Differences in Unix shell (), [] and [[]]

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The difference between the Unix shell (), [] and [[]]   1. First of all, although very similar, but conceptually, they are different levels of things. "[[" is a keyword, and many shells (such as ash BSH) do not support this approach. Ksh, Bash (reportedly introduced support for [[support] from 2.02 onwards. "[" is a command that is equivalent to test and is supported by most shells. In most modern SH implementations, "[" and "Test" are internal (builtin) commands, in other words, "test"/"[" is not called when/some/path/to/test such an external command (if there is such a command).   2. [[]] The structure is more generic than the bash version of []. All characters between [[and]] are not separated by a file extension or a tag, but there are parameter references and command substitutions.   with [[...]] Test structure than with [...] To prevent many logic errors in the script. For example, &&,| |,< and > Operators can be passed in a [[]] test, but errors will occur in the [] structure.   3. (()) structure expands and computes the value of an arithmetic expression. If the expression value is 0, it returns 1 or false as the exit status code. An expression of a value other than 0 returns a 0 or True as an exit status code. This structure is just the opposite of the previous Test command and [] structure.   4. [ ... ] As a shell command, so the expression in it should be its command-line argument, so the string comparison operator ">" and "<" must be escaped, otherwise it becomes an IO redirect operator (see the example above in 2). in [[<] and > do not need to escape, because "[" is the keyword, does not do the command line extension, so the relative syntax is slightly stricter. For example in [...] You can enclose the operators in quotation marks, because they are removed when you do a command-line extension, and in [[...] It is not allowed to do so.   5. [[ ... ]] Perform arithmetic expansion, while [...] Do not do   1) The operation of the test number in Ksh can use Let, (()), where the operation does not require the variable $ symbol, the operator is + 、-、 *,/,%, is not recommended to use the comparison of expr numbers (()), its operators >, >=, <, < =, = =,! = can use arithmetic extensions, such as: ((99+1 <= 101)) A comparison of character expressions using [[]], whose operator =,! =,-n,-Z file expressions are tested using [[]] , its operator-R,-L,-W,-X,-F,-D,-S,-nt,-ot logical expressions are tested using [[]], its operators!, &&, | | Numeric comparisons, character comparisons, and logic tests can be combined, such as $ [["a"! = "B" && 4-GT 3]] to support the wildcard extension in bash, such as: [[Hest = h?? T]], [hest = H*t]], when using (()), no spaces are required to separate the values and operators, and [[]] are used to separate the values and operators with a space.   2) The [[]] symbol can also be used in bash in redhat9, unlike Ksh [[]]. But it is recommended to use it strictly in accordance with the above principles. In bash, the comparison of numbers is best used (()), although [[]] is used, but if the operator >, >=, <, <=, = =,! = are used inside, the result is often wrong, but if you use the operator in [] in [[]]-eq,-ne,- Le,-lt,-GT,-ge "and so on, have not yet found fault. So a combination such as $ [["a"! = "B" && 4 > 3]] (see above) is also not available in bash, with a high error rate. Example: [["a"! = "B" && > 2]] "judgment result is not normal." such as [2 \< 10], [[2 < 10]] are not used. It is best to use arithmetic extensions ((99+1 = = 100)) instead of [[99+1-eq 100]].  

Differences in the Unix shell (), [], and [[]]