使用awk或sed命令循环从文件中打印大量特定行

Question

I've got this big txt file with ID names. It has 2500 lines, one column. Let's call it file.txt

H3430
H3467
H9805

Also, I've got another file, index.txt, which has 390 numbers:

1
4
9
13
15

Those numbers are the number of lines (of IDs) I have to extract from file.txt. I need to generate another file, newfile.txt let's call it, with only the 390 IDs that are in the specific lines that index.txt demands (the first ID of the list, the fourth, the ninth, and so on).

So, I tried to do the following loop, but it didn't work.

num=$'index.txt'
for i in num
do
awk 'NR==i' "file.txt" > newfile.txt
done

I'm a noob regarding this matters... so, I need some help. Even if it is with my loop or with a new solution suggested by you. Thank you :)

Answer 1

Lets create an example file that simulates your 2500 line file with seq :

$ seq 2500 > /tmp/2500

And use the example you have for the line numbers to print in a file called 390:

$ echo "1
4
9
13
15" > /tmp/390

You can print line N in the file 2500 by reading the line numbers into an array and printing the lines if in that array:

$ awk 'NR==FNR{ a[$1]++; next} a[FNR]' /tmp/390 /tmp/2500

You can also use a sed command file:

$ sed 's/$/p/' /tmp/390 > /tmp/sed_cmd
$ sed -n -f /tmp/sed_cmd /tmp/2500

With GNU sed, you can do sed 's/$/p/' /tmp/390 | sed -n -f - /tmp/2500 sed 's/$/p/' /tmp/390 | sed -n -f - /tmp/2500 but that does not work on OS X :-(

You can do this tho:

$ sed -n -f <(sed 's/$/p/' /tmp/390) /tmp/2500

Answer 2

You can read the index.txt file in to a map and then compare it with the line number of file.txt. Redirect the output to another file.

awk 'NR==FNR{line[$1]; next}(FNR in line){print $1}' index.txt file.txt > newfile.txt

When you work with two files, using FNR is necessary as it gets reset to 1 when new file starts (on the contrary NR will continue to increment).

As Ed Morton suggests in the comments. The command could then be refined to further remove {print $1} since awk prints by default on truth.

awk 'NR==FNR{line[$1]; next} FNR in line' index.txt file.txt > newfile.txt

Answer 3

If index.txt is sorted, we could walk file.txt in order.
That reduces the number of actions to the very minimum (faster script):

awk 'BEGIN
     {  indexfile="index.txt"
        if ( (getline ind < indexfile) <= 0)
             { printf("Empty %s\n; exiting",indexfile);exit }
     }
     {  if ( FNR <  ind ) next
        if ( FNR == ind ) printf("%s %s\n",ind,$0)
        if ( (getline ind < indexfile) <= 0) {exit}
     }' file.txt

If the file is not actually sorted, get it quickly sorted with sort:

sort -n index.txt > temp.index.txt
rm index.txt
mv temp.index.txt index.txt