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

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

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.