Famously, ' [ ' is a program, not a piece of shell syntax, and it's also known as ' test ' ( which was the original name for it ). On many systems, this was and is implemented by ' [ ' being a hardlink to ' test ' (generally 'test' was the primary name for various reasons). However, today I found out that GNU Coreutils is an exception . Although the two names are built from the same source code ( src/test.c ), they're different binaries and the ' [ ' binary is larger than the ' test ' binary. What is ultimately going on here is a piece of ' test ' behavior that I had forgotten about, that of the meaning of running ' test ' with a single argument.

The POSIX specification for test is straightforward. A single argument is taken as a string, and the behavior is the same as for -n, although POSIX phrases it differently:

string

True if the string string is not the null string; otherwise, false.

The problem for GNU Coreutils is that GNU programs like to support options like --help and --version. Support for these is specifically disallowed for ' test ', where ' test --help ' and ' test --version ' must both be silently true. However, this is not disallowed by POSIX for ' [ ' if ' [ ' is invoked without the closing ' ] ':

$ [ --version
[ (GNU coreutils) 9.1
[...]
$ [ foo
[: missing ‘]’
$ [ --version ] && echo true
true

As we can see here, invoking 'test' as ' [ ' without the closing ' ] ' as an argument is an error, and GNU Coreutils is thus allowed to interpret the results of your error however it likes, including making ' [ --version ' and so on work.

(There's a comment about it in test.c .)

The binary size difference is presumably because the ' test ' binary omits the version and help text, along with the code to display it. But if you look at the relevant Coreutils test.c code , the relevant code isn't disabled with an #ifdef. Instead, LBRACKET is #defined to 0 when compiling the ' test ' binary. So it seems that modern C compilers are doing dead code elimination on the ' if (LBRACKET) { ...} ' section, which is a well established optimization, and then going on to notice that the called functions like ' usage() ' are never invoked and dropping them from the binary. Possibly this is set with some special link time magic flags.

PS: This handling of a single argument for test goes all the way back to V7, where test was actually pretty smart . If I'm reading the V7 test(1) manual page correctly, this behavior was also documented.

PPS: In theory GNU Coreutils is portable and you might find it on any Unix. In practice I believe it's only really used on Linux.