bashsyntax

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


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?


Solution

  • 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:

    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.

    Bare double parentheses ((...)) return 0 (true) if the value of the enclosed expression is non-zero, 1 (false) otherwise, so:

    $ ((2 + 3 - 5)) || echo "last command returned 1"
    last command returned 1
    $ ((2 + 3 - 7)) && echo "last command returned 0"
    last command returned 0
    

    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