The use of parentheses inside the shell

Middle Bracket

Brackets are divided into single brackets [], and double brackets [[]], and brackets are commonly used in expression judgments, and parentheses can also be used in expressions, as mentioned later.

Single brackets []

a=5b=3[$A-eq $B] determine if A is equal to b[${a}-eq ${b}] ["${a}"-eq "${b}" function as above

a=abc[$A = = "abc"] or write [${a} = = "abc"] or write ["${a}" = = "abc"] [$A \> "ABC"]

[-D./aaa.sh-a-E./bbb] Tests whether the aaa.sh is a directory and if BBB exists, returns true if all matches [-D./aaa.sh-o-E./bbb] Tests if aaa.sh is a directory and BBB exists, and returns if a condition is met Really

We can see from the above that there are some rules for using single brackets:

• There should be a space between the variable and the bracket

• There must be a space between the variable and the operator.

• Variables can be referenced directly with $, or can be referenced using ${} and "${}", although it is possible to use the ${} directly, but it is recommended to refer to the variable using "${}" or directly with $

• strings are enclosed in double quotation marks.

• String comparisons generally use = = to do the same test as strings, use < or > to compare sizes, and to compare characters greater than and less than in single brackets by transferring \> and \<. The comparison principle is comparison of ASCII.

• Logical operators such as-A and-O can also be used in single brackets

double brackets [[]]:

a=5b=3[[$A-eq $B]] "Determine if A is equal to b[[${a}-eq ${b}]] [[[" ${a} "-eq" ${b} "] as in the same role

a=abc[[$A = = "abc"] or write [[${a} = = "ABC]] or write [[" ${a} "= =" abc "]] or write [[$A = = ABC]][[$A > ABC]]

[[-D./aaa.sh &&-E./bbb]] Tests whether the aaa.sh is a directory and if BBB exists, returns true if all matches [[-D./aaa.sh | |-E./BBB]] Testing whether aaa.sh is a directory and BBB exists, and a condition is met to return true

a=abc[[$A =~ "a"]] to do pattern matching, this expression is true or written as [[${a} =~ "a"] or written as [["$[a]" =~ "A"]][[$A = A *]] expression is true [[$A = b*]] Expression is False

• The use of double brackets is similar to the rules for using single brackets

• The greater than less comparison of the middle string between the two brackets does not need to be escaped, and the string can be referenced without double quotes

• Double brackets inside can use &&, | | Do logical operators

• A fuzzy match can also be done inside the double brackets, using the =~ expression 2 can be a regular expression

[Expression 1-a expression 2] is equivalent to [[Expression 1 && expression 2]] equivalent to [expression 1] && [expression 2] equivalent to [[Expression 1]] && [[Expression 2]]

[Expression 1-o-expression 2] is equivalent to [[Expression 1 | | expression 2]] equivalent to [expression 1] | | [Expression 2] is equivalent to [[expression 1]] | | [[Expression 2]]

Parentheses

Single parenthesis

Single parentheses are used to represent the instruction group, which executes the command in parentheses sequentially through the child shell.

(ls./;echo "Hello"; mkdir-p/tmp/ccc)

The most common usage this is used in command substitution, command substitution can also use anti-quote ' '

echo "Current directory is: $ (PWD)" echo "current directory is: ' pwd '

Double parenthesis

It is commonly used in arithmetic operations and in the For loop.

a=$ ((5+3)) b=$ (($A +4))

The form of the For statement

1 to 10 loop execution

Notation 1:for i in (1..10);d odone notation 2:for i in SEQ 1 10;dodone notation 3:for ((i=1; i<=10; i++)); Dodone

The form of a while statement

1 to 10 cycles

I=1while [$I-le];d o let I++done

This article is from the "Little Demon's Home" blog, so be sure to keep this source http://littledevil.blog.51cto.com/9445436/1875711

The use of parentheses inside the shell