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  • 27
    The empty string (and many other) problem is solved by using quotes. The "x" is to cover another kind of problems: where operands may be taken as operators. Like when $foo is ! or ( or -n... That problem is not meant to be a problem with POSIX shells where the number of arguments (beside [ and ]) is not greater than four. Commented Jan 25, 2013 at 20:36
  • 38
    To clarify, [ x$foo = x$blah ] is just as plain wrong as [ $foo = $bar ]. [ "$foo" = "$bar" ] is correct in any POSIX compliant shell, [ "x$foo" = "x$bar" ] would work in any Bourne-like shell, but the x is not for cases where you have empty strings but where $foo may be ! or -n... Commented Mar 7, 2013 at 16:29
  • 11
    Interesting semantics in this answer. I'm not sure what you mean by it's *not* syntax. Of course, [ isn't precisely an alias for test. If it were, then test would accept a closing square bracket. test -n foo ]. But test does not, and [ requires one. [ is identical to test in all other respects, but that's not how alias works. Bash describes [ and test as shell builtins, but [[ as a shell keyword. Commented Oct 9, 2017 at 13:02
  • 10
    Well, in bash [ is a shell builtin, but /usr/bin/[ is also an executable. It is traditionally a link to /usr/bin/test, but in modern gnu coreutils is a separate binary. The traditional version of test examines argv[0] to see if it is invoked as [ and then looks for the matching ]. Commented Jan 9, 2019 at 20:03
  • 1
    My bash does not work with >= and <= Commented Jul 22, 2019 at 13:15