A single bracket ([) usually actually calls a program named [; man test or man [ for more info. Example: $ VARIABLE=abcdef $ if [ $VARIABLE == abcdef ] ; then echo yes ; else echo no ; fi yes The double bracket ([[) does the same thing (basically) as a single bracket, but is a bash builtin. $ VARIABLE=abcdef $ if [[ $VARIABLE == 123456 ]] ; then echo yes ; else echo no ; fi no Parentheses (()) are used to create a subshell. For example: $ pwd /home/user $ (cd /tmp; pwd) /tmp $ pwd /home/user As you can see, the subshell allowed you to perform operations without affecting the environment of the current shell. (a) Braces ({}) are used to unambiguously identify variables. Example: $ VARIABLE=abcdef $ echo Variable: $VARIABLE Variable: abcdef $ echo Variable: $VARIABLE123456 Variable: $ echo Variable: ${VARIABLE}123456 Variable: abcdef123456 (b) Braces are also used to execute a sequence of commands in the current shell context, e.g. $ { date; top -b -n1 | head ; } >logfile # 'date' and 'top' output are concatenated, # could be useful sometimes to hunt for a top loader ) $ { date; make 2>&1; date; } | tee logfile # now we can calculate the duration of a build from the logfile

尽管()有一个微妙的语法差异(参见bash参考);本质上是一个分号;在最后一个命令之后的大括号内是必须的，大括号{，}必须用空格包围。

在Bash中，test和[是shell内置的。

双括号是shell关键字，它支持额外的功能。例如，您可以使用&&和||来代替-a和-o，并且有一个正则表达式匹配操作符=~。

此外，在一个简单的测试中，双方括号的计算速度似乎比单方括号快得多。

$ time for ((i=0; i<10000000; i++)); do [[ "$i" = 1000 ]]; done

real    0m24.548s
user    0m24.337s
sys 0m0.036s
$ time for ((i=0; i<10000000; i++)); do [ "$i" = 1000 ]; done

real    0m33.478s
user    0m33.478s
sys 0m0.000s

大括号，除了分隔变量名外，还用于参数展开，这样你就可以做以下事情:

截断变量的内容 $ var = "中的";echo $ {var % d *} 美国广播公司 进行类似sed的替换 $ var = "中的";echo $ {var /德/ 12} abc12 使用默认值 默认美元=“你好”;未设置的var;echo $ {var: - $默认} 你好 还有更多

同样，大括号展开创建字符串列表，这些字符串通常在循环中迭代:

$ echo f{oo,ee,a}d
food feed fad

$ mv error.log{,.OLD}
(error.log is renamed to error.log.OLD because the brace expression
expands to "mv error.log error.log.OLD")

$ for num in {000..2}; do echo "$num"; done
000
001
002

$ echo {00..8..2}
00 02 04 06 08

$ echo {D..T..4}
D H L P T

请注意，前导零和增量特性在Bash 4之前是不可用的。

感谢gboffi提醒我关于大括号展开。

双括号用于算术运算:

((a++))

((meaning = 42))

for ((i=0; i<10; i++))

echo $((a + b + (14 * c)))

它们使您可以省略整数和数组变量上的美元符号，并在操作符周围包含空格以提高可读性。

单括号也用于数组下标:

array[4]="hello"

element=${array[index]}

右边的(most/all?)数组引用需要花括号。

Ephemient的评论提醒了我，括号也用于subshell。它们被用来创建数组。

array=(1 2 3)
echo ${array[1]}
2

我只是想从TLDP中添加这些:

~:$ echo $SHELL
/bin/bash

~:$ echo ${#SHELL}
9

~:$ ARRAY=(one two three)

~:$ echo ${#ARRAY}
3

~:$ echo ${TEST:-test}
test

~:$ echo $TEST


~:$ export TEST=a_string

~:$ echo ${TEST:-test}
a_string

~:$ echo ${TEST2:-$TEST}
a_string

~:$ echo $TEST2


~:$ echo ${TEST2:=$TEST}
a_string

~:$ echo $TEST2
a_string

~:$ export STRING="thisisaverylongname"

~:$ echo ${STRING:4}
isaverylongname

~:$ echo ${STRING:6:5}
avery

~:$ echo ${ARRAY[*]}
one two one three one four

~:$ echo ${ARRAY[*]#one}
two three four

~:$ echo ${ARRAY[*]#t}
one wo one hree one four

~:$ echo ${ARRAY[*]#t*}
one wo one hree one four

~:$ echo ${ARRAY[*]##t*}
one one one four

~:$ echo $STRING
thisisaverylongname

~:$ echo ${STRING%name}
thisisaverylong

~:$ echo ${STRING/name/string}
thisisaverylongstring

test，[和[]之间的区别在BashFAQ中有详细的解释。 (注:链接中有很多例子供比较)

To cut a long story short: test implements the old, portable syntax of the command. In almost all shells (the oldest Bourne shells are the exception), [ is a synonym for test (but requires a final argument of ]). Although all modern shells have built-in implementations of [, there usually still is an external executable of that name, e.g. /bin/[. [[ is a new, improved version of it, and it is a keyword, not a program. This has beneficial effects on the ease of use, as shown below. [[ is understood by KornShell and BASH (e.g. 2.03), but not by the older POSIX or BourneShell.

结论是:

什么时候使用新的测试命令[[，什么时候使用旧的测试命令[? 如果要考虑到POSIX或BourneShell的可移植性/一致性，则应该考虑旧语法 被使用。如果脚本需要BASH、Zsh或KornShell， 新的语法通常更加灵活。

括号

if [ CONDITION ]    Test construct  
if [[ CONDITION ]]  Extended test construct  
Array[1]=element1   Array initialization  
[a-z]               Range of characters within a Regular Expression
$[ expression ]     A non-standard & obsolete version of $(( expression )) [1]

[1] http://wiki.bash-hackers.org/scripting/obsolete

花括号

${variable}                             Parameter substitution  
${!variable}                            Indirect variable reference  
{ command1; command2; . . . commandN; } Block of code  
{string1,string2,string3,...}           Brace expansion  
{a..z}                                  Extended brace expansion  
{}                                      Text replacement, after find and xargs

括号

( command1; command2 )             Command group executed within a subshell  
Array=(element1 element2 element3) Array initialization  
result=$(COMMAND)                  Command substitution, new style  
>(COMMAND)                         Process substitution  
<(COMMAND)                         Process substitution 

双括号

(( var = 78 ))            Integer arithmetic   
var=$(( 20 + 5 ))         Integer arithmetic, with variable assignment   
(( var++ ))               C-style variable increment   
(( var-- ))               C-style variable decrement   
(( var0 = var1<98?9:21 )) C-style ternary operation

函数定义中的括号

括号()在函数定义中被使用:

function_name () { command1 ; command2 ; }

这就是在命令形参中也必须转义括号的原因:

$ echo (
bash: syntax error near unexpected token `newline'

$ echo \(
(

$ echo () { command echo The command echo was redefined. ; }
$ echo anything
The command echo was redefined.

Truncate the contents of a variable

$ var="abcde"; echo ${var%d*}
abc

Make substitutions similar to sed

$ var="abcde"; echo ${var/de/12}
abc12

Use a default value

$ default="hello"; unset var; echo ${var:-$default}
hello

关于如何使用括号对表达式进行分组和展开的其他信息: (它列在链接语法括号中)

这里有一些要点:

在子shell中分组命令:() (列表)

将当前shell中的命令分组:{} {列表;｝

Test -返回一个表达式的二进制结果:[[]] [表达式]]

算术扩张 算术展开的格式是: $(表达式)

简单算术求值的格式是: (表达)

组合多个表达式 (表达式) (expr1 && expr2))

括号、圆括号和大括号的一些常见而方便的用法

如上所述，有时您希望在不丢失返回值的情况下显示消息。这是一个方便的片段:

$ [ -f go.mod ] || { echo 'File not found' && false; }

这不会产生输出，如果文件运行，返回值为0 (true)。Mod在当前目录中存在。测试结果:

$ echo $? 
0

如果文件不存在，你会得到消息，但返回值也为1 (false)，这也可以测试:

$ [ -f fake_file ] || { echo 'File not found'; false; }
File not found

$ echo $?
1

你也可以简单地创建一个函数来检查文件是否存在:

fileexists() { [ -f "$1" ]; }

或者如果文件是可读的(未损坏，有权限等):

canread() { [ -r "$1" ]; }

如果它是一个目录:

isdir() { [ -d "$1" ]; }

或当前用户可写:

canwrite() { [ -w "$1" ]; }

或者如果一个文件存在并且不是空的(就像一个包含内容的日志文件…)

isempty() { [ -s "$1" ]; }

有更多的细节:TLDP

你还可以查看一个程序是否存在并且在路径上可用:

exists () { command -v $1 > /dev/null 2>&1; }

这在脚本中很有用，例如:

# gitit does an autosave commit to the current
# if Git is installed and available.
# If git is not available, it will use brew 
# (on macOS) to install it.
#
# The first argument passed, if any, is used as 
# the commit message; otherwise the default is used.
gitit() {
    $(exists git) && { 
        git add --all; 
        git commit -m "${1:-'GitBot: dev progress autosave'}"; 
        git push; 
    } || brew install git; 
}