Bash provides one-dimensional indexed and associative array variables.
Any variable may be used as an indexed array;
the declare builtin will explicitly declare an array.
There is no maximum
limit on the size of an array, nor any requirement that members
be indexed or assigned contiguously.
Indexed arrays are referenced using integers (including arithmetic
expressions (see Shell Arithmetic) and are zero-based;
associative arrays use arbitrary strings.
An indexed array is created automatically if any variable is assigned to using the syntax
name[subscript]=value
The subscript is treated as an arithmetic expression that must evaluate to a number greater than or equal to zero. To explicitly declare an array, use
declare -a name
The syntax
declare -a name[subscript]
is also accepted; the subscript is ignored.
Associative arrays are created using
declare -A name.
Attributes may be
specified for an array variable using the declare and
readonly builtins. Each attribute applies to all members of
an array.
Arrays are assigned to using compound assignments of the form
name=(value1 ... valuen)
where each
value is of the form [subscript]=string.
Indexed array assignments do not require the bracket and subscript.
When assigning to indexed arrays, if
the optional subscript is supplied, that index is assigned to;
otherwise the index of the element assigned is the last index assigned
to by the statement plus one. Indexing starts at zero.
When assigning to an associative array, the subscript is required.
This syntax is also accepted by the declare
builtin. Individual array elements may be assigned to using the
name[subscript]=value syntax introduced above.
Any element of an array may be referenced using
${name[subscript]}.
The braces are required to avoid
conflicts with the shell's filename expansion operators. If the
subscript is ‘@’ or ‘*’, the word expands to all members
of the array name. These subscripts differ only when the word
appears within double quotes.
If the word is double-quoted,
${name[*]} expands to a single word with
the value of each array member separated by the first character of the
IFS variable, and ${name[@]} expands each element of
name to a separate word. When there are no array members,
${name[@]} expands to nothing.
If the double-quoted expansion occurs within a word, the expansion of
the first parameter is joined with the beginning part of the original
word, and the expansion of the last parameter is joined with the last
part of the original word.
This is analogous to the
expansion of the special parameters ‘@’ and ‘*’.
${#name[subscript]} expands to the length of
${name[subscript]}.
If subscript is ‘@’ or
‘*’, the expansion is the number of elements in the array.
Referencing an array variable without a subscript is equivalent to
referencing with a subscript of 0.
An array variable is considered set if a subscript has been assigned a value. The null string is a valid value.
The unset builtin is used to destroy arrays.
unset name[subscript]
destroys the array element at index subscript.
Care must be taken to avoid unwanted side effects caused by filename
expansion.
unset name, where name is an array, removes the
entire array. A subscript of ‘*’ or ‘@’ also removes the
entire array.
The declare, local, and readonly
builtins each accept a -a option to specify an indexed
array and a -A option to specify an associative array.
The read builtin accepts a -a
option to assign a list of words read from the standard input
to an array, and can read values from the standard input into
individual array elements. The set and declare
builtins display array values in a way that allows them to be
reused as input.