We’ve covered using variables in Bash previously – this article will explain Bash array variables and provide some example usage.
What is an Array
An array is a type of variable that can hold multiple values. It’s a list of values you can loop through and perform operations on each individual value.
For example, you might want to perform an action on a list of files. By storing that list as an array, you can loop through the file names in it and perform the action on each one.
Arrays are indexed, with the position of each item in the array being represented by a number starting at 0.
Bash arrays do not require that array elements be indexed contiguously – or adjacent without gaps – you can have values at array positions 0, 3, 17 with no values in between, for example.
You can find out more about Bash arrays at the Linux Documentation Project.
Creating Bash Arrays
Bash arrays can contain any type of bash variable – paths, strings, numbers – even other arrays.
Why You Should Wrap File Paths in Strings in Your Shell Scripts
There are several methods for declaring arrays, outlined below.
Indirect Declaration
Arrays can be declared indirectly by assigning the array element’s value – the array will be created along with the array element with the given value.
ARRAYNAME[INDEX]=VALUE
For example:
#!/bin/bash peopleArray[3]="Tom"
The array peopleArray will be created automatically (indirectly) when a value in the array is assigned – in this case, the value “Tom” is assigned to index 3 in the newly created peopleArray.
Direct Declaration
The declare command can also be used to define an array:
declare -a ARRAYNAME
For example:
declare -a peopleArray
This would create an empty array called peopleArray.
Creating via Compound Assignment
This is the method you’ll probably use most. It creates an array with the values already assigned.
ARRAYNAME=(value1 value2 ...)
This assigns the value of the array to the variable name. The array values are contained in () (standard brackets) and separated by spaces. Values containing spaces should be quoted.
For example:
peopleArray=("Tom", "Bill", "Theodore Baker Sr.")
Assigning Values to an Array
Adding a New Value to the End of an Array
To add a value to an array, assign the value with no index specified:
ARRAYNAME[]=VALUE
For example:
peopleArray[]="Tony"
Replacing Value in an Array
To replace a value, overwrite the value at the current value’s index.
For example, if you have an array with a value at index 7 which you wish to replace:
peopleArray[7]="Tony"
Deleting Value in an Array
Delete a value from an array by using the unset command and specifying the array index:
unset ARRAYNAME[INDEX]
For example:
unset peopleArray[1]
…will delete the second value in the array peopleArray.
Accessing Array Values
To access the value in an array, you must use curly braces – otherwise, Bash will not interpret the brackets containing the array index correctly:
echo ${ARRAYNAME[INDEX]}
For example:
#!/bin/bash
peopleArray=("Tom", "Bill", "Theodore Baker Sr.")
echo ${peopleArray[0]}
…will output:
Tom
Whereas if the last line were:
echo $peopleArray[0]
