Shell script----[] and [[]] the difference and contact

a , [] single and double brackets

Basic elements:

[] Two symbols should be separated by a space around

There is a space between the internal operator and the action variable: such as ["a" = "B"]

String comparisons in,> < need to be written \> \< escaped

[] in the string or ${} variables as far as possible using "" "double quotation marks, to avoid the value does not define a reference and a good way to error

[] can use –a–o for logical operation

[] is a bash built-in command: [is a shell builtin

[] is actually shorthand for the test command in bash. That is, all [expr] equals test expr.

two , [[]] both brackets

Basic elements:

[[]] Two symbols should be separated by a space

There is a space between the internal operator and the action variable: [["A" = "B"]]

in string comparisons, you can use the > < directly without escaping

[[]] the string or ${} variable as far as possible if not used "" double quotation marks, pattern and meta-character matching are performed , for example: if * is not enclosed by double quotes, all characters can be matched

For example:

[[Email protected] lxjwork]$ [["AB" =a*]] && echo "OK"

Ok

[[ ]] internal can use && | | For logical Operation

[[]] is the bash keyword:[[is a shell keyword

[[]] other usages are the same as []

[[]] and [] both can and! Use together

Priority level

! > && > | |

Logical operators < relational operators

Logical operator:! && | | -a-o

Relational operators: < > \> \< = =!=–eq–ne-gt-ge–lt-le

Third, Comparison

The [[]] operator is simply an extension of the [] operator. [[]] can support the <,> symbol operation does not require an escape character, [] must be escaped:\> \<.

[[]] is the size of the string comparison.  Inside support logical operator:&& | | ,-a-o is no longer used.

For the test command, use-eq to make a numeric comparison, and when you pass in the string, you get an error.

[[expr]] is a real conditional judgment statement in Bash whose syntax is more in line with programming habits (such as &&, | | use), in [[]] deliberately passed the string to-eq will not error, bash implementation directly converts a non-integer string directly to 0 (you can self-verify, in [[]], any need for an integer, but the supply is another can not be converted to an integer string, has become 0). This should be the result of a bash implementation that does not check for content on either side of the-EQ operator in [[]]. For us, there is no need to drill these things, and we should make sure that we always provide integers when we need to use integers in our code.

Iv. [[]] advantages over []

①[[is a key word for the Bash programming language. is not a command, [[]] structure is more general than [] structure. There is no filename extension or word splitting between all characters in [[and]], but parameter extensions and command substitution occur.

② supports pattern matching of strings, and even supports the shell's regular expressions when using the =~ operator. String comparisons can be made to the right as a pattern, not just a string, such as [[Hello = = Hell]], the result is true. Matches a string or wildcard character in [[]] without the need for quotation marks.

③ using [[...]] The conditional judgment structure, rather than [...], can prevent many logic errors in the script. For example,,&&, | |, <, and > operators can normally exist in the [[]] conditional judgment structure, but if they appear in the [] structure, an error will be found.

④bash the expression in double brackets as a separate element and returns an exit status code.

use [[...]] The conditional judgment structure, rather than [...], can prevent many logic errors in the script. For example, &&, | |, <, and > operators can normally exist in [[]] conditional judgment structures, but if they appear in the [] structure, they will get an error.

Shell script----[] and [[]] the difference and contact