You need to search for a string (e.g., “error”) in a log file, and you want to do it caseinsensitively to catch all occurrences.
Use the -i option on grep to ignore case:
$ grep -i error logfile.msgs
A case-insensitive search finds messages written “ERROR”, “error”, “Error,” as well as ones like “ErrOR” and “eRrOr.”
This option is particularly useful for finding words anywhere that you might have mixed-case text, including words that might be capitalized at the beginning of a sentence or email addresses.