bash 2 variables in a for loop

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Using multiple variables in a for loop

I want to use two variables in a for loop like this (This is for example, I'm not going to execute seq like operations)

Expected output is

1 www.google.com 2 www.yahoo.com

• command-line

3 Answers 3

If i was to just be a number that incremented with each string, you could try a for loop and increment i with each iteration.

For example:

• why did you, use double braces in $((i++)) . I think one braces is enought for this. Please correct me , If i said wrong. –  SuperKrish Nov 5, 2016 at 8:32
• 1 @SuperKrish $(( . . .)) is arithmetic expansion, which is what makes i++ work. With single braces that's $(. . .) command substitution , so i++ won't work there. If you were trying to catch output of another program, say $( df ) for example, you could use single braces. If you need calculation, then you use $(( . . . )) structure –  Sergiy Kolodyazhnyy Nov 5, 2016 at 10:30
• Great! @jake My doubt is clear –  SuperKrish Nov 5, 2016 at 10:47

Lets create variable to point to the file location

FILE="/home/user/myfile"

The file content:

To get the output of:

It can be done by one of the following methods below:

Using counter variable:

Using cat -n (number all output lines)

Using array:

If your file already with line numbers, example:

Loop and split every line to array:

For more info:

How to increment a variable in bash?

Loop Through Array of Strings in Bash Script

Arrays Syntax

Here are some alternative ways, two short and simple ones:

which both output:

and the slightly more complex:

that outputs:

In that second suggestion, I'm using an array named s created with the line s=(xx yy)

The ${#s[@]} syntax is the number of elements in the array, here 2 and the ${s[i-1]} is element at offset i-1 from the beginning of the array, thus ${s[1-1]} is ${s[0]} and then is www.google.com , etc.

• I dont know what is means ${#s[@]} `${s[i-1]}1. Here s is the variable loaded with some values.Can you clear my doubt? –  SuperKrish Nov 5, 2016 at 14:15
• Answer edited with explanations added. –  jlliagre Nov 5, 2016 at 15:26

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How to pass multiple variable to a for loop as a argument in a bash script? [closed]

I am new to linux and i am writing a bash script... In the script, I have 2 variables(inside the varibles contents are there). I am trying to pass these 2 varibales in a same for loop and perform some actions.

But when i pass 2 varibles in a same for loop, am getting error. In the below code. to make it ease I am passing two paramerters but in actual it will vary. I am getting output of these varibales from a command

Below is my code:

Could any one pls help me to pass 2 arguments in the for loop at the same time.

Argument should pass like in the for loop

Below is the error message

Kindly help!

Below is my output

Below is my curl coomand

• Please post the error as well –  cutrightjm May 4, 2020 at 5:33
• @cutrightjm Syntax error in for loop –  clarie May 4, 2020 at 5:36
• It is helpful when you ask the question to include the running of the script and the error. –  cutrightjm May 4, 2020 at 5:40
• 1 you're missing a do after the first for –  pLumo May 4, 2020 at 6:01
• 1 Please edit your question to explain what you want to achieve as well how you think you should do it. Also show where your input data comes from and provide an example of it. There are already two possible answers that have been invalidated because you didn't give us the necessary detail in the question . For example, in your question you're reading data from files but in response to one answer you say the data is in environment variables. There are people who want to help you so make it easy for them to give you a plausible answer. –  roaima May 4, 2020 at 6:52

4 Answers 4

Here's how I'd write it:

It reads the two variables from the heredoc that is supplied at the end.

• In my case I have 100 parametrs and it will vary. Its not possible to give the variables at the end. –  clarie May 4, 2020 at 6:14
• @clarie Beginning, end… What's the difference? Do you mean the values cannot be hardcoded in the script or what? Are they passed via already existing environment variables? What condition are you not telling us? Robin's code works for me. –  Kamil Maciorowski May 4, 2020 at 6:25
• @KamilMaciorowski yes, variables are passed in the existing environment variables. We can't hardcode it –  clarie May 4, 2020 at 6:36
• @KamilMaciorowski I have editied my question –  clarie May 4, 2020 at 6:49

The percent sign question mark %? chops off the / that you didn't seem to want.

Put that in a file named myscript.sh then run it:

Update: Output with printf replaced by echo curl ... :

• I tried the above code...As expected loop are executing twice.But Both times sames valus are coming in. 2019_06/ 100 –  clarie May 4, 2020 at 6:48
• I edited my answer to show the output I get. It is not the same values repeated. –  Robin A. Meade May 4, 2020 at 6:51
• yes when i run as updtaed, in the terminal values are not repaeted. But when i use curl to update in the influxdb.only first value is getting inserted. –  clarie May 4, 2020 at 6:59
• I suggest you echo the curl invocation to the terminal to check it. Put echo before curl . Once you are sure the curl command looks right, remove the echo at the beginning of that line to execute curl for real. You might need to post a separate question about your curl invocation. –  Robin A. Meade May 4, 2020 at 7:05
• I used echo and i also posted the output in the question sction. –  clarie May 4, 2020 at 7:14

Putting multiple values in one variable, using it unquoted and relying on word splitting to separate the values again is not a good practice. Use arrays. You can retrieve values from two or more arrays "in parallel" by using the same index; this should solve your problem.

Prepare two arrays of equal size and loop like this:

Replace echo … with the desired (and adjusted) if … curl … block.

If FILES and FILESIZE are variables through which your script receives data and you cannot change this and you must rely on word splitting, then create arrays from the variables:

The variables are not quoted, word splitting (and filename generation ) occurs at this stage.

• In this case..Size is not fixed..It will vary... I think its not feasible to decleae one by one.. Could u pls help me –  clarie May 4, 2020 at 6:10
• @clarie What do you mean? The loop depends on the length of the file array ( ${#file[@]} ), so it will work for longer arrays as well. –  Kamil Maciorowski May 4, 2020 at 6:16
• Yes both file and filesize will have the same length. and the loop should go through till the end –  clarie May 4, 2020 at 6:16
• @clarie the loop should go through till the end – And it does. Have you tried my code with longer arrays? –  Kamil Maciorowski May 4, 2020 at 6:28
• @clarie See the second part of my answer. –  Kamil Maciorowski May 4, 2020 at 6:53

Often you will want todo S3 stuff in parallel:

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How-To Geek

9 examples of for loops in linux bash scripts.

How to repeat yourself, elegantly.

Quick Links

The for loop, simple for loops, c-like for loops, for loops using word arrays, for loops using associative arrays, iterating over the output of commands, the dazzling for loop.

The versatile Bash for loop does much more than loop around a set number of times. We describe its many variants so you can use them successfully in your own Linux scripts.

All scripting and programming languages have some way of handling loops. A loop is a section of code that you want to have executed repeatedly. Rather than type the same set of instructions into your script , again and again, a loop will repeat one section of code over and over for you.

Related: How to Use Bash If Statements (With 4 Examples)

The Bash  for loop is very flexible. It can work with numbers, words, arrays, command line variables, or the output of other commands. These are used in the header of the loop. The header dictates what the loop is working with---numbers or strings, for example---and what the end condition is that will stop the looping.

The body of the loop contains the code that you want to have repeated. It holds what you want the loop to do. The loop body can contain any valid script command.

A variable called the loop counter or iterator is used to step through a range of values or a list of data items. For each loop, the iterator takes on the value of the next number, string, or whatever data type the loop is iterating over. This allows the loop to work with the values of each of the data items in turn, or even in some cases to manipulate the data items themselves.

If you're looking to write your first for loop, these simple examples will get you started.

for Loops using Numerical Lists

You can run a for loop on the command line. This command creates and executes a simple for loop. The iterator is a variable called i . We're going to assign i to be each of the values in the list of numbers, in turn. The body of the loop is going to print that value to the terminal window. The condition that ends this loop is when i has iterated across the entire list of numbers.

for i in 1 2 3 4 5; do echo $i; done

It's important to note here that the variable i is increased by one each time the loop spins round,  but that's because the list of numbers goes up by one each time.

This list of numbers starts at 3 and goes up in steps of two, then arbitrarily leaps to 44.

for i in 3 5 7 9 11 44; do echo $i; done

It makes no difference to the for loop. It starts at one end of the list and uses each value in turn, until all the values in the list have been used.

Nor do the numbers need to be in ascending order. They can be in any order.

for i in 3 43 44 11 9; do echo $i; done

for Loops Using Word Lists

We can just as easily do the same with words. Copy the text of the script into an editor and save it as "word-list.sh."

#!/bin/bash

for word in This is a sequence of words

  echo $word

You'll need to use chmod to make the script executable, and any other script you copy out of this article. Just substitute the name of the script each time you use the chmod command.

chmod +x word-list.sh

Let's run the script.

./word-list.sh

Just as it did with the numbers, the iterator---in this example, the variable word ---works its way through the list of data items until it reaches the end of the list. The loop body accesses the value in the word variable and so each word in the list gets processed.

for Loops with Number Ranges

If you wanted a for loop to run 100 times it would be a pretty tiresome affair to have to type in a sequence of 100 numbers in the loop header. Number ranges let you specify the first and last number only.

This script is "number-range.sh."

for i in {1..10}

  echo "Loop spin:" $i

The number range is defined within curly brackets " {} " with two periods " .. " separating the numbers that start and end the range. Make sure you don't include any whitespace in the range definition.

This is how it runs:

./number-range.sh

You can include another number that defines the step size the iterator should use to walk through the numbers in the range. This script, "number-range2.sh" will use a range of 0 to 32, and a step size of 4.

for i in {0..32..4}

The iterator steps through the number range in jumps of four.

./number-range2.sh

for Loops Using Filenames

Because we can process lists of words, we can get our scripts to work with filenames. This script is called "filenames.sh."

for file in word-list.sh number-range.sh number-range2.sh filenames.sh

  ls -lh "$file"

It would be pretty pointless to have a script that only does what ls can do, but it does demonstrate how to access filenames inside the loop body.

./filenames.sh

In a similar way to using the number range, we can use a file pattern in the loop header to specify the files we want to process. This avoids a lot of typing and means we don't need to know in advance the names of the files.

This script is called "filenames2.sh." We've replaced the list of filenames with the filename pattern "*.sh" to have the script report on all script files in the current directory.

for file in *.sh

Here's the output.

./filenames2.sh

for Loops Using Command Line Parameters

We can add some more flexibility by passing in the filename pattern on the command line. The $* variable represents all of the command line parameters passed to the script.

This is "filenames3.sh."

for file in $*

We'll ask for filenames that begin with "n" and have an SH extension.

./filenames3.sh n*.sh

We can also pass in more than one pattern at a time.

./filenames3.sh n*.sh .bashrc

The iterator variable file takes on the value of each of the command line parameters. Filename patterns are expanded, and all of the filenames are processed in the loop body.

Related: How to Work with Variables in Bash

Bash supports the classic three-term for loop, such as those found in the C programming language. They're called three-term for loops because there are three terms in the loop header.

• The initial value of the loop iterator.
• The test for whether the loop continues or ends.
• The incrementing---or decrementing---of the iterator.

This script is "c-like.sh."

The iterator I is set to 1 at the start of the loop, and the loop will run for as long as the statement " i<=10 " is true. As soon as i reaches 11, the for loop will stop. The iterator is being increased by one, every revolution of the loop.

for (( i=1; i<=10; i++ ))

  echo "Loop number:" $i

Let's run this script.

./c-like.sh

The C-like for loop permits the easy creation of for loops that have slightly odd requirements. This loop starts at 15, and counts backward in steps of 3. This is "c-like2.sh"

for (( i=15; i>0; i-=3 ))

When we run it, it should jump backward in steps of three.

./c-like2.sh

Infinite for Loops

You can also use this format of for loop to create an infinite loop. All you need do is remove all of the elements from the loop header, like this. This is "infinite.sh."

for (( ; ; ))

  echo "Press Ctrl+C to stop..."

You'll need to hit Ctrl+C to stop the loop.

./infinite.sh

We can easily iterate through an array of words. We need to provide the name of the array in the loop header, and the iterator will walk through all entries in the array. This is "word-array.sh."

distributions=("Ubuntu Fedora Manjaro Arch EndeavourOS Garuda")

for distro in $distributions

  echo $distro

All the distributions are listed for us.

./word-array.sh

The continue Command

If you want the loop to step over a particular entry, test whether the iterator matches that entry and use the continue command. The continue command abandons the current spin of the loop. It increments the iterator and starts the next spin of the loop---assuming the entry you want to skip over isn't the last item in the list.

This is "word-array2.sh." It steps over the "Arch" array entry but processes all other array members.

  if [[ "$distro" == "Arch" ]] ;

    continue

"Arch" doesn't appear in the terminal window.

./word-array2.sh

The break Command

The break command breaks out of the loop and prevents any more processing.

This is "word-array3.sh." It's the same as the previous script with continue replaced by break .

When the iterator contains "Arch" the for loop abandons any more processing.

./word-array3.sh

In Bash 4 and higher, associative arrays allow you to create lists of key-value pairs that can be searched by the key or by the value. Because of the two-way relationship between the key and the value, they're also called data dictionaries.

Related: What Are Bash Dictionaries on Linux, and How Do You Use Them?

We can iterate through an associative array using a for loop. This script is "associative.sh." It defines an associative array with four entries in it, one for each of "dog", "cat", "robin" , and "human." These are the keys. The values are the (default) number of legs they each have.

declare -A animals=( [dog]=Four-legged [cat]=Four-legged [robin]=Two-legged [human]=Two-legged )

for legs in ${!animals[@]}

  if [ ${animals[$legs]} == "Two-legged" ]; then

    echo ${legs}

The iterator is called legs . Note that the loop header contains an " ! " exclamation point. This is not acting as the logical NOT operator, it's part of the associative array syntax. It is required to search through the array.

The body of the loop performs a string comparison test. If the value of the array member is "Two-legged", it prints the key value to the terminal window. When we run it, the script prints the two-legged creatures.

./associative.sh

If you have a command or sequence of commands that produce a list of something, such as filenames, you can iterate through them with a for loop. You need to watch out for unexpected filename expansions, but in simple cases it is fine.

This script is "command.sh." it uses ls and wc to provide a sorted list of script file names, together with their line, word, and byte counts.

for i in $(ls *.sh | sort);

  echo $(wc $i)

When we run it we get the statistics for each file, with the files listed in alphabetical order.

./command.sh

The for loop is a versatile and easily understood scripting tool. But as flexible as it is, don't forget that other loops exist for a reason. Don't be dazzled into thinking the for loop is all you'll ever need.

Related: How to Process a File Line by Line in a Linux Bash Script

The while loop, for example, is much better suited for certain things than the for loop, such as reading lines from a file .

Writing good scripts means using the most suitable tool for the task at hand.  The for loop is a great tool to have in your toolbox of tricks.

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Bash: Two for loops at once?

I am not sure how to do this at all.

I have two text files, FILE1 and FILE2 .

I would like to run a for loop for each file at the same time and display the contents next to each other.

For example,

for $i in $(cat FILE1); do echo $i; done

for $j in $(cat FILE2); do echo $j; done

I would like to combine these two commands, so I can run both files at the same time and have an output like $i $j

• 3 That is precisely what paste does. –  rici Aug 18, 2018 at 5:25
• 1 If you don't want paste (though I guess you do) maybe look at Looping over pairs of values in Bash –  tripleee Aug 18, 2018 at 5:51

4 Answers 4

Use the paste command

paste FILE1 FILE2

Details for paste command

Another resource

You can do this if they have the same number of lines.

You can extend it to what you want for unequal number of lines.

• 3 Solution 2 is horribly inefficient, and should never be used. –  chepner Aug 18, 2018 at 13:13

You shouldn't be using for loops at all; see Bash FAQ 001 . Instead, use two read commands in a single while loop.

Each read command reads from a separate file descriptor. In this version, the loop will exit when the shorter of the two files is exhausted.

There are two different questions being asked here. Other answers address the question of how to display the contents of the file in 2 columns. Running two loops simultaneously (which is the wrong way to address the first problem) can be done by running them each asynchronously: for i in ${seqi?}; do ${cmdi?}; done & for j in ${seqj?}; do ${cmdj?}; done & wait

Although you could also implement paste -d ' ' file1 file2 with something like:

• I just noticed that @chepner gave nearly this same solution, but I'll not delete mine since it addresses the issue of the files having different line lenghts. But this is totally a hack and should never be done. –  William Pursell Aug 18, 2018 at 13:31

Another option, in bash v4+ is to read the two files into 2 arrays, then echo the array elements side-by-side:

Or change the echo to printf if you want the columns to line up:

I'm not suggesting you do this if your files are 100s of megabytes.

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Command line wizardry, part two: variables and loops in bash, learn to process thousands of items reliably and repeatably in this installment..

Jim Salter - Sep 20, 2021 11:15 am UTC

In our first  tutorial on command line wizardry, we covered simple redirection and the basics of sed, awk, and grep. Today, we're going to introduce the concepts of simple variable substitution and loops—again, with a specific focus on the Bash command line itself, rather than Bash  scripting.

If you need to write a script for repeated use—particularly one with significant logical branching and evaluation—I strongly recommend a "real language" instead of Bash. Luckily, there are plenty of options. I'm personally a big fan of Perl, in part because it's available on pretty much any *nix system you'll ever encounter. Others might reasonably choose, say, Python or Go instead, and I wouldn't judge.

The real point is that we're focusing on the command line itself. Everything below is something you can easily learn to think in and use in real time with a little practice.

Setting and getting values in Bash

Bash handles variables a bit oddly compared to any of the "real" languages I've used. In particular, you must refer to the same variable with different syntax when setting its value versus when retrieving it.

Let's take a look at a very simple example:

If you've never worked with variables in Bash before, you might think there's a typo in the above example. We set the value of hello  but read back its value as $hello . That's not a mistake—you don't use the leading  $  when defining/setting a variable, but you must use it when reading from the same variable later.

Destroying variables

If we need to clear that variable, we can use the unset  command—but, again, we must refer to hello  instead of $hello :

Thinking about the error messages we get when trying incorrectly to unset $hello  should make the problem clear. When we unset $hello , we're passing the  value of hello  to unset  rather than passing the variable name itself. That means exactly what you think it does:

Although the switch back and forth between referencing hello  and $hello  is deeply unsettling if you're only familiar with "real" languages, Bash is at least relatively consistent about when it uses each style. With a little practice, you'll get used to this—even if you never feel truly  comfortable about it.

Variable scope

Before we move on from the basics of variables, we need to talk about variable scope. Basically, a Bash variable is limited in scope to only the current process. It won't pass your variables on to any child processes you initiate. We can see this in action most easily by calling  bash   from bash :

In our original Bash session, we set the variable hello  to equal "Hello, world!" and access it successfully with the echo  command. But when we start a new bash  session, hello  is not passed on to that child session. So when we again try to echo $hello , we get nothing. But after exiting the child shell, we can once again successfully echo $hello  and receive the value we originally set.

If you need to carry variables set in one session into a child process, you need to use the export  command:

As you can see, export  successfully flagged our variable hello  for passing down to child processes—in this case, another instance of bash . But the same technique works, in the same way, for calling  any child process that references environment variables. We can see this in action by writing a very small Perl script:

Just like our earlier example of a child session of bash , our Perl script can't read our hello  environment variable unless we first export  it.

The last thing we'll say about export  is that it can be used to set the value of the variable it's exporting, in one step:

Now that we understand how to set and read values of variables on the command line, let's move on to a very useful operator: $() .

Capturing command output with $()

The $()  command substitution operator captures the output of a command. Sounds simple enough, right? In the above example, we demonstrate that the output of the command echo test is, of course, "test"—so when we echo $(echo test)  we get the same result.

Backticks can provide the same result—eg, echo `echo test` —but $()  is generally preferred, since unlike backticks, the  $()  operator can be nested.

We can take this a step further and instead assign the output of a command into a variable:

Finally, we can combine this with the lessons from our first tutorial and strip the first column from du 's output to assign to our variable:

At this point, we understand how variables and the $() operator work. So let's talk about simple looping structures.

For loops in Bash

The above example concisely demonstrates the way a simple for loop works in Bash. It iterates through a supplied sequence of items, assigning the current item from the list to a loop variable. We used semicolons to separate the for loop itself, the do command that marks the interior of the loop, and the done , which lets us know the loop is over. Alternatively, we could also enter them as technically separate lines.

In this example, the leading angle brackets aren't something you type—they're a prompt supplied by Bash itself. This lets you know you're still inside a loop despite having hit Enter . And that's important, because you can have as many commands as you'd like inside the loop itself!

As you can see, we can string as many commands as we'd like together inside our for loop. Within the loop, each command may refer to the loop variable if it likes—or it can ignore the loop variable entirely.

So far, we've only looked at numbers in sequences, but the for loop doesn't care about that a bit.  Any sequence of items, in any order, will work:

Finally, you don't have to supply the list with a variable; you can also embed it directly in the loop... but be careful about how you use quotes, because bash considers a quoted "list" to be a single item:

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Introduction to Linux Bash programming: 5 `for` loop tips

%t min read | by Nathan Lager (Sudoer, Red Hat) , Ricardo Gerardi (Editorial Team, Sudoer alumni, Red Hat)

Every sysadmin probably has some skill they've learned over the years that they can point at and say, "That changed my world." That skill, or that bit of information, or that technique just changed how I do things. For many of us, that thing is looping in Bash. There are other approaches to automation that are certainly more robust or scalable. Most of them do not compare to the simplicity and ready usability of the for loop, though.

If you want to automate the configuration of thousands of systems, you should probably use Ansible. However, if you're trying to rename a thousand files, or execute the same command several times, then the for loop is definitely the right tool for the job.

[ You might also like:  Mastering loops with Jinja templates in Ansible ]

If you already have a programming or scripting background, you're probably familiar with what for loops do. If you're not, I'll try to break it down in plain English for you.

The basic concept is: FOR a given set of items, DO a thing.

The given set of items can be a literal set of objects or anything that Bash can extrapolate to a list. For example, text pulled from a file, the output of another Bash command, or parameters passed via the command line. Converting this loop structure into a Bash script is also trivial. In this article, we show you some examples of how a for loop can make you look like a command line hero, and then we take some of those examples and put them inside a more structured Bash script.

Basic structure of the for loop

First, let's talk about the basic structure of a for loop, and then we'll get into some examples.

The basic syntax of a for loop is:

The variable name will be the variable you specify in the do section and will contain the item in the loop that you're on.

The list of items can be anything that returns a space or newline-separated list.

Here's an example:

That's about as simple as it gets and there isn't a whole lot going on there, but it gets you started. The variable $name will contain the item in the list that the loop is currently operating on, and once the command (or commands) in the do section are carried out, the loop will move to the next item. You can also perform more than one action per loop. Anything between do  and done  will be executed. New commands just need a ; delimiting them.

Now for some real examples.

Renaming files

This loop takes the output of the Bash command ls *.pdf and performs an action on each returned file name. In this case, we're adding today's date to the end of the file name (but before the file extension).

To illustrate, run this loop in a directory containing these files:

The files will be renamed like this:

In a directory with hundreds of files, this loop saves you a considerable amount of time in renaming all of them.

Extrapolating lists of items

Imagine that you have a file that you want to scp to several servers. Remember that you can combine the for loop with other Bash features, such as shell expansion, which allows Bash to expand a list of items that are in a series. This can work for letters and numbers. For example:

Assuming your servers are named in some sort of pattern like, web0 , web1 , web2 , web3 , you can have Bash iterate the series of numbers like this:

This will iterate through web0 , web1 , web2 , web3 , and so forth, executing your command on each item.

You can also define a few iterations. For example:

You can also combine iterations. Imagine that you have two data centers, one in the United States, another in Canada, and the server's naming convention identifies which data center a server HA pair lived in. For example, web-us-0 would be the first web server in the US data center, while web-ca-0 would be web 0's counterpart in the CA data center. To execute something on both systems, you can use a sequence like this:

In case your server names are not easy to iterate through, you can provide a list of names to the for loop:

You can also combine some of these ideas for more complex use cases. For example, imagine that you want to copy a list of files to your web servers that follow the numbered naming convention you used in the previous example.

You can accomplish that by iterating a second list based on your first list through nested loops. This gets a little hard to follow when you're doing it as a one-liner, but it can definitely be done. Your nested for loop gets executed on every iteration of the parent for loop. Be sure to specify different variable names for each loop.

To copy the list of files file1.txt, file2.txt, and file3.txt to the web servers, use this nested loop:

More creative renaming

There might be other ways to get this done, but remember, this is just an example of things you can do with a for loop. What if you have a mountain of files named something like FILE002.txt , and you want to replace FILE with something like TEXT . Remember that in addition to Bash itself, you also have other open source tools at your disposal, like sed , grep , and more. You can combine those tools with the for loop, like this:

Adding a for loop to a Bash script

Running for loops directly on the command line is great and saves you a considerable amount of time for some tasks. In addition, you can include for loops as part of your Bash scripts for increased power, readability, and flexibility.

For example, you can add the nested loop example to a Bash script to improve its readability, like this:

When you save and execute this script, the result is the same as running the nested loop example above, but it's more readable, plus it's easier to change and maintain.

You can also increase the flexibility and reusability of your for loops by including them in Bash scripts that allow parameter input. For example, to rename files like the example More creative renaming above allowing the user to specify the name suffix, use this script:

In this script, the user provides the source file's prefix as the first parameter, the file suffix as the second, and the new prefix as the third parameter. For example, to rename all files starting with FILE , of type .txt to TEXT , execute the script like this :

This is similar to the original example, but now your users can specify other parameters to change the script behavior. For example, to rename all files now starting with TEXT to NEW , use the following:

[ A free course for you: Virtualization and Infrastructure Migration Technical Overview . ] 

Hopefully, these examples have demonstrated the power of a for loop at the Bash command line. You really can save a lot of time and perform tasks in a less error-prone way with loops. Just be careful. Your loops will do what you ask them to, even if you ask them to do something destructive by accident, like creating (or deleting) logical volumes or virtual disks.

We hope that Bash for loops change your world the same way they changed ours.

Check out these related articles on Enable Sysadmin

Nathan Lager

Nate is a Technical Account Manager with Red Hat and an experienced sysadmin with 20 years in the industry.  He first encountered Linux (Red Hat 5.0) as a teenager, after deciding that software licensing was too expensive for a kid with no income, in the late 90’s.  Since then he’s run More about me

Ricardo Gerardi

Ricardo Gerardi is Technical Community Advocate for Enable Sysadmin and Enable Architect. He was previously a senior consultant at Red Hat Canada, where he specialized in IT automation with Ansible and OpenShift.  More about me

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