grep '*foo*'

grep 'foo'   # or more explicitly,  grep '.*foo.*'

In globs, * matches any number of any character.

In regex, * matches any number of the preceding character.

grep uses regex, not globs, so this means that grep '*foo' is nonsensical because there's no preceding character for *.

If the intention was to match "any number of characters followed by foo", use '.*foo'. Also note that since grep matches substrings, this will match "fishfood". Use anchors to prevent this, e.g. foo$.

This also means that f* will match "hello", because f* matches 0 (or more) "f"s and there are indeed 0 "f" characters in "hello". Again, use grep 'f' to find strings containing "f", or grep '^f' to find strings starting with "f".

If you're aware of the differences between globs and regex, you can ignore this message.