How to use the Array Variable in Bash?

About

Bash provides a one-dimensional array variables. See also: Bash Shell and (Unix|Linux) Utilities (XCU)

When calling a function, quote the variable otherwise bash will not see the string as atomic. The separator is variable $IFS.

There is:

no maximum limit on the size of an array,
nor any requirement that members be indexed or assigned contiguously.

Management

Assignment

One value

Implicit

Individual array elements may be assigned to using the

name[subscript]=value

where:

subscript is treated as an arithmetic expression that must evaluate to a number greater than or equal to zero. Indexing: Arrays are indexed using integers and are zero-based.
name is the name of the array variable
value is the value of the variable for name[subscript]

Explicit

The declare builtin will explicitly declare an array.

To explicitly declare an array, use

declare -a name
declare -a name[subscript] # is also accepted but the subscript is ignored

#Example
declare -a arr=("element1" "element2" "element3")

The following builtin command accept a -a option to specify an array

Associative

By default, if not defined, the array is indexed by integer. If you want to have an associative array, you need to declare it

declare -A arr

declare -A name=(key1 value1 key2 value2 … )
declare -A name=( [key1]=value1 [key2]=value2 … )

Multiple values - string word splitting

With IFS being a space, you can transform a string to an array thanks to word splitting

Explicit:

string="a b c";
array=( $string )
echo ${array[0]} # a
echo ${array[1]} # b

Implicit: The read builtin accepts a -a option to assign a list of words read from the standard input to an array.

# string
IFS=', ' read -r -a array <<< "element1, element2, element3"
# file (every value on one line)
IFS=$'\n' read -r -a array -d $'\c0' < /path/to/file

Multiple values

Arrays are assigned using compound assignments of the form

# Implicit
name=([subscript1]=string1... [subscriptN]=stringN)
array=("element1" "element2" "element3")

# Explicit
declare -a array=("element1" "element2" "element3")
IFS=', ' read -r -a array <<< "element1, element2, element3"

where:

Only string is required.
If the optional brackets and subscript are 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.

This syntax is also accepted by the declare builtin.

Show the array definition with typeset

typeset -p myarray

declare -A myarray=([author]="jon" )

Other

The set and declare builtins display array values in a way that allows them to be reused as assignments.

Append

In the context where an assignment statement is assigning a value to a array index, the += operator, the variable’s value is not unset (as it is when using =), and new values are appended to the array beginning at one greater than the array’s maximum index.

Example

declare -a myArray
myArray+=(first)
myArray+=(second)

Attributes

Attributes may be specified for an array variable using the:

declare
and readonly builtins.

Each attribute applies to all members of an array.

Reference

Index

Any element of an array may be referenced using

${name[subscript]}

where:

subscript is the index (default 0, may be negative). Referencing an array variable without a subscript is equivalent to referencing element zero.
The braces are required to avoid conflicts with pathname expansion.

Example with the builtin BASH_VERSINFO array.

echo ${BASH_VERSINFO[0]}
# same as:
echo ${BASH_VERSINFO}

Last element

declare -a args=("element1" "element2" "element3")
echo ${args[-1]}

element3

Special subscript @ or *

If subscript is @ or *, the word expands to all members of name.

These subscripts differ only when the word appears within double quotes.

If the word is double-quoted:

Array[*] expands to a single word with the value of each array member separated by the first character of the IFS special variable
Array[@] expands each element of name to a separate word.

When there are no array members, Array[@] 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 @.

Number of elements

${#name[subscript]} expands to the length of ${name[subscript]}.

This is a parameter expansion mechanism.

If the subscript is * or @, the expansion is the number of elements in the array. Example with the builtin BASH_VERSINFO array.

echo ${#BASH_VERSINFO[*]}

List of indices

Through the indices parameter expansion mechanism, it's possible to get a list of indices.

Default

name[0]=value0
name[1]=value1
name[3]=value3
echo ${!name[*]}

Output

0 1 3

Associative Array

declare -A colors
colors['yellow']=jaune
colors['red']=rouge
echo ${!colors[*]}

yellow red

Slicing

See parameter_expansion substring_expansion

Example with the BASH_VERSINFO, we can slice it to get the element 1 to 4

echo ${BASH_VERSINFO[@]:1:4}

3 46 1 release

Unset (Destroy)

The unset builtin is used to destroy arrays.

To destroy the array element at index subscript.

unset name[subscript]

Care must be taken to avoid unwanted side effects caused by filename generation.

The following statement removes the entire array.

unset name # where name is an array
# or 
unset name[*]
unset name[@]

Looping

index looping with Bash - For Statement

## declare an array variable
IFS=' ' read -r -a array <<< "element1 element2 element3"

## Loop
for i in "${!array[@]}"
do
  printf "$i - ${array[$i]}\n"
done

0 - element1
1 - element2
2 - element3

name looping

## declare an array variable
declare -a array=("element1" "element2" "element3")

## 
for name in "${array[@]}"
do
  printf "$name\n"
done

element1
element2
element3

Print / Explode

Set the IFS separator and print in a subshell to not set IFS in the current session

(IFS=$'\n'; echo "${ARGS[*]}")

example

MY_ARRAY=("element1" "element2" "element3")
(IFS=$'\n'; echo "${MY_ARRAY[*]}")

element1
element2
element3

Contains

To check that an array contains a variable, you can:

declare the value as key and check

declare -A myArray=(  [value1]=1  [value2]=1  [value3]=1 )
x=value1
[[ -n "${array1[$x]}" ]]

a regular expression on the concatenation

[[ "${array1[*]}" =~ "value" ]] && echo contains || echo does not contains

Documentation / Reference

man bash