Diff head of files

diff -e bigger smaller will do the trick, but requires some interpretation, as the output is a "valid ed script".

I made two files, "bigger" and "smaller", where the contents of "smaller" is identical to lines 5 through 9 of "bigger" doing `diff -e bigger smaller" got me:

% diff -e bigger smaller
10,15d
1,4d

Which means "delete lines 10 through 15 of 'bigger', and then delete lines 1 through 4, to get 'smaller'". That means "smaller" is lines 5 through 9 of "bigger".

Reversing the file names got me something more complicated. If "smaller" truly constitutes a subset of "bigger", only 'd' (for delete) commands will show up in the output.

You can do this visually with meld. Unfortunately, it is a GUI tool but if you just want to do this once, and on a relatively small file, it should be fine:

The image below is the output of meld a b:

If the files are small enough, you can slurp them both into Perl and have its regex engine do the trick:

perl -0777e '
        open "$FILE1","<","file_1";
        open "$FILE2","<","file_2";
        $file_1 = <$FILE1>;
        $file_2 = <$FILE2>;
        print "file_2 is", $file_1 =~ /\Q$file_2\E/ ? "" : "not";
        print " a subset of file_1\n";
'

The -0777 switch instructs Perl to set its input record separator $/ to the undefined value so as to slurp files completely.

If the files are text files and smaller, within bigger starts at the beginning of a line, it's not too difficult to implement with awk:

awk -v i=0 'NR==FNR{l[n++]=$0;next}
    {if ($0 == l[i]) {if (++i == n) {print FNR-n+1;exit}} else i=0}
    ' smaller bigger

Your question is "Diff head of files". If you really mean that one file is the head of the other, then a simple cmp will tell you that:

cmp big_file small_file
cmp: EOF on small_file

That tells you that a difference between the two files was not detected until end-of-file was reached while reading small_file.

If however you mean that the entire text of small file can occur anywhere inside big_file, then assuming you can fit both files in memory, you can use

perl -le '
   use autodie;
   undef $/;
   open SMALL, "<", "small_file";
   open BIG, "<", "big_file";
   $small = <SMALL>;
   $big = <BIG>;
   $pos = index $big, $small;
   print $pos if $pos >= 0;
'

This will print the offset within big_file where the contents of small_file are located (e.g. 0 if small_file matches at the beginning of big_file). If small_file does not match inside big_file, then nothing will be printed. If there is an error, the exit status will be non-zero.