bin/post command not working when run from crontab

I'm working with a script where I want to send a command to delete all elements in an index; notably,

/opt/solr/bin/post -c <my collection> -d  "<delete><query>:</query></delete>"

When run interactively, this works fine.

However, when run automatically as a cron job, it gives this interesting output:

  Unrecognized argument:   "<delete><query>:</query></delete>"
 If this was intended to be a data file, it does not exist relative to /root

The culprit seems to be these lines, 143-148:

if [[ ! -t 0 ]]; then
     MODE="stdin"
else
   #when no stdin exists and -d specified, the rest of the arguments
   #are assumed to be strings to post as-is
   MODE="args"
 

This code seems to be doing the opposite of what the comment says - it sets MODE="stdin" if stdin is NOT a terminal, but if it IS (i.e., there IS an stdin) it assumes the rest of the args can be posted as-is.

On the other hand, if the condition is reversed, my command will fail interactively but not when run as a cron job. Both options are, of course, unsatisfactory.

It will actually work in both cases, if instead the command to delete the contents of the index is written as:

echo "<delete><query>:</query></delete>" | /opt/solr/bin/post -c departments -d

I've confirmed this bug in SOLR v. 7.7.1 and 7.5 - it is presumably present in more versions.

I've raised the issue on the solr-user mailing list, where I was asked to file a Jira report.