When do we need curly braces around shell variables

Question

In shell scripts  when do we use    when expanding variables   For example  I have seen the following   var 10          Declare variable  echo    var     One use of the variable echo   var      Another use of the variable   Is there a significant difference  or is it just style  Is one preferred over the other

User · Answer

Variables are declared and assigned without $ and without {}. You have to use

var=10

to assign. In order to read from the variable (in other words, 'expand' the variable), you must use $.

$var      # use the variable
${var}    # same as above
${var}bar # expand var, and append "bar" too
$varbar   # same as ${varbar}, i.e expand a variable called varbar, if it exists.

This has confused me sometimes - in other languages we refer to the variable in the same way, regardless of whether it's on the left or right of an assignment. But shell-scripting is different, $var=10 doesn't do what you might think it does!

User · Answer

You use    for grouping  The braces are required to dereference array elements  Example   dir                 store the contents of the directory into an array echo    dir 0       get the first entry  echo   dir 0        incorrect

User · Answer

The end of the variable name is usually signified by a space or newline  But what if we don t want a space or newline after printing the variable value  The curly braces tell the shell interpreter where the end of the variable name is   Classic Example 1  - shell variable without trailing whitespace  TIME 10    WRONG  no such variable called  TIMEsecs  echo  Time taken    TIMEsecs     What we want is  TIME followed by  secs  with no whitespace between the two  echo  Time taken     TIME secs    Example 2  Java classpath with versioned jars    WRONG - no such variable LATESTVERSION src CLASSPATH hibernate- LATESTVERSION src zip hibernate  LATEST VERSION jar    RIGHT CLASSPATH hibernate-  LATESTVERSION  src zip hibernate  LATEST VERSION jar    Fred s answer already states this but his example is a bit too abstract

User · Answer

Following SierraX and Peter s suggestion about text manipulation  curly brackets    are used to pass a variable to a command  for instance   Let s say you have a sposi txt file containing the first line of a well-known Italian novel    gt  sposi  somewhere myfolder sposi txt   gt  cat  sposi   Ouput  quel ramo del lago di como che volge a mezzogiorno  Now create two variables     Search the 2nd word found in the file that  sposi  variable points to  gt  word   cat  sposi   cut -d     -f 2     This variable will replace the word  gt  new word  filone    Now substitute the word variable content with the one of new word  inside sposi txt file   gt  sed -i  s   word    new word  g   sposi  gt  cat  sposi   Ouput  quel filone del lago di como che volge a mezzogiorno  The word  ramo  has been replaced

User · Answer

You are also able to do some text manipulation inside the braces   STRING    folder subfolder file txt  echo   STRING    STRING         Result     folder subfolder file txt   folder   or  STRING  This is a string  echo   STRING         Result   This is a string   You are right in  regular variables  are not needed    But it is more helpful for the debugging and to read a script

User · Answer

In this particular example  it makes no difference  However  the    in     are useful if you want to expand the variable foo in the string     foo bar    since   foobar  would instead expand the variable identified by foobar   Curly braces are also unconditionally required when    expanding array elements  as in   array 42   using parameter expansion operations  as in   filename      remove extension  expanding positional parameters beyond 9    8  9   10    11     Doing this everywhere  instead of just in potentially ambiguous cases  can be considered good programming practice  This is both for consistency and to avoid surprises like  foo  bar jpg  where it s not visually obvious that the underscore becomes part of the variable name

User · Answer

Curly braces are always needed for accessing array elements and carrying out brace expansion   It s good to be not over-cautious and use    for shell variable expansion even when there is no scope for ambiguity   For example   dir log prog foo path  var   dir    prog         excessive use of     not needed since   can t be a part of a shell variable name logfile   path    prog  log     same as above    can t be a part of a shell variable name path copy   path                   is totally unnecessary archive   logfile  arch            is needed since   can be a part of shell variable name   So  it is better to write the three lines as   path  var  dir  prog logfile  path  prog log path copy  path   which is definitely more readable   Since a variable name can t start with a digit  shell doesn t need    around numbered variables  like  1   2 etc   unless such expansion is followed by a digit   That s too subtle and it does make to explicitly use    in such contexts   set app        set  1 to app fruit  1le     sets fruit to apple  but confusing fruit   1 le   sets fruit to apple  makes the intention clear     See    Allowed characters in Linux environment variable names

[bash] When do we need curly braces around shell variables?

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