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

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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

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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

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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

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The difference between test    and    is explained in great details in the BashFAQ      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  which 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 to the BourneShell is a concern  the old syntax should   be used  If on the other hand the script requires BASH or KornShell    the new syntax is much more flexible

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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  amp  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    gt  COMMAND                          Process substitution    lt  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 lt 98 9 21    C-style ternary operation

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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      gt logfile     date  and  top  output are concatenated     could be useful sometimes to hunt for a top loader        date  make 2 gt  amp 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

User · Answer

In Bash  test and   are shell builtins   The double bracket  which is a shell keyword  enables additional functionality  For example  you can use  amp  amp  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 lt 10000000  i      do      i    1000     done  real    0m24 548s user    0m24 337s sys 0m0 036s   time for   i 0  i lt 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 lt 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

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

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