I've already removed and reinstalled grep on Ubuntu but still the same error persists.
Whatever grep command I type I always see the following output:
grep: invalid option -- '='
Usage: grep [OPTION]... PATTERN [FILE]...
Try grep --help for more information.`
results in this error, or
/bin/grep has the same byte size as it does on my other identical hosts, where it works fine, so it's likely to be some environmental variable or local setting.