Unbound variable not causing exit from subshell when set -eu

Question

$ cat test.sh
set -eu
echo "`wc -l < $DNE`"
echo should not get here

$ /bin/bash test.sh
test.sh: line 2: DNE: unbound variable

should not get here

I'm running bash version 4.1.2. Is there a way to make sure all such usage of unbound variables in subshells cause the script to exit without having to modify each call involving a subshell?

Answer 1

The better solution to ensure variable sanitisation

#!/usr/bin/env bash

set -eu

if [[ ${1-} ]]; then
  DNE=$1
else
  echo "ERROR: Please enter a valid filename" 1>&2
  exit 1
fi

By putting a hyphen in to the variable name inside curly braces like this allows bash to sanely handle the variable being undefined. I also highly recommend looking at the Google shell style guide, it's a great reference https://google.github.io/styleguide/shell.xml

[[ -z ${variable-} ]] \
  && echo "ERROR: Unset variable \${variable}" \
  && exit 1 \
  || echo "INFO: Using variable (${variable})"

Answer 2

Use a temporary variable so as to let test.sh process know about the failure of wc. You can change it to:

#!/bin/bash
set -eu
out=$(wc -l < $DNE)
echo $out
echo should not get here

Now, you won't see the should not get here if wc fails.