Thursday, March 7, 2013

brackets, parentheses, curly braces in BASH

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*}
  • Make substitutions similar to sed
    $ var="abcde"; echo ${var/de/12}
  • Use a default value
    $ default="hello"; unset var; echo ${var:-$default}
  • and several more
Double parentheses are used for arithmetic operations:

((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:

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]}

  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
  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
  3. Parentheses (()) are used to create a subshell. For example:
    $ pwd
    $ (cd /tmp; pwd)
    $ pwd
    As you can see, the subshell allowed you to perform operations without affecting the environment of the current shell.
4a. Braces ({}) are used to unambiguously identify variables. Example:
    $ VARIABLE=abcdef
    $ echo Variable: $VARIABLE
    Variable: abcdef
    $ echo Variable: $VARIABLE123456
    $ echo Variable: ${VARIABLE}123456
    Variable: abcdef123456
4b. 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.



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