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