12-30-2008
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Originally Posted by
methyl
I agree with fpmurphy. [[ ]] is supported in Posix and required though I personally have only found it useful in the context of "while [[ condition ]]".
Please reread the section of the POSIX specification that was posted. It says that a POSIX shell may implement [[, but it is not required to. Therefore there is no guarantee that a POSIX shell will support it, and indeed there are POSIX-compliant shells that do not support it.
For those shells that do support it, POSIX says absolutely nothing about what it should do. That's what "unspecified results" means. It could legitimately do anything at all, including wipe your hard drive.
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[[ and ]] are keywords with strict syntax rules.
Their rules are specified only by the shells that implement them.
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A keyword starts a command and should not be used in the wrong context or be quoted (or you can get unpredictable results).
[ and ] are commands. (On early unix they were executables).
[ is a command that is builtin to all modern shells, and on all POSIX systems, it also an executable. It is synonymous with test, which is also both a builtin and an executable.
] is not a command.
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More importantly.
What goes between [[ and ]] is a "conditional expression" with one set of syntax.
And that syntax is specified only by the shells that implement it; it is not defined in the POSIX specs.
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What goes between [ and ] is a simple "test" with a simpler syntax. See "man test" where it you'll see that [ ] is an equivalent to "test".
Better than [i]man test[/t], which will tell you what the executable supports, your shell documentation will tell you what the builtin command supports: on bash, help test.
The two syntaxes are covered in the man page for a Posix shell.
Yes, they are covered in the shell's documenation only; in the POSIX specs the behaviour of [[ ]] is unspecified.
For a POSIX shell that does not support [[ ]], see /bin/sh on *BSD systems or ash or dash on Linux.
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