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I'd use awk, but not store the whole content of L.txt in memory and do unnecessary hash look ups ;-). list=L.txt file=F.txt LIST="$list" awk ' function nextline() { if ((getline n < list) <=0) exit } BEGIN{ list = ENVIRON["LIST"] nextline() } NR == n { print nextline() }' < "$file"


grep -n | sort | sed | cut ( export LC_ALL=C grep -n '' | sort -t: -nmk1,1 ./L - | sed /:/d\;n | cut -sd: -f2- ) <./F That should work pretty quickly (some timed tests are included below) with input of any size. Some notes on how: export LC_ALL=C Because the point of the following operation is to get the entire file of ./F stacked ...


I'd use awk: awk 'NR==FNR {a[$1]; next}; FNR in a' L.txt F.txt Update: I've done performance measures; it seems this version scales even better with very large data sets (as is the case with the stated requirements), since the comparison is very fast and overcompensates the effort necessary to build up the hash table.


With C omitting meaningful error messages: #include <stdio.h> #include <stdlib.h> int main (int argc, char *argv[]) { FILE *L; FILE *F; unsigned int to_print; unsigned int current = 0; char *line = NULL; size_t len = 0; if ((L = fopen(argv[1], "r")) == NULL) { return 1; } else if ((F = fopen(argv[2], ...


I wrote a simple Perl script to do that: Usage: script.pl inputfile_f inputfile_f #!/usr/bin/env perl $number_arguments = $#ARGV + 1; if ($number_arguments != 2) { die "Usage: script.pl inputfile_f inputfile_l\n"; } open($f, '<', $ARGV[0]) or die "$ARGV[0]: Not found\n"; open($l, '<', $ARGV[1]) or die "$ARGV[1]: Not found\n"; ...


Just for completeness: we can merge the excellent awk script in the answer by St├ęphane Chazelas, and the perl script in the answer by kos but without keeping the entire list in memory, in the hope that perl might be faster than awk. (I've changed the order of args to match the original question). #!/usr/bin/env perl use strict; die "Usage: $0 l f\n" if ...


You can use the usual awk de-duplicating technique, on the first field only (fields are separated by spaces): awk '!count[$1]++'


This perl solution is faster than the other awk or perl solutions by 20% or so, but oviously not as fast as the solution in C. perl -e ' open L, shift or die $!; open F, shift or die $!; exit if ! ($n = <L>); while (1) { $_ = <F>; next if $. != $n; print; exit if ! ($n = <L>); } ' -- L F

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