Assigning variables in bash is easily done and extremely
useful, but like other programming languages, bash can also use arrays. This is
particularly handy when you want to read the contents of a file into an array or
simply keep your scripts more organized and logical.

There are two ways of declaring an array:

declare -a FOO

This creates an empty array called FOO. You can also declare
an array by assigning values to it:

FOO[2] = 'bar'

This assigns the third element of the array to the value
‘bar’. In this instance, FOO[0] and FOO[1] are also
created, but their values are empty.

To populate an array, use:

FOO=( bar string 'some text' )

This assigns the first element (FOO[0])
to ‘bar’, the second (FOO[1]) to ‘string’ and the final element (FOO[3]) to
‘some text’. Notice that the array elements are separated by a blank space, so
if a value contains white spaces it must be quoted.

To use an array, it is referred to as $FOO[2]
but it also needs to be surrounded in curly braces, otherwise bash will not
expand it correctly:

$ echo {$FOO[2]}
some text

To loop through an array, you can use a piece of shell code
like the following:

FOO=( bar string 'some text')
for ((i=0;i<$foonum;i++)); do
   echo ${FOO[${i}]}

Here we loop through each item of the array and print out its
value. Each array element is accessed by number, so we use the special variable
${#FOO} which gives the number of elements in the array (in the above case, it
would return the number 3). That value is then used in the for loop to determine how many
times to loop. By accessing the array in this manner, you can easily generate
arrays from external data or command-line arguments, and process each element
one at a time.

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