How to use double or single brackets, parentheses, curly braces

In Bash, test and [ are shell builtins.

The double bracket, which is a shell keyword, enables additional functionality. For example, you can use && and || instead of -a and -o and there's a regular expression matching operator =~.

Also, in a simple test, double square brackets seem to evaluate quite a lot quicker than single ones.

$ 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

The braces, in addition to delimiting a variable name are used for parameter expansion so you can do things like:

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

and several more

Also, brace expansions create lists of strings which are typically iterated over in loops:

$ 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

Note that the leading zero and increment features weren't available before Bash 4.

Thanks to gboffi for reminding me about brace expansions.

Double parentheses are used for arithmetic operations:

((a++))

((meaning = 42))

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

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

and they enable you to omit the dollar signs on integer and array variables and include spaces around operators for readability.

Single brackets are also used for array indices:

array[4]="hello"

element=${array[index]}

Curly brace are required for (most/all?) array references on the right hand side.

ephemient's comment reminded me that parentheses are also used for subshells. And that they are used to create arrays.

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

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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

There is a subtle syntactic difference with ( ), though (see bash reference) ; essentially, a semicolon ; after the last command within braces is a must, and the braces {, } must be surrounded by spaces.

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Brackets

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

Curly Braces

${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

Parentheses

( 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 

Double Parentheses

(( 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

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I just wanted to add these from TLDP:

~:$ 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

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The difference between test, [ and [[ is explained in great details in the BashFAQ. (Note: The link shows many examples for comparison)

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.

And the conclusion:

When should the new test command [[ be used, and when the old one [? If portability/conformance to POSIX or the BourneShell is a concern, the old syntax should be used. If on the other hand the script requires BASH, Zsh, or KornShell, the new syntax is usually more flexible.

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Parentheses in function definition

Parentheses () are being used in function definition:

function_name () { command1 ; command2 ; }

That is the reason you have to escape parentheses even in command parameters:

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

$ echo \(
(

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

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Some common and handy uses for brackets, parenthesis, and braces

As mentioned above, sometimes you want a message displayed without losing the return value. This is a handy snippet:

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

This produced no output and a 0 (true) return value if the file go.mod exists in the current directory. Test the result:

$ echo $? 
0

If the file does not exist, you get the message but also a return value of 1 (false), which can also be tested:

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

$ echo $?
1

You can also simply create a function to check if a file exists:

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

or if a file is readable (not corrupted, have permissions, etc.):

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

or if it is a directory:

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

or is writable for the current user:

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

or if a file exists and is not empty (like a log file with content...)

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

There are more details at: TLDP

You can also see if a program exists and is available on the path:

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

This is useful in scripts, for example:

# 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; 
}

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Additional info about How to use parentheses to group and expand expressions:
(it is listed on the link syntax-brackets)

Some main points in there:

Group commands in a sub-shell: ( ) (list)

Group commands in the current shell: { } { list; }

Test - return the binary result of an expression: [[ ]] [[ expression ]]

Arithmetic expansion The format for Arithmetic expansion is: $(( expression ))

The format for a simple Arithmetic Evaluation is: (( expression ))

Combine multiple expressions ( expression ) (( expr1 && expr2 ))

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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

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