Shell Programming and Scripting

30910   rs7468327
36587   rs10814410
91857   rs9408752
105797  rs1133715
146659  rs2262038
152695  rs2810979
181843  rs3008128
182129  rs3008131
192118  rs3008170

30910 1.9415219673 0
36431 1.3351312477 0.0107191428
36587 1.3169171182 0.0109274233
37123 1.3181466012 0.0116332908
38515 1.1211025231 0.0134681509
44551 1.5498135416 0.0202351257
47327 1.5694610726 0.0245374081
48265 1.5556343019 0.0260095626
68775 1.5538580867 0.0579156221

# single pass across each file, but requires the entire first file
# to be held in memory which might not be realistic.
# order is preserved based on file2
awk '
    NR == FNR { cache[$1] = $0; next; }
    $1 in cache {
        printf( "%s", cache[$1] );
        $1 = "";
        print;
    }
' file1 file2 >output

# multiple passes across the data, but memory requirement is eliminated
# order of file2 is not preserved.
(
    sed 's/^/a /' file1
    sed 's/^/b /' file2
) | sort -k 2n,2 -k 1,1  awk '
    $1 == "a" {
        x = $2;
        $1 = "";
        cache = $0;
        next;
    }
    $2 == x {
        $1 = $2 ="";
        printf( "%s%s\n", substr( cache, 2 ), $0 );
    }
'

join FileA.txt FileB.txt