, and I am tempted to file a bug report - I generally try myself to respect files that mightn't have the trailing new line. I came across this when using the -d option to read. read -d " " will split on spaces instead of newlines, but it will miss out on the last entry unless it has a trailing space.

(Ubuntu. GNU bash, version 4.1.5(1)-release)

回答1:

$ man bash
   read [-ers] [-a aname] [-d delim] [-i text] [-n nchars] [-N nchars] [-p prompt] [-t timeout] [-u fd] [name ...]
          One line is read from the standard input, ...

I think the key is: How to define "One line".
Does text without a '\n' at the end makes One line?
I guess read don't think so.

回答2:

If you want the above loop to process the incomplete line, do this:

echo -n $'a\nb\nc' | while read x || [[ $x ]]; do echo = $x =; done

which gives:

= a =
= b =
= c =

When read encounters the incomplete line, it does read that into the variable (x in this case) but returns a non-zero exit code which would end the loop, and || [[ $x ]] takes care of running the loop for the incomplete line as well. When read is called the next time, there is nothing to read and it exits with 1, setting x to an empty string as well, which ensures that we end the loop.