Pad a file with zeros

Bash + coreutils, 13

truncate -s$@

Input from the command-line - the first parameter is the file size and the second is the filename.

From man truncate:

If a FILE is shorter, it is extended and the extended part (hole) reads as zero bytes.

Try it online.

Groovy, 54 47 43 41 bytes

{a,b->(c=new File(a))<<'\0'*(b-c.size())}

-6 thanks to manatwork's idea of removing the loop; that was my original idea, don't know why I thought it wouldn't work and opted for a loop... It definitely works, just tested it.

Ungolfed:

{
    a,b->                    // Two inputs: a is the File name, b is the desired size.
    File c = new File(a)     // Make and store the file.
    c << '\0'*(b-c.size())   // Append the difference between b and c.size() in nullbytes.
}
                             // Usually a close would be required, but, in Groovy,
                             // because private data isn't so protected, if a File
                             // Object goes out of scope, the attached Stream is 
                             // automatically flushed to disk.

APL (Dyalog), 33 17 bytes

⎕⎕NRESIZE⍞⎕NTIE¯1

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