Diff for "BashFAQ/005"

Anchor(faq5)

How can I use array variables?

BASH and KornShell already have one-dimensional arrays indexed by a numerical expression, e.g.

•  host[0]="micky"
   host[1]="minnie"
   host[2]="goofy"
   i=0
   while (($i < ${#host[@]} ))
   do
       echo "host number $i is ${host[i++]}"
   done

The indexing always begins with 0.

The awkward expression  ${#host[@]}  returns the number of elements for the array host. Also noteworthy is the fact that inside the square brackets, i++ works as a C programmer would expect. The square brackets in an array reference force an ArithmeticExpression.

It's possible to assign multiple values to an array at once, but the syntax differs from Bash to KornShell:

•  # Bash
   array=(one two three four)
  
   # KornShell
   set -A array -- one two three four

Bash also lets you initialize an array using a [:glob:]:

•  oggs=(*.ogg)

Using array elements en masse is one of the key features. In exactly the same way that "$@" is expanded for positional parameters, "${arr[@]}" is expanded to a list of words, one array element per word. For example,

•  for x in "${arr[@]}"; do
     echo "next element is '$x'"
   done

This works even if the elements contain whitespace. You always end up with the same number of words as you have array elements.

If one simply wants to dump the full array, "${arr[*]}" will cause the elements to be concatenated together, with the first character of IFS (a space by default) between them. As it happens, "$*" is expanded the same way for positional parameters.

•  arr=(x y z)
   IFS=/; echo "${arr[*]}"; unset IFS
   # prints x/y/z

BASH's arrays are also sparse. Elements may be added and deleted out of sequence.

•  arr=(0 1 2 3)
   arr[42]="what was the question?"
   unset arr[2]
   echo "${arr[*]}"
   # prints 0 1 3 what was the question?

BASH 3.0 added the ability to retrieve the list of index values in an array, rather than just iterating over the elements:

•  echo ${!arr[*]}
   # using the previous array, prints 0 1 3 42

[:BashFAQ#faq73:Parameter Expansions] may be performed on array elements en masse as well:

•  arr=(abc def ghi jkl)
   echo "${arr[@]#?}"          # prints bc ef hi kl
   echo "${arr[@]/[aeiou]/}"   # prints bc df gh jkl

Parameter Expansion can also be used to extract elements from an array:

•  echo "${arr[@]:1:3}"        # three elements starting at #1 (second element)
   echo "${arr[@]:(-2)}"       # last two elements
   echo "${@:(-1)}"            # last positional parameter
   echo "${@:(-2):1}"          # second-to-last positional parameter

The @ array (the array of positional parameters) can be used just like any regularly named array.

For examples of loading data into arrays, see [:BashFAQ#faq1:FAQ #1]. For examples of using arrays to hold complex shell commands, see [:BashFAQ#faq50:FAQ #50] and [:BashFAQ#faq40:FAQ #40].