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

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I am confused by the usage of
brackets, parentheses, curly braces in Bash, as well as the difference between their
double or single forms. Is there a clear explanation?

Answer

In Bash,
test and [ are shell
builtins.

The href="http://mywiki.wooledge.org/BashFAQ/031" rel="noreferrer">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 href="http://tiswww.case.edu/php/chet/bash/bashref.html#Brace-Expansion"
rel="noreferrer">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
rel="noreferrer">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 rel="noreferrer">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