Using awk to pull specific lines from a file

Question

I have two files, one file is my data, and the other file is a list of line numbers that I want to extract from my data file. Can I use awk to read in my lines file, and the

Answer 1

One way with sed:

sed 's/$/p/' linesfile | sed -n -f - datafile

You can use the same trick with awk:

sed 's/^/NR==/' linesfile | awk -f - datafile

Edit - Huge files alternative

With regards to huge number of lines it is not prudent to keep whole files in memory. The solution in that case can be to sort the numbers-file and read one line at a time. The following has been tested with GNU awk:

extract.awk

BEGIN {
  getline n < linesfile
  if(length(ERRNO)) {
    print "Unable to open linesfile '" linesfile "': " ERRNO > "/dev/stderr"
    exit
  }
}

NR == n { 
  print
  if(!(getline n < linesfile)) {
    if(length(ERRNO))
      print "Unable to open linesfile '" linesfile "': " ERRNO > "/dev/stderr"
    exit
  }
}

Run it like this:

awk -v linesfile=$linesfile -f extract.awk infile

Testing:

echo "2
4
7
8
10
13" | awk -v linesfile=/dev/stdin -f extract.awk <(paste <(seq 50e3) <(seq 50e3 | tac))

Output:

2   49999
4   49997
7   49994
8   49993
10  49991
13  49988