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

1. 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
   

2. 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
   

3. 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.

4. (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.

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; 
}

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 ))

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