Re: manpage note? weird strings that appear to be equal but create haywire comparisons?

[Greg Wooledge]

On Wed, Aug 26, 2009 at 02:45:39AM -0700, Linda Walsh wrote:
> I was scripting and fixing some permissions in Win,
> and had a var 'c' set to (no quotes in the var):   'C\windows\system32'

How did you assign this value?  Did you read it from a file?  Did you
type a specific bash command?

>  # echo  $v |hexdump -C
>  00000000  43 3a 5c 77 69 6e 64 6f  77 73 5c 73 79 73 74 65  

This isn't trustworthy, because you did not quote $v.  The variable
might have leading or trailing whitespace, which the unquoted parameter
expansion would remove before echo gets it.

There's also glob expansion to worry about.  Your variable might contain
C:\windows\sy?tem3* for all we know.

> # printf -v v "%q" $c

Same problem here -- unquoted $c means you're not necessarily getting
the variable's exact contents.

> # echo  $v |hexdump -C

And here.

> TestProg: (interactive shell...)
> 
> {
> c='C:\Windows\System32'
> printf -v v "%q" "$c"

Finally, quotes!

Here's what your v variable should contain after that:

$ c='C:\Windows\System32'; printf -v v %q "$c"; echo "$v"
C:\\Windows\\System32

But your test script says:

> export c v 
> echo  for c=$c, and v=$v

And your alleged output says:

> for c=C:\Windows\System32, and v=C:\Windows\System32

Even with the unquoted $v here, you should have seen the double backslashes:

$ echo $v
C:\\Windows\\System32

So your output doesn't match your script.  Something strange is afoot.

> Note that [[ and [ return different results when the vars are unquoted.

Since we don't really know WHAT your variables contain, any analysis
of that output is a waste of time, although Pierre gave some decent
pointers about the general difference between [ and [[.


On Wed, Aug 26, 2009 at 01:17:07PM +0300, Pierre Gaston wrote:
> Now I'm not too sure why var='"*'"; [[ \* = $var ]] is false while
> var='\*' [[ \* = $var ]] is true

Assuming the first part was supposed to be var='"*"' ...

The bash command [[ \* = $var ]] returns true if $var contains a glob
pattern against which a literal asterisk * can be matched.  (By the way,
you don't need the \ there.  No glob expansion is done inside [[...]] so
you could use a plain * on the left hand side.)

If $var contains \* then the match is successful.  \* as a glob describes
a literal asterisk, which is what we're trying to match.

$ var='\*'; [[ * = $var ]]; echo $?
0

If $var contains "*" (double quote, asterisk, double quote), then
the double quotes are considered part of the glob pattern.  A bare
asterisk won't match that glob, because it doesn't have double quotes
attached to it.

imadev:~$ var='"*"'; [[ * = $var ]]; echo $?
1

It only matches double quote, asterisk, double quote:

$ var='"*"'; [[ \"*\" = $var ]]; echo $?
0