On Tue, Apr 05, 2011 at 10:07:46PM +0200, Nicolas de Pesloüan wrote:
> On the same kind of things, the construct if test "x$foo" = "x" is
> pointless. It is a very strange heritage from DOS, where IF x%A = x
> was a common construct, because DOS (command.com) lack quoting. (Is
> wasn't possible to write IF "%a" = ""). In shell, "" is an empty
> string, but a real argument to commands. The following construct is
> the good one : if test "$foo" = "". The test command will receive
> three arguments: the value of $foo, the = sign and an empty argument
> and will return true if $foo happens to be empty.
Actually, it's more relevantly a heritage from pre-POSIX shells, which
became habit for many GNU programmers via Autoconf. Here's what the
Autoconf documentation has to say on the subject:
`test' (strings)
Posix says that `test "STRING"' succeeds if STRING is not null,
but this usage is not portable to traditional platforms like
Solaris 10 `/bin/sh', which mishandle strings like `!' and `-n'.
Posix also says that `test ! "STRING"', `test -n "STRING"' and
`test -z "STRING"' work with any string, but many shells (such as
Solaris, AIX 3.2, UNICOS 10.0.0.6, Digital Unix 4, etc.) get
confused if STRING looks like an operator:
$ test -n =
test: argument expected
$ test ! -n
test: argument expected
Similarly, Posix says that both `test "STRING1" = "STRING2"' and
`test "STRING1" != "STRING2"' work for any pairs of strings, but
in practice this is not true for troublesome strings that look
like operators or parentheses, or that begin with `-'.
It is best to protect such strings with a leading `X', e.g., `test
"XSTRING" != X' rather than `test -n "STRING"' or `test !
"STRING"'.
Solaris 10 is not that old, and this has long been a problem for users
of /bin/sh on Solaris in particular. Note that GRUB works on Solaris.
I would be interested to know if OpenSolaris, or even Illumos, still
uses a pre-POSIX /bin/sh.
In most cases, of course, this doesn't matter since it only affects
certain strings. But on packages that run on Solaris I am always
extremely reticent to change existing code to assume POSIX shell, even
if I might write new code that way.
When I can assume POSIX shell, your version:
if test "$foo" = ""
is needlessly verbose. The standard stipulates that this works just
fine:
if test "$foo"
> Writing solid code imply - in particular - knowing the exact behaviors
> of the programming language :-)
On Unix-like systems, though, and particularly when it comes to shell,
this often involves knowing about historical behaviour as well as
current behaviour.
--
Colin Watson [cjwat...@ubuntu.com]
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