Bash Scripting - Bracket

Updated: 2019-01-03
Name Other Name
[] Brackets
() Round Brackets
{} Curly brackets
<> Angle brackets

In Bash, test and are builtins. The double bracket enables additional functionality. For example, you can use && and || instead of -a and -o and there's a regular expression matching operator =~.

In Bash, test and [ are builtins.

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

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.

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.

Parentheses are also used for subshells. And that they are used to create arrays.

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

[ vs [[: http://mywiki.wooledge.org/BashFAQ/031