I find it confusing that I can read once from an empty string, but not from an empty file:
while read -r foo; do echo ${#foo}; done <<< '' # prints '0'
while read -r foo; do echo ${#foo}; done < /dev/null # prints nothing
Is there a way to write a string literal that cannot be read from even once? Conversely, what contents does a file need to have, in order to be able to read exactly one empty string from it, and nothing else?
Refer to the bash manual description of here strings:
3.6.7 Here Strings
A variant of here documents, the format is:
[n]<<< wordThe word undergoes tilde expansion, parameter and variable expansion, command substitution, arithmetic expansion, and quote removal. Filename expansion and word splitting are not performed. The result is supplied as a single string, with a newline appended, to the command on its standard input (or file descriptor n if n is specified).
Note "with a newline appended". Even if your string starts out empty, it ends up containing at least a newline when used in this fashion. A string or file containing just a newline will be interpreted as one blank line by the while read loop.
