Arrays

Array Assignments

List Assignment

If you are familiar with Perl, C, or Java, you might think that Bash would use commas to separate array elements, however this is not the case; instead, Bash uses spaces:

 # Array in Perl
 my @array = (1, 2, 3, 4);

 # Array in Bash
 array=(1 2 3 4)

Create an array with new elements:

array=('first element' 'second element' 'third element')

Subscript Assignment

Create an array with explicit element indices:

array=([3]='fourth element' [4]='fifth element')

Assignment by index

array[0]='first element'
array[1]='second element'

Assignment by name (associative array)

declare -A array
array[first]='First element'
array[second]='Second element'

Dynamic Assignment

Create an array from the output of other command, for example use seq to get a range from 1 to 10:

array=(`seq 1 10`)

Assignment from script’s input arguments:

array=("$@")

Assignment within loops:

while read -r; do
    #array+=("$REPLY")     # Array append
    array[$i]="$REPLY"     # Assignment by index
    let i++                # Increment index 
done < <(seq 1 10)  # command substitution
echo ${array[@]}    # output: 1 2 3 4 5 6 7 8 9 10

where $REPLY is always the current input

Accessing Array Elements

Print element at index 0

echo "${array[0]}"

Print last element using substring expansion syntax

echo "${arr[@]: -1 }"

Print last element using subscript syntax

echo "${array[-1]}"

Print all elements, each quoted separately

echo "${array[@]}"

Print all elements as a single quoted string

echo "${array[*]}"

Print all elements from index 1, each quoted separately

echo "${array[@]:1}"

Print 3 elements from index 1, each quoted separately

echo "${array[@]:1:3}"

String Operations

If referring to a single element, string operations are permitted:

array=(zero one two)
echo "${array[0]:0:3}" # gives out zer (chars at position 0, 1 and 2 in the string zero)
echo "${array[0]:1:3}" # gives out ero (chars at position 1, 2 and 3 in the string zero)

so ${array[$i]:N:M} gives out a string from the Nth position (starting from 0) in the string ${array[$i]} with M following chars.

Array Length

${#array[@]} gives the length of the array ${array[@]}:

array=('first element' 'second element' 'third element')
echo "${#array[@]}" # gives out a length of 3

This works also with Strings in single elements:

echo "${#array[0]}"    # gives out the lenght of the string at element 0: 13

Array Modification

Change Index

Initialize or update a particular element in the array

array[10]="elevenths element"    # because it's starting with 0

Append

Modify array, adding elements to the end if no subscript is specified.

array+=('fourth element' 'fifth element')

Replace the entire array with a new parameter list.

array=("${array[@]}" "fourth element" "fifth element")

Add an element at the beginning:

array=("new element" "${array[@]}")

Insert

Insert an element at a given index:

arr=(a b c d)
# insert an element at index 2
i=2
arr=("${arr[@]:0:$i}" 'new' "${arr[@]:$i}")
echo "${arr[2]}" #output: new

Delete

Delete array indexes using the unset builtin:

arr=(a b c)
echo "${arr[@]}"   # outputs: a b c
echo "${!arr[@]}"  # outputs: 0 1 2
unset -v 'arr[1]'
echo "${arr[@]}"   # outputs: a c
echo "${!arr[@]}"  # outputs: 0 2

Merge

array3=("${array1[@]}" "${array2[@]}")

This works for sparse arrays as well.

Re-indexing an array

This can be useful if elements have been removed from an array, or if you’re unsure whether there are gaps in the array. To recreate the indices without gaps:

array=("${array[@]}")

Array Iteration

Array iteration comes in two flavors, foreach and the classic for-loop:

a=(1 2 3 4)
# foreach loop
for y in "${a[@]}"; do
    # act on $y
    echo "$y"
done
# classic for-loop
for ((idx=0; idx < ${#a[@]}; ++idx)); do
    # act on ${a[$idx]}
    echo "${a[$idx]}"
done

You can also iterate over the output of a command:

a=($(tr ',' ' ' <<<"a,b,c,d")) # tr can transform one character to another
for y in "${a[@]}"; do
    echo "$y"
done

Destroy, Delete, or Unset an Array

To destroy, delete, or unset an array:

unset array

To destroy, delete, or unset a single array element:

unset array[10]

Associative Arrays

Declare an associative array

declare -A aa

Declaring an associative array before initialization or use is mandatory.

Initialize elements

You can initialize elements one at a time as follows:

aa[hello]=world
aa[ab]=cd
aa["key with space"]="hello world"

You can also initialize an entire associative array in a single statement:

aa=([hello]=world [ab]=cd ["key with space"]="hello world")

Access an associative array element

echo ${aa[hello]}
# Out: world

Listing associative array keys

echo "${!aa[@]}"
#Out: hello ab key with space

Listing associative array values

echo "${aa[@]}"
#Out: world cd hello world

Iterate over associative array keys and values

for key in "${!aa[@]}"; do
    echo "Key:   ${key}"
    echo "Value: ${array[$key]}"
done

# Out:
# Key:   hello
# Value: world
# Key:   ab
# Value: cd
# Key:   key with space
# Value: hello world

Count associative array elements

echo "${#aa[@]}"
# Out: 3

List of initialized indexes

Get the list of inialized indexes in an array

$ arr[2]='second'
$ arr[10]='tenth'
$ arr[25]='twenty five'
$ echo ${!arr[@]}
2 10 25

Looping through an array

Our example array:

arr=(a b c d e f)

Using a for..in loop:

for i in "${arr[@]}"; do
    echo "$i"
done

Using C-style for loop:

for ((i=0;i<${#arr[@]};i++)); do
    echo "${arr[$i]}" 
done

Using while loop:

i=0
while [ $i -lt ${#arr[@]} ]; do
    echo "${arr[$i]}"
    i=$((i + 1))
done

Using while loop with numerical conditional:

i=0
while (( $i < ${#arr[@]} )); do
    echo "${arr[$i]}"
    ((i++))
done

Using an until loop:

i=0
until [ $i -ge ${#arr[@]} ]; do
    echo "${arr[$i]}"
    i=$((i + 1))
done

Using an until loop with numerical conditional:

i=0
until (( $i >= ${#arr[@]} )); do
    echo "${arr[$i]}"
    ((i++))
done

Array from string

stringVar="Apple Orange Banana Mango"
arrayVar=(${stringVar// / })

Each space in the string denotes a new item in the resulting array.

echo ${arrayVar[0]} # will print Apple
echo ${arrayVar[3]} # will print Mango

Similarly, other characters can be used for the delimiter.

stringVar="Apple+Orange+Banana+Mango"
arrayVar=(${stringVar//+/ })
echo ${arrayVar[0]} # will print Apple
echo ${arrayVar[2]} # will print Banana

Array insert function

This function will insert an element into an array at a given index:

insert(){
    h='
################## insert ########################
# Usage:
#   insert arr_name index element
#
#   Parameters:
#       arr_name    : Name of the array variable
#       index       : Index to insert at
#       element     : Element to insert
##################################################
    '
    [[ $1 = -h ]] && { echo "$h" >/dev/stderr; return 1; }
    declare -n __arr__=$1   # reference to the array variable
    i=$2                    # index to insert at
    el="$3"                 # element to insert
    # handle errors
    [[ ! "$i" =~ ^[0-9]+$ ]] && { echo "E: insert: index must be a valid integer" >/dev/stderr; return 1; }
    (( $1 < 0 )) && { echo "E: insert: index can not be negative" >/dev/stderr; return 1; }
    # Now insert $el at $i
    __arr__=("${__arr__[@]:0:$i}" "$el" "${__arr__[@]:$i}")
}

Usage:

insert array_variable_name index element

Example:

arr=(a b c d)
echo "${arr[2]}" # output: c
# Now call the insert function and pass the array variable name,
# index to insert at
# and the element to insert
insert arr 2 'New Element'
# 'New Element' was inserted at index 2 in arr, now print them
echo "${arr[2]}" # output: New Element
echo "${arr[3]}" # output: c

Reading an entire file into an array

Reading in a single step:

IFS=$'\n' read -r -a arr < file

Reading in a loop:

arr=()
while IFS= read -r line; do
  arr+=("$line")
done

Using mapfile or readarray (which are synonymous):

mapfile -t arr < file
readarray -t arr < file