How to check if the matched EMPTY line is the LAST line of a file in a while IFS read - bash

Question

I have a while IFS read loop to check for different matches in the lines.
I check for and empty/blank line like this:
while IFS= read -r line; do
[[ -z $line ]] && printf some stuff
I also want to check if the matched empty/blank is also the last line of the file. I am going to run this script on a lot of files, they all:
-end with an empty line
-they are all a DIFFERENT LENGTH so I cannot assume anything
-they have other empty lines but not necessarily at the very end (this is why I have to differentiate)
Thanks in advance.

Answer 1

As chepner has noted, in a shell line-reading loop the only way to know whether a given line is the last one is to try to read the next one.
You can emulate "peeking" at the next line using the code below, which allows you to detect the desired condition while still processing the lines uniformly.
This solution may not be for everyone, because the logic is nontrivial and therefore requires quite a bit of extra, non-obvious code, and processing is slowed down as well.
Note that the code assumes that the last line has a trailing \n (as all well-formed multiline text input should have).
#!/usr/bin/env bash
eof=0 peekedChar= hadEmptyLine=0 lastLine=0
while IFS= read -r line || { eof=1; (( hadEmptyLine )); }; do
# Construct the 1-2 element array of lines to process in this iteration:
# - an empty line detected in the previous iteration by peeking, if applicable
(( hadEmptyLine )) && aLines=( '' ) || aLines=()
# - the line read in this iteration, with the peeked char. prepended
if (( eof )); then
# No further line could be read in this iteration; we're here only because
# $hadEmptyLine was set, which implies that the empty line we're processing
# is by definition the last one.
lastLine=1 hadEmptyLine=0
else
# Add the just-read line, with the peeked char. prepended.
aLines+=( "${peekedChar}${line}" )
# "Peek" at the next line by reading 1 character, which
# we'll have to prepend to the *next* iteration's line, if applicable.
# Being unable to read tells us that this is the last line.
IFS= read -n 1 peekedChar || lastLine=1
# If the next line happens to be empty, our "peeking" has fully consumed it,
# so we must tell the next iteration to insert processing of this empty line.
hadEmptyLine=$(( ! lastLine && ${#peekedChar} == 0 ))
fi
# Process the 1-2 lines.
ndx=0 maxNdx=$(( ${#aLines[#]} - 1 ))
for line in "${aLines[#]}"; do
if [[ -z $line ]]; then # an empty line
# Determine if this empty line is the last one overall.
thisEmptyLineIsLastLine=$(( lastLine && ndx == maxNdx ))
echo "empty line; last? $thisEmptyLineIsLastLine"
else
echo "nonempty line: [$line]"
fi
((++ndx))
done
done < file