Working with Arrays in Linux Shell Scripting – Part 8

We cannot imagine a programming language without the concept of arrays. It doesn’t matter how they are implemented among various languages. Instead arrays help us in consolidating data, similar or different, under one symbolic name.

Working With Arrays in Shell Scripting

Working With Arrays in Shell Scripting

Here as we are concerned about shell scripting, this article will help you in playing around with some shell scripts which make use of this concept of arrays.

Array Initialization and Usage

With newer versions of bash, it supports one-dimensional arrays. An array can be explicitly declared by the declare shell-builtin.

declare -a var  

But it is not necessary to declare array variables as above. We can insert individual elements to array directly as follows.

var[XX]=<value>

where ‘XX’ denotes the array index. To dereference array elements use the curly bracket syntax, i.e.

${var[XX]}

Note: Array indexing always start with 0.

Another convenient way of initializing an entire array is by using the pair of parenthesis as shown below.

var=( element1 element2 element3 . . . elementN )

There is yet another way of assigning values to arrays. This way of initialization is a sub-category of the previously explained method.

array=( [XX]=<value> [XX]=<value> . . . )

We can also read/assign values to array during the execution time using the read shell-builtin.

read -a array

Now upon executing the above statement inside a script, it waits for some input. We need to provide the array elements separated by space (and not carriage return). After entering the values press enter to terminate.

To traverse through the array elements we can also use for loop.

for i in “${array[@]}”
do
	#access each element as $i. . .
done 

The following script summarizes the contents of this particular section.

#!/bin/bash 

array1[0]=one 
array1[1]=1 
echo ${array1[0]} 
echo ${array1[1]} 

array2=( one two three ) 
echo ${array2[0]} 
echo ${array2[2]} 

array3=( [9]=nine [11]=11 ) 
echo ${array3[9]} 
echo ${array3[11]} 

read -a array4 
for i in "${array4[@]}" 
do 
	echo $i 
done 

exit 0

Various Operations on Arrays

Many of the standard string operations work on arrays . Look at the following sample script which implements some operations on arrays (including string operations).

#!/bin/bash 

array=( apple bat cat dog elephant frog ) 

#print first element 
echo ${array[0]} 
echo ${array:0} 

#display all elements 
echo ${array[@]} 
echo ${array[@]:0} 

#display all elements except first one 
echo ${array[@]:1} 

#display elements in a range 
echo ${array[@]:1:4} 

#length of first element 
echo ${#array[0]} 
echo ${#array} 

#number of elements 
echo ${#array[*]} 
echo ${#array[@]} 

#replacing substring 
echo ${array[@]//a/A} 

exit 0

Following is the output produced on executing the above script.

apple 
apple 
apple bat cat dog elephant frog 
apple bat cat dog elephant frog 
bat cat dog elephant frog 
bat cat dog elephant 
5 
5 
6 
6 
Apple bAt cAt dog elephAnt frog

I think there is no significance in explaining the above script in detail as it is self-explanatory. If necessary I will dedicate one part in this series exclusively on string manipulations.

Command Substitution with Arrays

Command substitution assigns the output of a command or multiple commands into another context. Here in this context of arrays we can insert the output of commands as individual elements of arrays. Syntax is as follows.

array=( $(command) )

By default the contents in the output of command separated by white spaces are plugged into array as individual elements. The following script list the contents of a directory, which are files with 755 permissions.

#!/bin/bash 

ERR=27 
EXT=0 

if [ $# -ne 1 ]; then 
	echo "Usage: $0 <path>" 
	exit $ERR 
fi 

if [ ! -d $1 ]; then 
	echo "Directory $1 doesn't exists" 
	exit $ERR 
fi 

temp=( $(find $1 -maxdepth 1 -type f) ) 

for i in "${temp[@]}" 
do 
	perm=$(ls -l $i) 
	if [ `expr ${perm:0:10} : "-rwxr-xr-x"` -eq 10 ]; then 
		echo ${i##*/} 
	fi 
done 

exit $EXT

Simulating Two-dimensional Arrays

We can easily represent a 2-dimensional matrix using a 1-dimensional array. In row major order representation elements in each row of a matrix are progressively stored in array indexes in a sequential manner. For an mXn matrix, formula for the same can be written as.

matrix[i][j]=array[n*i+j]

Look at another sample script for adding 2 matrices and printing the resultant matrix.

#!/bin/bash 

read -p "Enter the matrix order [mxn] : " t 
m=${t:0:1} 
n=${t:2:1} 

echo "Enter the elements for first matrix" 
for i in `seq 0 $(($m-1))` 
do 
	for j in `seq 0 $(($n-1))` 
	do 
		read x[$(($n*$i+$j))] 
	done 
done 

echo "Enter the elements for second matrix" 
for i in `seq 0 $(($m-1))` 
do 
	for j in `seq 0 $(($n-1))` 
	do 
		read y[$(($n*$i+$j))] 
		z[$(($n*$i+$j))]=$((${x[$(($n*$i+$j))]}+${y[$(($n*$i+$j))]})) 
	done 
done 

echo "Matrix after addition is" 
for i in `seq 0 $(($m-1))` 
do 
	for j in `seq 0 $(($n-1))` 
	do 
		echo -ne "${z[$(($n*$i+$j))]}\t" 
	done 
	echo -e "\n" 
done 

exit 0 

Even though there are limitations for implementing arrays inside shell scripting, it becomes useful in a handful of situations, especially when we handle with command substitution. Looking from an administrative point of view, the concept of arrays paved the way for development of many background scripts in GNU/Linux systems.

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12 Responses

  1. swww says:

    Good collection of examples, especially command output assigning is nice. I wrote somewhat similar but more extensive article on arrays in BASH. Check it out here: wojnowski.net.pl/main/index/arrays-in-bash.

    Regards

  2. kumar says:

    Hi,

    I want print only first and third element of the array.

    what is the echo command?

  3. Gulab says:

    how to import multiple directory in array(run time log /var/log/ ) in runtime and check if directory is present or not ?

    • Anoop C S says:

      Hi Gulab,

      Sorry for being late..Got stuck with some other work. I would assume that by this time you might have found an answer to your question. Anyway following is my pov.

      You can always insert elements into array by referring to the index numbers as I have mentioned in the article. By run time if you mean to have some kind of looping, then you could increment index number for the array within the loop and insert elements one by one. Checking for directory existence is not at all a complex task and you have various methods to do so (easy way: use if conditional statement with test expression parameter -d).

  4. BV Naagabaabu says:

    is there any way to change the default index value 0 of array to 1..

    • Anoop C S says:

      Can you please specify the use case?

      • BV Naagabaabu says:

        actually we can access an array element like below

        array_name=(“name_1″,”name_2″,”name_3″,”name_4″,”name_5”)
        echo array_name[0]

        the above echo command prints “name_1”

        but i would like to print name_1 by using below echo command
        echo array_name[1]

        Here my intention is change the array default index value 0 to 1. so that i can print first value of array by using array_name[1] instead of using array_name[0]

        • Anoop C S says:

          Sorry for the late reply:

          Some corrections to your example mentioned below:
          * Your array initialization is wrong. I assume your intention was to initialize array as follows,
          # array_name=(name_1 name_2 name_3 name_4 name_5)
          Array elements are by default separated by one or more white spaces.

          * Your de-referencing of array elements is wrong. See the correct usage below,
          # echo ${array_name[0]}

          Now coming to your question:
          Yes, it is possible. You need to initialize the array by referencing the index as,
          # array_name=([1]=name_1 name_2 name_3 name_4 name_5)
          This means
          array_name[1] = name_1
          array_name[2] = name_2
          array_name[3] = name_3
          array_name[4] = name_4
          array_name[5] = name_5

          Hope this solution is helpful.

          • Thanks for your reply. I got it what exactly i want. Actually i’m facing some issues while using arrays in for loop and while loop. In those for and while loops the loop count is starting from 1, at that time if i want to use an array inside that loop, then i need to increase array index value to 1 manually. Now your solution solves this. Now i can directly use array with in a loop which has loop count start from 1.

            Thanks.

  5. Syed Aqhib says:

    Excellent

  6. skumar says:

    Your examples are so good. Thanks.

  7. santosh loke says:

    nice to know the functionality of arrays.

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