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6.7 Arrays

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 section 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


The subscript is treated as an arithmetic expression that must evaluate to a number. If subscript evaluates to a number less than zero, it is used as an offset from one greater than the array’s maximum index (so a subcript of -1 refers to the last element of the array). 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. If both options are supplied, ‘-A’ takes precedence. 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.

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