Bash for loop redirect output to variable filename

Question

In the following bash loop, I want to redirect content into a variable filename.

for i in {1..22};do echo $i > $i_output.txt; done

Unfortunately the files are not created. What I originally want to do is to grep on chromosomes in a file and create a file per chromosome. But this is less testable:

for i in {1..22}; grep -E "[a-z]+[0-9]+\t$i\t" rawdata.txt > $i_chr.txt;done

How can we redirect output in a for loop to a variable filename?

Answer 1

You have to use curly braces or double-quotes around your variable name. The following loop works.
Otherwise the shell will try to expand the variable $i_output.

for i in {1..22};do echo ${i} > ${i}_output.txt; done
for i in {1..22};do echo "$i" > "$i"_output.txt; done

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In your case curly braces might be the better choise, as you already use double-quotes and your shell will have a problem with sorting out the inner and outer double-quotes.

for i in {1..22}; grep -E "[a-z]+[0-9]+\t${i}\t" rawdata.txt > ${i}_chr.txt;done