How to make a script fail when there is an error in here string?

Question

I have a script similar to this:

#!/bin/bash
set -euo pipefail
IFS=$'\n\t'
while read -r l; do
    echo "${l}"
done <<< "$(cat input.txt)"
echo Success

The command cat input.txt is just an example here to simplify my question.

I expected that if input.txt does not exist, the script would exit immediately thanks to set -euo pipefail. But it's not the case, instead the script ends successfully with the following output:

cat: input.txt: No such file or directory
Success

Is there a way to make the script fail as I expected?

Answer 1

^{I understand here cat input.txt is a place holder for some arbitrary command. For cat input.txt specifically, the solution would be easy: just use input redirection from input.txt instead.}

The exit code will be available in next expansions in the here string. For instance:

cat <<< "$(exit 3)$?"

Outputs 3. So you could do something like:

#! /bin/bash -
set -o nounset -o pipefail -o errexit
unset -v ret
{
  [ "$ret" -eq 0 ] || exit "$ret"
  while IFS= read -r line || [ -n "$line" ]; do
    printf '%s\n' "$line" || exit
  done
} <<< "$(cat input.txt)"${ret[1+(ret=$?)]}"
echo success

Which appears to work with bash 5.0 at least.

Or you could use a temporary variable:

#! /bin/bash -
set -o nounset -o pipefail -o errexit
output_minus_trailing_newlines=$(cat input.txt)
while IFS= read -r line || [ -n "$line" ]; do
  printf '%s\n' "$line" || exit
done <<< "$output_minus_trailing_newlines"
echo success

In either case, if the command fails, none of its output will be processed.

You could also use a pipe and the lastpipe option if you don't want the loop to run in a subshell.

#! /bin/bash -
set -o nounset -o pipefail -o errexit
shopt -s lastpipe
cat input.txt |
  while IFS= read -r line || [ -n "$line" ]; do
    printf '%s\n' "$line" || exit
  done
echo success

Then the output will be processed as it comes. It's only after the pipeline exits that the shell will exit if the command failed thanks to the combination of errexit (-e) and pipefail. In that case, trailing empty lines are preserved.

Same using process substitution:

#! /bin/bash -
set -o nounset -o pipefail -o errexit
{
  pid="$!"
  while IFS= read -r line || [ -n "$line" ]; do
    printf '%s\n' "$line" || exit
  done
  wait "$pid"
} < <(cmd input.txt)
echo success

Or you could use a tempfile (here docs and here strings used to be implemented with temp files in bash).

#! /bin/bash -
set -o nounset -o pipefail -o errexit
tmp=$(mktemp)
{
  rm -f -- "$tmp"
  cat input.txt >&3
  exec 3>&-
while IFS= read -r line || [ -n "$line" ]; do
    printf '%s\n' "$line" || exit
  done
  wait "$pid"
} 3> "$tmp" < "$tmp"
echo success

This way, the output is not processed if the command failed and you don't store the output (several times) in memory and the trailing empty lines are preserved.

In any case, see Why is using a shell loop to process text considered bad practice?

Answer 2

Like this:

#!/bin/bash
set -euo pipefail
IFS=$'\n\t'
[[ -s input.txt ]]
while read -r l; do
    echo "${l}"
done <<< "$(cat input.txt)"
echo Success