test is surprisingly smart

November 20, 2013

Via Hacker News I would up reading Common shell script mistakes. When I read this, I initially thought that it contained well-intentioned but mistaken advice about test (aka '[ ... ]'). Then I actually checked what test's behavior is and got a bunch of surprised. It turns out that test is really quite smart, sometimes disturbingly so.

Here's two different versions of a test expression:

[ x"$var" = x"find" ] && echo yes
[ "$var" = "find" ] && echo yes

In theory, the reason the first version has an 'x' in front of both sides is to deal with the case where someone sets $var to something that is a valid test operator, like '-a' or '-x' or even '('; after all, '[ -a = find ]' doesn't look like a valid test expression. But if you actually check, it turns out that the second version works perfectly well too.

What's going on is that test is much smarter than you might think. Rather than simply processing its arguments left to right, it uses a much more complicated process of actually parsing its command line. When I started writing this entry I thought it was just modern versions that behaved this way, but in fact the behavior is much older than that; it goes all the way back to the V7 version of test, which actually implements a little recursive descent parser (in quite readable code). This behavior is even specified in the Single Unix Specification page for test where you can read the gory details for yourself (well, most of them).

(The exception is that the SuS version of test doesn't include -a for and or -o for or. This is an interesting exclusion since it turns out they were actually in the V7 version of test per eg the manpage.)

Note that this cleverness can break down in extreme situations. For example, '[ "$var1" -a "$var2" -a "$var3" ]' is potentially dangerous; consider what happens if $var2 is '-r'. And of course you still really want to use "..." to force things to be explicit empty arguments, because an outright missing argument can easily completely change the meaning of a test expression. Consider what happens to '[ -r $var ]' if $var is empty.

(It reduces to '[ -r ]', which is true because -r is not the empty string. You probably intended it to be false because a zero-length file name is considered unreadable.)

Written on 20 November 2013.
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Last modified: Wed Nov 20 23:05:59 2013
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