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

Question

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 1

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

Answer 2

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.

Answer 3

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

Answer 4

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

Answer 5

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.

Answer 6

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.

Answer 7

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;
}

Answer 8

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 ))

Answer 9

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