Differences between (), [] and [] in unixshell

In unixshell, (), [] and [[] are different. 1. although they are very similar, they are conceptually different levels. & Quot; [[& quot;, is a keyword. many shells (such as ashbsh) do not support this method. Ksh, bash (it is said that [[support) is introduced from 2.02. & Quot; [& quot; yes... in unix shell, (), [] and [[] are different. 1. first of all, although very similar, the two are conceptually different levels of things. "[[", Is a keyword. many shells (such as ash bsh) do not support this method. Ksh, bash (it is said that [[support) is introduced from 2.02. "[" Is a command, equivalent to test, which is supported by most shells. In most modern sh implementations, "[" and "test" are internal (builtin) commands, in other words, when "test"/"[" is executed, external commands such as/some/path/to/test are not called (if such commands exist ). 2. the [[] structure is more common than the [] structure of the Bash version. All characters between [and] will not be extended or marked or separated by files, but there will be parameter reference and command replacement. Using [[...] to test the structure can prevent many logical errors in the script better than using. For example, &, |, <和> The operator can pass a [[] test, but an error occurs in the [] structure. 3. () structure extension and calculation of the value of an arithmetic expression. If the expression value is 0, 1 or false is returned as the exit status code. A non-zero expression returns a 0 or a real exit code. This structure is the opposite of the previous test command and [] structure. 4. [...] is a shell command, so the expression in it should be its command line parameter, so the string comparison operators ">" and "<" must be escaped, otherwise, it becomes the IO conversion operator (see the example in 2 above ). In [[, "<" and ">" do not need to be escaped. because "[" is a keyword and does not support command line extension, the relative syntax is slightly stricter. For example, in [...] you can use quotation marks to enclose operators, because these quotation marks are removed when you perform command line extension, and in [[...] this is not allowed. 5. [[...] while [...] 1) you can use let, () for the test number operation in ksh. the variable $ is not required for the operation. the operators are +,-, *,/, and %, we do not recommend that you use () to compare expr numbers. the operators >,>=, <=, =, and ,! = Arithmetic extensions can be used, for example: (99 + 1 <= 101) comparison of character expressions uses [[], and its operators are = ,! =,-N,-z file expression test using [[], the test uses operators-r,-l,-w,-x,-f,-d,-s,-nt, and-ot logical expressions [[], its operator!, &, | Numeric comparison, character comparison, and logical testing can be combined, for example, $ [[""! = "B" & 4-gt 3] supports wildcard extensions in bash, such as [[hest = h ?? T], [hest = h * t] when using (), no spaces are required to separate values and operators, when [[] is used, separate values and operators with spaces. 2) the [[] symbol can also be used in bash of redhat9 different from [[] in ksh. However, we recommend that you strictly follow the above principles. In bash, it is best to use () for numbers. although [[] can be used, however, if you use operators>, >=, <, <=, =, and ,! =, The result is often incorrect, however, if you use the [] operator "-eq,-ne,-le,-lt,-gt, and-ge" in [[], no error has been found. Therefore, such as $ [""! = "B" & 4> 3] such combinations (see above) cannot be used in bash. the error rate is very high. Example: [[""! = "B" & 10> 2] The result is not normal. For example, [2 \ <10] and [2 <10] are not used. It is best to use arithmetic extensions (99 + 1 = 100) instead of [[99 + 1-eq 100].