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 [[ "$i" = 1000 ]]; donereal    0m24.548suser    0m24.337ssys 0m0.036s$ time for ((i=0; i<10000000; i++)); do [ "$i" = 1000 ]; donereal    0m33.478suser    0m33.478ssys 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}dfood feed fad$ mv error.log{,.OLD}(error.log is renamed to error.log.OLD because the brace expressionexpands to "mv error.log error.log.OLD")$ for num in {000..2}; do echo "$num"; done000001002$ 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 ; fiyes

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 ; fino

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: $VARIABLEVariable: abcdef$ echo Variable: $VARIABLE123456Variable:$ echo Variable: ${VARIABLE}123456Variable: 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.

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