I've run into some strange behaviour today:
#!/bin/bash
set -ex
export a=$(cat foo)
echo "It's a living thing"
b=$(cat foo)
echo "This won't make it"
This won't stop on cat foo even though foo does not exist. But without the export, it fails. Why?
In:
a=$(cmd)
That's a simple command without command part, so the exit status is that of the command substitution instead. In:
a=$(cmd) export a
or
export a="$(cmd)"
There is a command part here (even if in some shells, export is midway between a command and a keyword), so its exit status is returned. If cmd fails and produces no output export a= will still be run and succeed, so export will return a 0 exit status.
Here you'd want:
a=$(cmd)
export a
Or even better, get rid of that unreliable set -e and do proper error handling by hand:
a=$(cmd) || exit
export a
See e.g. BashFAQ 015: "Why doesn't set -e do what I expected?" for further examples of why and how set -e can produce unexpected results.
