GNU tools often output error and warning messages with parts of the line (e.g. filenames, syntax errors, invalid dates, etc) quoted with non-matching quotes: ` and '
e.g. From the GNU coreutils FAQ:
$ date -d "2006-04-02 02:30:00" date: invalid date `2006-04-02 02:30:00' and
rm: cannot remove `backup.tar': Value too large for defined data Why do they do this? Is there any benefit to it? What's the history behind it?
With old X fonts, `..' looked symmetrical like ‛..’
Also given `..' latex generates correct left and right single quotes like ‘..’
So `..' was a hack and in 2012 coreutils changed to shell like quoting '..'
Now it wasn't full shell syntax however and would generate corrupted output if there were embedded "\r" chars etc. present. Also inconsistently in some cases no quoting was used, and sometimes unicode ‘..’ quoting was used (which uses quotes as defined for the locale and caters for escaping these control characters).
A very recent (2015) patch set was introduced to coreutils to make this more consistent, by quoting all file name output in shell compat format, which also escapes control characters appropriately, and allows for easier copy and pasting back to other commands. Other items in diagnostics use "full unicode" quoting as mentioned above.
date(date (GNU coreutils) 8.23) outputsdate: invalid date ‘2006-02-31 02:30:00’which isdate: invalid date \342\200\2302006-02-31 02:30:00\342\200\231$, unicode left and right single quotation marks. the faq entry above uses a backtick and an apostrophe.