bash assign value to an array element

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How can I assign a value to an array in Bash?

I am trying to read a list of values from a text file, hello.txt , and store them in an array.

I am not able to assign values to the array Unix_Array[] ... The echo statement does not print the contents of the array.

5 Answers 5

There are a few syntax errors here, but the clear problem is that the assignments are happening, but you're in an implied subshell . By using a pipe, you've created a subshell for the entire while statement. When the while statement is done, the subshell exits and your Unix_Array ceases to exist.

In this case, the simplest fix is not to use a pipe:

By the way, you don't really need the counter. An easier way to write this might be:

If you are using Bash v4 or higher, you can use mapfile to accomplish this:

Otherwise, this should work:

+1 for calling out mapfile. I always forget about the new shell features. – kojiro Jun 18, 2012 at 17:42

The best way I found is:

And then consume it with:

2 I think you mean declare -a ; the uppercase version declares an assoclative array (Bash 4+). – tripleee Sep 20, 2021 at 8:38

Instead of this:

You can just do this:

Without reassigning IFS , this will create multiple elements for any line with a space in it. – kojiro Jun 18, 2012 at 17:41

It’s a solution:

This suffers from the same problem as Jon Lin's solution, namely, that you have to set IFS to a newline or you'll split on any whitespace in hello.txt . – kojiro Jun 19, 2012 at 12:14

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Assigning array values to variable [closed]

Can anybody please tell me how to assign array value to a variable?

Why am I getting errors while trying the above scenario?

shell-script

2 Answers 2

You were very close.

There must not be any whitespace around = in variable declaration.

bash arrays index starts at 1 – Ahmed Shehab Nov 24, 2021 at 8:27

in bash you cannot operate on variables like you do in any other language. however this page may help you

https://stackoverflow.com/questions/15691942/bash-print-array-elements-on-separate-lines

Not the answer you're looking for? Browse other questions tagged shell-script or ask your own question .

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You don't know Bash: An introduction to Bash arrays

WOCinTech Chat. Modified by Opensource.com. CC BY-SA 4.0

Although software engineers regularly use the command line for many aspects of development, arrays are likely one of the more obscure features of the command line (although not as obscure as the regex operator =~ ). But obscurity and questionable syntax aside, Bash arrays can be very powerful.

Programming and development

Wait, but why?

Writing about Bash is challenging because it's remarkably easy for an article to devolve into a manual that focuses on syntax oddities. Rest assured, however, the intent of this article is to avoid having you RTFM.

A real (actually useful) example

To that end, let's consider a real-world scenario and how Bash can help: You are leading a new effort at your company to evaluate and optimize the runtime of your internal data pipeline. As a first step, you want to do a parameter sweep to evaluate how well the pipeline makes use of threads. For the sake of simplicity, we'll treat the pipeline as a compiled C++ black box where the only parameter we can tweak is the number of threads reserved for data processing: ./pipeline --threads 4 .

The first thing we'll do is define an array containing the values of the --threads parameter that we want to test:

In this example, all the elements are numbers, but it need not be the case—arrays in Bash can contain both numbers and strings, e.g., myArray=(1 2 "three" 4 "five") is a valid expression. And just as with any other Bash variable, make sure to leave no spaces around the equal sign. Otherwise, Bash will treat the variable name as a program to execute, and the = as its first parameter!

Now that we've initialized the array, let's retrieve a few of its elements. You'll notice that simply doing echo $allThreads will output only the first element.

To understand why that is, let's take a step back and revisit how we usually output variables in Bash. Consider the following scenario:

Say the variable $type is given to us as a singular noun and we want to add an s at the end of our sentence. We can't simply add an s to $type since that would turn it into a different variable, $types . And although we could utilize code contortions such as echo "Found 42 "$type"s" , the best way to solve this problem is to use curly braces: echo "Found 42 ${type}s" , which allows us to tell Bash where the name of a variable starts and ends (interestingly, this is the same syntax used in JavaScript/ES6 to inject variables and expressions in template literals ).

So as it turns out, although Bash variables don't generally require curly brackets, they are required for arrays. In turn, this allows us to specify the index to access, e.g., echo ${allThreads[1]} returns the second element of the array. Not including brackets, e.g., echo $allThreads[1] , leads Bash to treat [1] as a string and output it as such.

Yes, Bash arrays have odd syntax, but at least they are zero-indexed, unlike some other languages (I'm looking at you, R ).

Looping through arrays

Although in the examples above we used integer indices in our arrays, let's consider two occasions when that won't be the case: First, if we wanted the $i -th element of the array, where $i is a variable containing the index of interest, we can retrieve that element using: echo ${allThreads[$i]} . Second, to output all the elements of an array, we replace the numeric index with the @ symbol (you can think of @ as standing for all ): echo ${allThreads[@]} .

Looping through array elements

With that in mind, let's loop through $allThreads and launch the pipeline for each value of --threads :

Looping through array indices

Next, let's consider a slightly different approach. Rather than looping over array elements , we can loop over array indices :

Let's break that down: As we saw above, ${allThreads[@]} represents all the elements in our array. Adding an exclamation mark to make it ${!allThreads[@]} will return the list of all array indices (in our case 0 to 7). In other words, the for loop is looping through all indices $i and reading the $i -th element from $allThreads to set the value of the --threads parameter.

This is much harsher on the eyes, so you may be wondering why I bother introducing it in the first place. That's because there are times where you need to know both the index and the value within a loop, e.g., if you want to ignore the first element of an array, using indices saves you from creating an additional variable that you then increment inside the loop.

Populating arrays

So far, we've been able to launch the pipeline for each --threads of interest. Now, let's assume the output to our pipeline is the runtime in seconds. We would like to capture that output at each iteration and save it in another array so we can do various manipulations with it at the end.

Some useful syntax

But before diving into the code, we need to introduce some more syntax. First, we need to be able to retrieve the output of a Bash command. To do so, use the following syntax: output=$( ./my_script.sh ) , which will store the output of our commands into the variable $output .

The second bit of syntax we need is how to append the value we just retrieved to an array. The syntax to do that will look familiar:

The parameter sweep

Putting everything together, here is our script for launching our parameter sweep:

What else you got?

In this article, we covered the scenario of using arrays for parameter sweeps. But I promise there are more reasons to use Bash arrays—here are two more examples.

Log alerting

In this scenario, your app is divided into modules, each with its own log file. We can write a cron job script to email the right person when there are signs of trouble in certain modules:

API queries

Say you want to generate some analytics about which users comment the most on your Medium posts. Since we don't have direct database access, SQL is out of the question, but we can use APIs!

To avoid getting into a long discussion about API authentication and tokens, we'll instead use JSONPlaceholder , a public-facing API testing service, as our endpoint. Once we query each post and retrieve the emails of everyone who commented, we can append those emails to our results array:

Note here that I'm using the jq tool to parse JSON from the command line. The syntax of jq is beyond the scope of this article, but I highly recommend you look into it.

As you might imagine, there are countless other scenarios in which using Bash arrays can help, and I hope the examples outlined in this article have given you some food for thought. If you have other examples to share from your own work, please leave a comment below.

But wait, there's more!

Since we covered quite a bit of array syntax in this article, here's a summary of what we covered, along with some more advanced tricks we did not cover:

One last thought

As we've discovered, Bash arrays sure have strange syntax, but I hope this article convinced you that they are extremely powerful. Once you get the hang of the syntax, you'll find yourself using Bash arrays quite often.

Bash or Python?

Which begs the question: When should you use Bash arrays instead of other scripting languages such as Python?

To me, it all boils down to dependencies—if you can solve the problem at hand using only calls to command-line tools, you might as well use Bash. But for times when your script is part of a larger Python project, you might as well use Python.

For example, we could have turned to Python to implement the parameter sweep, but we would have ended up just writing a wrapper around Bash:

Since there's no getting around the command line in this example, using Bash directly is preferable.

Time for a shameless plug

This article is based on a talk I gave at OSCON , where I presented the live-coding workshop You Don't Know Bash . No slides, no clickers—just me and the audience typing away at the command line, exploring the wondrous world of Bash.

This article originally appeared on Medium and is republished with permission.

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Update Element of an Array at Specific Index in Bash

Update element of an array.

To update element of an array in Bash, access the element using array variable and index, and assign a new value to this element using assignment operator.

The syntax to update or assign a new value to an element in array at specific index is

In the following script, we take an array arr with three elements and update the element with index=1.

In this Bash Tutorial , we have learnt how to update or set a new value for element of an array at specific index in Bash shell, with examples.

Bash Array – How to Declare an Array of Strings in a Bash Script

Bash scripts give you a convenient way to automate command line tasks.

With Bash, you can do many of the same things you would do in other scripting or programming languages. You can create and use variables, execute loops, use conditional logic, and store data in arrays.

While the functionality may be very familiar, the syntax of Bash can be tricky. In this article, you will learn how to declare arrays and then how to use them in your code.

How to Declare an Array in Bash

Declaring an array in Bash is easy, but pay attention to the syntax. If you are used to programming in other languages, the code might look familiar, but there are subtle differences that are easy to miss.

To declare your array, follow these steps:

Your array declaration will look something like this:

That's it! It's that simple.

How to Access an Array in Bash

There are a couple different ways to loop through your array. You can either loop through the elements themselves, or loop through the indices.

How to Loop Through Array Elements

To loop through the array elements, your code will need to look something like this:

To break that down: this is somewhat like using forEach in JavaScript. For each string (str) in the array (myArray), print that string.

The output of this loop looks like this:

Note : The @ symbol in the square brackets indicates that you are looping through all of the elements in the array. If you were to leave that out and just write for str in ${myArray} , only the first string in the array would be printed.

How to Loop Through Array Indices

Alternatively, you can loop through the indices of the array. This is like a for loop in JavaScript, and is useful for when you want to be able to access the index of each element.

To use this method, your code will need to look something like the following:

The output will look like this:

Note : The exclamation mark at the beginning of the myArray variable indicates that you are accessing the indices of the array and not the elements themselves. This can be confusing if you are used to the exclamation mark indicating negation, so pay careful attention to that.

Another note : Bash does not typically require curly braces for variables, but it does for arrays. So you will notice that when you reference an array, you do so with the syntax ${myArray} , but when you reference a string or number, you simply use a dollar sign: $i .

Bash scripts are useful for creating automated command line behavior, and arrays are a great tool that you can use to store multiple pieces of data.

Declaring and using them is not hard, but it is different from other languages, so pay close attention to avoid making mistakes.

Veronica is a librarian by trade with a longtime computer programming habit that she is currently working on turning into a career. She enjoys reading, cats, and programming in React.

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Assigning values to an array.

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Jan 29, 2023

How to Use Bash Arrays

Bash is the go-to choice for automation for Linux users. Since automation often deals with thousands of elements, it’s essential to know what a bash array is.

With bash arrays, managing VPS or physical servers is much easier. This tutorial will show you different types of bash arrays and provide useful examples.

What Is a Bash Array?

A bash array is a data structure designed to store information in an indexed way. In other words, a bash array is a large group of variables. Unlike typical arrays used in other programming languages, bash arrays can store different types of elements. For example, you can use a bash array to store both strings and numbers.

There are two types of bash arrays:

Remember that bash does not support multidimensional arrays, so it’s not possible to add an array within an array.

How to Declare Array in Bash

There are a few ways to declare indexed and associative arrays in bash. It’s worth noting that the bash array size does not need to be declared beforehand because bash arrays have no upper limit on the number of elements they can store.

Indexed Array

We will start with a simple bash indexed array. For example, we’ll use it to create a list of different means of transportation.

The first option is to declare an array by using the shell builtin declare with the -a flag and give the array its elements:

The same can be achieved without the declare builtin:

Or, make it even simpler by going with:

Remember that indexing starts at 0 , so the above example will assign the car element of the array to the 0 index.

However, there is an option to set an array with indices:

An interesting feature of bash arrays is that following index numbers in order is not necessary. For example, you can declare only the first and third elements while leaving the second element of an array empty:

Associative Array

While indexed arrays don’t require the declare builtin, it won’t be possible to create an associative bash array without declaring it first:

Next, add the values. Keep in mind that the key must be a string:

An alternative way would be:

How to Add a Variable to a Bash Array

Easily add bash variables using the += operator. For example, the process for an indexed array would look like this:

The indexed array has a new element now. Remember that this method appends to the end of an array. Therefore, the motorcycle element will be added as the last element.

For associative arrays, the process is very similar. Except, you need to specify the keys along with all the elements:

How to Reference and Print an Array Element

Users can reference bash array values using the element index or key. To do this, create an indexed array:

To reference the first array variable, use the following syntax:

Combine it with echo , and you will get the following:

The output will show you the first element. In this case, it’s car . The same logic applies when referencing and printing an associative array:

The output will be car as well.

To print the whole array, use @ as an index. The full script looks like this:

Bash script to create and print out an indexed array

You can also print the keys of an array instead. To do this, add an exclamation mark before the array name:

Command-line script to print out all the keys of an associative array. To print all the values instead, only the exclamation point would need to be removed

How to Remove Bash Array Elements

Deleting array elements is similar to referencing them. Use an index or a key combined with the unset builtin to delete an array element.

Here’s an example of deleting a single element from an indexed array:

Bash script to show unset command which is used to delete elements from an array

Bash script showcasing unset command for associative arrays, which is used to remove an element from the array

To delete an entire array, specify unset with the array name as shown here:

Nothing is shown after trying to print the array elements because the unset builtin deleted them.

How to Loop Through an Array

Creating bash loops is a fundamental aspect of learning bash scripting basics . You can use loops with arrays as well. For example, the most common use case is to iterate over each array item:

Shell script with a for loop to iterate and print an array

You can also combine keys with the array elements and print them all together like this:

Command-line script to print out both elements and keys for an associative array

How to Pass an Array to a Function

Functions save a considerable amount of time when scripting. Instead of writing the same code repeatedly, you can call out an already written function. We will combine the previously mentioned iteration loop and make a function out of it:

Running it on the command line will get you the following result:

Bash is one of the most popular shells and command languages for virtual servers as well as physical Linux-based servers. With bash scripting and arrays, users can automate their work and save hundreds of hours manually doing tasks.

In this tutorial, we’ve covered the majority of array operations:

We also provided some examples that you can use when tinkering around with bash. If you have any questions or comments, leave them below.

How to Use Bash Arrays FAQ

Bash array vs string.

A bash array stores a set of elements – for example, a set of numbers along with a list of words. On the other hand, a string can be considered an array, but it can only store characters and nothing else.

Bash Array vs List

A bash list is a sequence of one or more pipelines separated by one of the operators. As such, lists are not related to arrays. However, indexed arrays are sometimes referred to as lists.

What Do {} Mean in Bash?

Curly braces without a $ sign are considered brace expansions and are used to create arbitrary strings. You can use braces to build arrays. For example, echo {0..100} will print numbers from zero to 100.

Is Bash a Language?

Yes, bash is a command language. It is used to either automate tasks or run commands on the command line. Compared to most programming languages, the bash command language is easier to learn, and the syntax is relatively straightforward.

How to Echo a Bash Array?

To echo an array, use the format echo ${Array[0]} . Array is your array name, and 0 is the index or the key if you are echoing an associative array. You can also use @ or * symbols instead of an index to print the entire array.

Ignas takes great satisfaction in helping people tackle even the most complex technical issues. His current goal is to write easy-to-follow articles so that these issues will not happen at all. During his free time, Ignas likes to play video games and fix up things around his house.

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A Complete Guide on How To Use Bash Arrays

The Bash array variables come in two flavors, the one-dimensional indexed arrays , and the associative arrays . The indexed arrays are sometimes called lists and the associative arrays are sometimes called dictionaries or hash tables . The support for Bash Arrays simplifies heavily how you can write your shell scripts to support more complex logic or to safely preserve field separation.

This guide covers the standard bash array operations and how to declare ( set ), append , iterate over ( loop ), check ( test ), access ( get ), and delete ( unset ) a value in an indexed bash array and an associative bash array . The detailed examples include how to sort and shuffle arrays.

👉 Many fixes and improvements have been made with Bash version 5, read more details with the post What’s New in GNU Bash 5?

Difference between Bash Indexed Arrays and Associative Arrays

How to declare a bash array.

Arrays in Bash are one-dimensional array variables. The declare shell builtin is used to declare array variables and give them attributes using the -a and -A options. Note that there is no upper limit (maximum) on the size (length) of a Bash array and the values in an Indexed Array and an Associative Array can be any strings or numbers, with the null string being a valid value.

👉 Remember that the null string is a zero-length string, which is an empty string. This is not to be confused with the bash null command which has a completely different meaning and purpose.

Bash Indexed Array (ordered lists)

You can create an Indexed Array on the fly in Bash using compound assignment or by using the builtin command declare . The += operator allows you to append a value to an indexed Bash array.

With the declare built-in command and the lowercase “ -a ” option, you would simply do the following:

Bash Associative Array (dictionaries, hash table, or key/value pair)

You cannot create an associative array on the fly in Bash. You can only use the declare built-in command with the uppercase “ -A ” option. The += operator allows you to append one or multiple key/value to an associative Bash array.

⚠️ Do not confuse -a (lowercase) with -A (uppercase). It would silently fail. Indeed, declaring an Indexed array will accept subscript but will ignore it and treat the rest of your declaration as an Indexed Array, not an Associative Array.

👉 Make sure to properly follow the array syntax and enclose the subscript in square brackets [] to avoid the " Bash Error: must use subscript when assigning associative array " .

When to use double quotes with Bash Arrays?

A great benefit of using Bash Arrays is to preserve field separation. Though, to keep that behavior, you must use double quotes as necessary. In absence of quoting, Bash will split the input into a list of words based on the $IFS value which by default contain spaces and tabs.

Array Operations

How to iterate over a bash array (loop).

As discussed above, you can access all the values of a Bash array using the * (asterisk) notation. Though, to iterate through all the array values you should use the @ (at) notation instead.

How to get the Key/Value pair of a Bash Array? (Obtain Keys or Indices)

When looping over a Bash array it’s often useful to access the keys of the array separately of the values. This can be done by using the ! (bang) notation.

How to get a Bash Array size? (Array length)

Another useful aspect of manipulating Bash Arrays is to be able to get the total count of all the elements in an array. You can get the length (i.e. size) of an Array variable with the # (hashtag) notation.

How to remove a key from a Bash Array or delete the full array? (delete)

The unset bash builtin command is used to unset (delete or remove) any values and attributes from a shell variable or function. This means that you can simply use it to delete a Bash array in full or only remove part of it by specifying the key. unset take the variable name as an argument, so don’t forget to remove the $ (dollar) sign in front of the variable name of your array. See the complete example below.

Detailed Examples & FAQ

How to shuffle the elements of an array in a shell script.

There are two reasonable options to shuffle the elements of a bash array in a shell script. First, you can either use the external command-line tool shuf that comes with the GNU coreutils, or sort -R in older coreutils versions. Second, you can use a native bash implementation with only shell builtin and a randomization function. Both methods presented below assume the use of an indexed array, it will not work with associative arrays.

The shuf command line generates random permutations from a file or the standard input. By using the -e option, shuf would treat each argument as a separate input line. Do not forget to use the double-quote otherwise elements with whitespaces will be split . Once the array is shuffled we can reassign its new value to the same variable.

How to sort the elements of an Array in a shell script?

How to get a subset of an array.

The shell parameter expansions works on arrays which means that you can use the substring Expansion ${string:<start>:<count>} notation to get a subset of an array in bash. Example: ${myArray[@]:2:3} .

The notation can be use with optional <start> and <count> parameters. The ${myArray[@]} notation is equivalent to ${myArray[@]:0} .

How to check if a Bash Array is empty?

How to check if a bash array contains a value.

In order to look for an exact match, your regex pattern needs to add extra space before and after the value like (^|[[:space:]])"VALUE"($|[[:space:]]) .

With the Bash Associative Arrays, you can extend the solution to test values with [[ -z "${myArray[$value]}" ]] .

How to store each line of a file into an indexed array?

The easiest and safest way to read a file into a bash array is to use the mapfile builtin which read lines from the standard input. When no array variable name is provided to the mapfile command, the input will be stored into the $MAPFILE variable. Note that the mapfile command will split by default on newlines character but will preserve it in the array values, you can remove the trailing delimiter using the -t option and change the delimiter using the -d option.

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Full Guide to Bash Arrays
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How to Use Arrays in Bash Shell Scripts

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COMMENTS

How can I assign a value to an array in Bash?
... assign the entire file to an array $ echo "${some_array[2]}" # Get the third element of the array c $ echo "${#some_array[@]}" # Get the
Assigning array values to variable [closed]
in bash you cannot operate on variables like you do in any other language. however this page may help you.
10.2. Array variables
Each value is then in the form of [indexnumber=]string. The index number is optional. If it is supplied, that index is assigned to it; otherwise the index of
You don't know Bash: An introduction to Bash arrays
${#arr[@]}, Calculate array size ; arr[0]=3, Overwrite 1st element ; arr+=(4), Append value(s) ; str=$(ls), Save ls output as a string.
How to use bash array in a shell script
Declare, in bash, it's used to set variables and attributes. In this case, since we provided the -a option, an indexed array has been created
Update Element of an Array in Bash
To update element of an array in Bash, access the element using array variable and index, and assign a new value to this element using assignment operator.
Bash Array
How to Declare an Array in Bash · Give your array a name · Follow that variable name with an equal sign. The equal sign should not have any spaces
Assigning values to an array
The way I've been using arrays currently have been: pre { overflow:scroll; margin:2px; padding:15px; border:3px inset; margin-right:10px; } Code: #!
Learning Bash Arrays
A bash array is a variable containing a few elements or strings. ... above example will assign the car element of the array to the 0 index.
A Complete Guide on How To Use Bash Arrays
Bash Associative Array (dictionaries, hash table, or key/value pair) ... You cannot create an associative array on the fly in Bash. You can only use the declare