I have a file like this:


I want to make file a.seq which contains sequence AGTACTTCCAGGAACGGTGCACTCTCC. Similarly b.seq contains ATGGATTTTTGGAGCAGGGAGATGGAATAGGAGCATGCTCCAT. In short, Column1 should be used as output file name with extension .seq and then it should have corresponding column2 sequence in it. I can do this by writing a perl script but anything on command line will be helpful. Hope to hear soon.

4 Answers 4


My snap response would have been awk but if you're processing lots of lines —and I'm talking about millions— you'll likely see a real benefit from switching to a "real" programming language.

With that in mind (and awk already being taken as an answer) I wrote a few implementations in different languages and benchmarked them on the same 10,000-line dataset on a PCI-E SSD.

me* (C)                0m1.734s
me (C++)               0m1.991s
me (Python/Pypy)       0m2.390s
me (perl)              0m3.024s
Thor+Glenn (sed|sh)    0m3.353s
me (python)            0m3.359s
jasonwryan+Thor (awk)  0m3.779s
rush (while read)      0m6.011s
Thor (sed)             1m30.947s
me (parallel)          4m9.429s

At a glance the C looks best but it was a pig to get to run that fast. Pypy and C++ are much easier to write and perform well enough unless you're talking about many billions of lines. If that were the case, an upgrade to doing this all in RAM or on an SSD might be a better investment than a code improvement.

Obviously in the time I've spent going through these you could probably have processed a few hundred million records in the slowest option. If you can only write awk or Bash loops, do that and get on with life. I clearly had too much spare time today.

I did also test some multi-threaded options (in C++ and Python and hybrids with GNU parallel) but the overhead of threads completely outweighs any benefit for such a simple operation (string splitting, writing).


awk (gawk here) would honestly be my first port of call for testing data like this but you can do fairly similar things in Perl. Similar syntax but with a slightly better writing handle.

perl -ane 'open(my $fh, ">", $F[0].".seq"); print $fh $F[1]; close $fh;' infile


I like Python. It's my day job language and it's just a nice, solid and incredibly readable language. Even a beginner could probably guess what's happening here.

with open("infile", "r") as f:
    for line in f:
        id, chunk = line.split()
        with open(id + ".seq", "w") as fw:

You have to remember that your distribution's python binary isn't the only implementation of Python out there. When I ran this same test through Pypy, it was faster than C without any further logic optimisation. Keep that in mind before writing Python off as a "slow language".


I started this example to see what we could really get my CPU to do but frankly, C is a nightmare to code if you haven't touched it in a long time. This has the added downside of being limited to 100-char lines though it's very simple to expand that, I just didn't need it.

My original version was slower than C++ and pypy but after blogging about it I got some help from Julian Klode. This version is now the fastest because of its tweaked IO buffers. It's also a lot longer and more involved than anything else.

#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include <stdlib.h>

#define BUFLEN (8 * 1024)

int main(void) {
    FILE *fp;
    FILE *fpout;

    char line[100];
    char *id;
    char *token;
    char *buf = malloc(BUFLEN);

    fp = fopen("infile", "r");

    setvbuf ( fp , buf , _IOLBF, BUFLEN );
    while (fgets(line, 100, fp) != NULL) {
        id = strtok(line, "\t");
        token = strtok(NULL, "\t");

        char *fnout = malloc(strlen(id)+5);
        fnout = strcat(fnout, id);
        fnout = strcat(fnout, ".seq");

        fpout = fopen(fnout, "w");
        setvbuf ( fpout , NULL , _IONBF , 0 );
        fprintf(fpout, "%s", token);

    return 0;


Performs well and is much easier to write than real C. You have all sorts of things that hold your hand (especially when it comes to strings and input). All that means you can actually simplify the logic down. strtok in C is a hog because it processes the entire string and then we need to do all that tiresome memory allocation. This just flits along the line until it hits the tab and we pull the segments out as we need them.

#include <fstream>
#include <string>
using namespace std;

int main(void) {
    ifstream in("infile");
    ofstream out;
    string line;

    while(getline(in, line)) {
        string::size_type tab = line.find('\t', 0);
        string filename = line.substr(0, tab) + ".seq";
        out << line.substr(tab + 1);


GNU Parallel

(Not the moreutils version). It's a nice concise syntax but OMGSLOW. I might be using it wrong.

parallel --colsep '\t' echo {2} \> {1}.seq <infile

Test harness generator

Here's my data-generator for 100000 lines of [ATGC]*64. It's not fast and improvements are very welcome.

cat /dev/urandom | tr -dc 'ATGC' | fold -w 64 | awk 'NR>100000{exit}{printf NR"\t"$0"\n"}' > infile
  • 2
    I should point out that enumerating all your options for performance can be as wasteful as just going with the first thing that springs to mind. awk is still a fine answer for anything less than tens of millions. Even if you [linearly] scale this up to a billion lines, C is only saving you 1.5 hours over Perl and 3.6 hours over awk.
    – Oli
    Nov 13, 2014 at 12:15
  • Now my C++ version is on there is so much faster, maybe I'd consider C++ for more simple text processing of huge data-sets. It's almost twice as fast and that's many hours difference when you get to billions of lines.
    – Oli
    Nov 13, 2014 at 15:30
  • 9
    xkcd.com/1445 Nov 13, 2014 at 17:26
  • 1
    also: Parkinson's law of triviality
    – rook
    Nov 14, 2014 at 9:55
  • 1
    I think the generation speed of your test harness is bound by the random number generator. You can make it faster by using every number it gives or generate an homogeneous distribution, e.g.: paste <(yes A) <(yes T) <(yes G) <(yes C) | head -n1600000 | tr '\t' '\n' | shuf | tr -d \\n | fold -w64 | cat -n > infile.
    – Thor
    Dec 2, 2015 at 11:35

Using awk:

awk '{printf "%s\n", $2>$1".seq"}' file

From the nominated file, print the second field in each record ($2) to a file named after the first field ($1) with .seq appended to the name.

As Thor points out in the comments, for a large dataset, you may exhaust the file descriptors, so it would be wise to close each file after writing:

awk '{printf "%s\n", $2>$1".seq"; close($1".seq")}' file
  • Hi This works Thanks a lot.. Can you explain the code a bit? Nov 12, 2014 at 21:36
  • @user3138373 Hope that helps...
    – jasonwryan
    Nov 12, 2014 at 21:39
  • It helps.. Thanks Why won't print work instead of printf?? Nov 12, 2014 at 21:41
  • 4
    If there are many lines, all available file descriptors will be used, so you should probably add a close($1".seq").
    – Thor
    Nov 13, 2014 at 8:46
  • 1
    @Thor, true. Some awk implementations like GNU's know how to work around that though. Nov 13, 2014 at 12:25

Pure shell implementation:

while read -r filename content ; do
    printf '%s\n' "$content" >> "${filename}.seq"
done < /source/file

Here is one way you could do it with GNU sed:

<infile sed -r 's:(\w+)\s+(\w+):echo \2 > \1.seq:e; d'

Or more efficiently, as suggested by glenn jackman:

<infile sed -r 's:(\w+)\s+(\w+):echo \2 > \1.seq:' | sh
  • 1
    While that is cool, it's quite inefficient, having to spawn an external command for every line. It would be a bit better having the sed output all the raw commands, and pipe the output into "sh" Nov 13, 2014 at 1:18
  • 1
    @glennjackman: This was just an interesting alternative way of doing it. If the input is large, awk is probably the most efficient tool to use. You are of course right about not spawning sh for each line, I have added the pipe-option as an alternative.
    – Thor
    Nov 13, 2014 at 8:48

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