How to call i in a for loop?

Question

So, this question seems a-specific. It is, because I'm not a BASH-programmer, rather a Biologist-turned-writing-some-useful-scripts-for-my-daily-work-scripter. Anyway. Say, I have a for loop, like so:

for CHR $(seq 1 22); do
    echo "Processing chromosome ${CHR}";
done

I used to write `seq 1 22` but now I've learned to write $(seq 1 22). Clearly there is a difference in terms of the way you write it. But what is the difference in terms in computer language and interpretation? Can someone explain that to me?

The other thing I learned by simply doing on the command line on our computer cluster, was to call "i" differently. I used to do: $CHR. But when I'd have a file name sometext_chr to which I'd like to add the number (sometext_chr$CHR) that wouldn't work. What does work is sometext_chr${CHR}. Why is that? Can someone help me explain the difference?

Again, I know the question is a bit a-specific - I simply didn't know how to otherwise frame it - but I hope someone can teach me the differences.

Thanks and best!

Sander

Also, please try to limit yourself to one question per post. I'm pretty sure that all of these have already been asked and answered before, although maybe not in this combination. — 5gon12eder, Dec 15 '15 at 16:31
http://stackoverflow.com/q/4708549/2088135 is one possible duplicate — Tom Fenech, Dec 15 '15 at 16:31
I'm voting to close this question as a duplicate of the question found by @TomFenech because that particular sub-question is the only one that is focused enough to be acceptable as a good question. — 5gon12eder, Dec 15 '15 at 16:36

score 1 · Accepted Answer · answered Dec 15 '15 at 16:50

1

The $(...) can be nested easily, as the parentheses clearly indicate where an expression starts and ends. Using `, nesting is not so simple, as the start and end symbols are the same.

Your second example is probably from memory, because it's incorrect. sometext$chr and sometext${chr} would both work the same way. Perhaps what you really meant was a situation like this:

$chr_sometext
${chr}_sometext

The key point here is that _ is a valid character in a variable name. As a result, $chr_sometext is interpreter as the value of the variable chr_sometext. In ${chr}_sometext the variable is clearly chr, and the _sometext that follows it is a literal string value. Just like if you wrote $chrsometext you wouldn't assume that the chr is somehow special. This is the reason you have to add the clarifying braces.

answered Dec 15 '15 at 16:50

janos

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Thanks for clear answer. Makes sense. This _ is exactly the problem. I'm now changing the scripts I'm using to the curly-brackets method. Thanks again. – Sander W. van der Laan Dec 17 '15 at 00:05
I don't have enough credits to up vote your answer...newbie that I am... :-( – Sander W. van der Laan Dec 17 '15 at 06:30
I know, no worries. Thanks for accepting! – janos Dec 17 '15 at 06:45

How to call i in a for loop?

1 Answers1