How to use parentheses in Shell

Brackets

Brackets are divided into single brackets [] and double brackets [[]]. Square brackets are generally used in expression judgment, and parentheses can also be used in expressions, which will be mentioned later.

Single square brackets []

A=5
B=3
[ $A -eq $B ]    判断A是否等于B
[ ${A} -eq ${B} ]  作用同上
[ "${A}" -eq "${B}" ]  作用同上

     

A=ABC
[ $A == "ABC" ]   或写成  [ ${A} == "ABC" ]   或写成  [ "${A}" == "ABC" ]
[ $A \> "ABC" ]

   

[ -d ./aaa.sh -a -e ./bbb ]  测试aaa.sh是否为目录并且bbb是否存在，如果都符合则返回真
[ -d ./aaa.sh -o -e ./bbb ]  测试aaa.sh是否为目录并且bbb是否存在，满足一个条件则返回真

 

We can see some rules for using single square brackets from the above:

There must be a space between the variable and the square bracket

variable There must also be a space between the operator and the operator

Variables can be referenced directly with $, or they can be referenced with ${} and "${}". Although ${} can be used directly, it is recommended to use "${} ", or use $ directly to quote variables

Strings should be enclosed in double quotes

String comparison generally uses == to test whether the strings are the same, and use < or > for size comparison, in a single To compare characters in parentheses for greater than and less than, use transfer > and <. The comparison principle is to compare ASCII.

In addition, you can also use logical operators like -a and -o in single square brackets

Double square brackets [[]]:

A=5
B=3
[[ $A -eq $B ]]    判断A是否等于B
[[ ${A} -eq ${B} ]]  作用同上
[[ "${A}" -eq "${B}" ]]  作用同上

                                                                   

A=ABC
[[ $A == "ABC" ]]   或写成  [[ ${A} == "ABC" ]]   或写成  [[ "${A}" == "ABC" ]] 或写成 [[ $A == ABC ]]
[[ $A > "ABC" ]]

The rules for using double square brackets are similar to the rules for using single square brackets

The greater-than-less comparison of strings between double square brackets does not need to be escaped, and the strings can be quoted without double quotes

Can be used inside double square brackets &&, || are used as logical operators

You can also do fuzzy matching inside double brackets, use =~Expression 2 can be a regular expression

[[ -d ./aaa.sh && -e ./bbb ]]  测试aaa.sh是否为目录并且bbb是否存在，如果都符合则返回真
[[ -d ./aaa.sh || -e ./bbb ]]  测试aaa.sh是否为目录并且bbb是否存在，满足一个条件则返回真

A=ABC
[[ $A =~ "A" ]]   做模式匹配，这个表达式为真
或写成  [[ ${A} =~ "A" ]]  或写成  [[ "$[A]" =~ "A" ]]
[[ $A = A* ]] 表达式为真  [[ $A = B* ]] 表达式为假

Single brackets

Brackets

Single parentheses used alone represent a command group, and the commands in the brackets will be executed sequentially through a subshell

[ 表达式1 -a 表达式2 ] 等效于 [[ 表达式1 && 表达式2 ]] 等效于 [ 表达式1 ] && [ 表达式2 ] 等效于 [[ 表达式1 ]] && [[ 表达式2 ]]

The most common usage is to use it in command substitution. Backticks can also be used for command substitution. ``

[ 表达式1 -o 表达式2 ] 等效于 [[ 表达式1 || 表达式2 ]] 等效于 [ 表达式1 ] || [ 表达式2 ] 等效于 [[ 表达式1 ]] || [[ 表达式2 ]]

Double parentheses

are commonly used in arithmetic operations and also used in for loops

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

For statement

1 to 10 Loop execution

echo "当前目录是：$(pwd)"
echo "当前目录是：`pwd`"

While statement form

1 to 10 loop

A=$((5+3))
B=$(($A+4))