You’ve got a few things going on to be broken down here.

And forgive me if anything I say here seems condescending, it’s not meant to be, I just like to be very explanatory with things like this and to assume the reader may not know anything about anything. (Not as an insult, but simply as a gap in knowledge).

Also, while I’m proficient at Bash, I’m no expert.

LIST=$(ls): Here you’ve stored the output of the ls command to the variable LIST, which gives you a list of items in the given directory, in this case, whichever directory the command is run from. It’s also a good idea to quote the variable assignment like this: "$(ls)".

for i in $LIST;: This is the first part of the for loop statement, which is an iterator, meaning, it will loop or iterate over every item in the given variable/parameter/group of iterable items.

The i here, as you said could be anything. You could say for file in $LIST; or for item in $LIST;. It doesn’t matter, because it’s just a variable name that you are using in the first part of the for statement.

So what bash will do with this is loop over the list, and take each item in the list, and assign it to the variable i, which will allow you do act upon that single item by calling the variable i in some other commands.

do echo "I found one!";: This is the next part of the for loop, which is the instruction set to be executed inside the for loop. Here is where you can act upon the items in your list that have been assigned to the variable i.

In your case, you’re just printing a statement to stdout (standard out), that says, “I found one!”

It’s like saying, for each item in this list, print “I found one!”

So if there are 20 items in the list, it will print that statement 20 times.

However, maybe you want to print the item itself as part of a statement. So instead of “I found one!”, you could do something like:

do echo "I found $i!"

Which then would print “I found some-filename-or-directory-here!” for each item in your list.

done: Finally, the done statement tells bash that this is the end of the for loop. So any commands after the done statement will only run once the for loop has iterated over all items in the list and executed the commands inside the for loop for each item on the list.

A couple of notes:

The ; is used as a command separator or terminator. So bash knows to first run LIST=$(ls) before it attempts to run whatever the next command might be.

In bash, it’s good practice to always quote your variables like so: for i in "$LIST";. This is to avoid errors for characters that might need escaping like whitespace, backslashes, and other special characters.

With that in mind, if you’re running a command like echo "I found $i!", you don’t need to quote the variable again, because it’s already inside a quote set.

Further, it’s not absolutely necessary, but it can also be a good idea to also enclose all of your variables in {}, so whenever you use a variable, you’d do something like: "${LIST}"

This not only more clearly identifies variables in your bash scripts/commands, but is necessary when using bash’s parameter expansion, which is pretty great.

for loops

Your code executes ls and records the results in a variable. The result is some text, a string of characters. (We call them “strings” and i is now a string variable.) Among the characters in a string variable might be spaces, tabs, or new line characters. I mention this because the special variable IFS is used by for loops, and it contains exactly one space, tab, and new line by default.

When you call for with a string as the input, it splits the string into units by splitting on each character in IFS. That is, it splits the big string into individual parts by splitting at each space, tab and new line. So this creates an array which is what is looped over. Each word in turn is assigned to your looping variable and then the code after the do is executed once per word.

(“word” has a sort of a special meaning here. When I say word, I mostly just mean a string that has no spaces in it. When you read text in English, there are words. They’re strings of characters separated by spaces. But words can also be separated by tabs, new lines, commas, semicolons, or whatever, but not by default when using for! You have to modify IFS to add those characters if you want them to be considered word separators.)

So, if any of the file system entries returned by ls have spaces in them, your loop is going to create more outputs than there are file system entries in the current directory.

For example:

file one.txt
file two.txt
my photo.jpg
notes (final).md
a b c d.txt

That would cause like 12 loops and 12 outputs in your code despite there only being five files.

If you instead overwrite IFS before running your loop, and only assign a single new line to the variable, then your loop will only be over the actual lines of the input text. Like this:

IFS=$'\n'

and then use your exact code above. Using my example of five files, this code will now only produce 5 outputs, not 12.

You can assign whatever characters you want to IFS.

(I have not tested any of this code, or examples.)

Variable names

The loop variable name i is just an identifier. Any valid variable name would work except you can’t use the reserved names like $1, $2 or any keywords as names. Also, there’s no way to escape an identifier. They are just literal names.

You also don’t want to use any built-in variable names or else you’ll overwrite their values for the duration of the current session. Bash will happily let you use them as your looping variable, but the rest of your code might have undesirable results. Variable names like IFS, for instance. :D

https://www.gnu.org/software/bash/manual/bash.html#Looping-Constructs

Anything immediately in the position after for is an assignment of whatever you put there as a temporary variable inside the loop. You can call it whatever you want. The “i” is just used a lot in examples in programming for “item” or “iterate”, but you can literally call it anything. Anything that refers to it later will have a single item from the list in $LIST assigned to it for each run through the loop.