Read numbers from control file and Extract matching line numbers from the data file

Question

I have a control file - cntl.txt

2
3
5

Data file - data.txt

red
blue
yellow
green
violet
orange

I need to reading the lines matching from control file, here the output expected is:

blue
yellow 
violet

Answer 1

Example of a very inefficient solution:

for i in $(<control.txt); do awk -v c=$i 'NR~c{ print $0 }' data.txt; done;

I report also a good solution I learned tonight:

awk 'FNR==NR{ z[$0]++;next }; FNR in z' control.txt data.txt

Answer 2

Using only POSIX specified features of Sed:

sed -n -e "$(sed '/./s/$/p/' cntl.txt)" data.txt

Of course if your cntl.txt file has lines besides numbers, you may get an error. But if it has empty lines these will be handled correctly (i.e. they will not affect the output).

Answer 3

Try this:

join <(nl data.txt|sort -k1b,1) <(cat cntl.txt|sort -k1b,1) | sort -nk1,1 | cut -d' ' -f2-

nl - will enumerate lines for you

 1  red
 2  blue
 3  yellow
 4  green
 5  violet
 6  orange

| sort -k1b,1 - will sort them by the line number (first field), lexicographically

cat cntl.txt| sort -k1b,1 - will sort the control file in the same order

2
3
5

join <() <() - will join the sorted (and numbered) "data" with the sorted "control", on the first field (i.e. line number)

2 blue
3 yellow
5 violet

|sort -nk1,1 - will re-sort the results numerically (to put the lines back in order)

| cut -d' ' -f2- - will drop the line number field

blue
yellow
violet

Answer 4

With sed only:

sed -n "$(sed -e 's/$/p;/' < cntl.txt)" data.txt

Answer 5

Another possible solution:

IFS=$'\n' read -d '' -r -a colors < 'data.txt'; unset IFS;

for i in $(<cntl.txt); do
        echo ${colors[i-1]} 
done

IFS line sets up internal file separator as newline and inserts each line from data.txt into array. After that you loop through lines in cntl.txt and print array elements with given index from it (minus 1 because you start your data.txt from 1, not from 0, otherwise it would be unnecessary).