Re: Why echo without quotes need to glob the file

[Greg Wooledge]

On Wed, Dec 14, 2022 at 10:51:09AM +0800, wang yuhang wrote:
> on Tue, 13 Dec 2022 09:15:53 -0500 Greg Wooledge via wrote:
> > [--hello] is an example of a "glob", which is a pattern that can match
> > one or more filenames.  Other examples of globs are *.txt or foobar.[ch]
> > Since your glob is not quoted, the shell expands it to a list of 
> filenames
> > that it matches in the current directory.
> > If you want [--hello] NOT to be expanded, put quotes around it.  Either
> > single quotes or double quotes will do.
> 
> I konw the right thing to do is put quotes around it, I just don't 
> understand that bash regards [-- hello] as a glob to match filenames in `echo`

Any argument that contains "*", or "?", or both of the "[" and "]"
characters in that order, will be treated as a glob, if those characters
are not quoted.

In a command like this:

  echo Do you want food?

The argument "food?" is a glob, because it contains an unquoted "?"
character.  It will be matched against files in the current directory,
and could be replaced by a list of filenames.

The same goes for "[--hello]" which contains the "[" and "]" characters.

The same goes for commands like this one:

  unset array[1]

The argument "array[1]" is treated as a glob.  IF there happens to be
a file named "array1" in the current directory, then the glob is
replaced, and you end up running this instead:

  unset array1

This has a different meaning, and doesn't do what you want.

To get the correct results, any string that *could* be seen as a glob
needs to be quoted.  Or at the very least, the special glob characters
need to be quoted.

  echo Do you want food\?     # This one is safe.  But weird.

  echo Do you want "food?"    # Also weird.

  echo "Do you want food?"    # Much more normal.

  unset "array[1]"            # Annoying?  Yes, very.  But necessary.

It's worth developing the habit of always putting quotes around messages
that you pass to echo for printing.  There's virtually never a time you
want any part of a message to undergo word splitting or globbing.