Re: Is this a bug in [[ -f ]]?

[Eric Blake]

On 08/19/2011 08:45 AM, Suvayu Ali wrote:
> I am trying to test if a file exists and then source it. My problem is
> the test succeeds even if the variable is empty! If I pass no argument
> at all, it still succeeds. To give you an example:
>
> $ unset bla
> $ [ -f $bla ]&&  echo yes
> yes
> $ [ -f  ]&&  echo yes
> yes

Both expected behaviors, and evidence of your lack of quoting.

Remember, the behavior of [] depends on how many arguments are present.

[ -f "$bla" ] (note the "") - guarantees that there are exactly two 
arguments, so it proceeds with the two-argument test where -f is the 
operator and "$bla" is the file name.

[ -f ] (which is the same as [ -f $bla ] if $bla is empty, note the lack 
of "") - there is exactly one argument, so it proceeds with the 
one-argument test of whether the argument (the literal string -f) is 
empty (it is not).

Furthermore, [ -f $bla ] is different than [[ -f $bla ]].  [ is a POSIX 
utility, and mandated to do all argument word expansion before [ ever 
gets a chance to see what arguments it was given - if $bla is empty or 
has spaces, you changed the number of arguments that are given to [.  [[ 
is a bash (and ksh) extension that is part of the shell syntax (similar 
to how () for subshells is part of the syntax), thus it knows how many 
words, _pre-expansion_, were present, and the fact that $bla was 
unquoted is not a problem, [[ -f $bla ]] is a safe way to check if $bla 
is a file even if $bla is empty or contains spaces.

--
Eric Blake   eblake@redhat.com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org