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I have two files with sizes 124665 and 124858 in bytes and want to check whether file1 is a prefix of file2 or not.

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up vote 11 down vote accepted

Supposing you have the size of file1 in the variable FILE1_SZ and your head implementation supports the (non-standard) -c option:

if head -c "$FILE1_SZ" file2 | cmp -s - file1; then
    echo "file1 is a prefix of file2"
    echo "file1 is not a prefix of file2"
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@StéphaneChazelas Can you please explain why cmp would be better than diff here? – Joseph R. Jun 7 '14 at 19:40
Because cmp does a simple byte to byte comparison, and returns as soon as it finds a difference, while diff is a text utility that is going to use a complex algorithm to show you all the differences between the two files which you don't care about. – Stéphane Chazelas Jun 7 '14 at 20:02

If your system has the cmp command from GNU diffutils, then one option is

cmp -n 124665 file1 file2

to compare at most the first 124665 bytes of the two files and report if they differ - or, more generally

cmp -n "$(wc -c < file1)" file1 file2
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@StephaneChazelas I'm second guessing myself here but would it have been better to suggest $(stat -c %s file1) for the size in bytes? Does wc actually open and process the whole file to get the byte count? – steeldriver Jun 7 '14 at 19:51
no, most wc implementations will optimise that case and do a fstat() (or/and a lseek(SEEK_END)) so will be as efficient as it gets. On the other hand, that stat -c is GNU specific. – Stéphane Chazelas Jun 7 '14 at 19:52
Ah thanks that makes sense. – steeldriver Jun 7 '14 at 19:59
Although if you're going to require the GNU-specific cmp, you might reasonably assume GNU-specific stat. – Barmar Jun 11 '14 at 19:04

GNU cmp can solve the problem in an easier way:

cmp file1 file2

There are four possible outputs (barring some sort of error).

  • No output: the files are identical.

  • cmp: EOF on file1: file1 is a prefix of file2.

  • cmp: EOF on file2: file2 is a prefix of file1.

  • file1 file2 differ: byte NNN, line MMM: Neither is a prefix of the other.

Unfortunately this is a little awkward to use in a script, since these cases don't seem to be distinguished in the exit code. Moreover, the EOF on file1 messages go to stderr, while the file1 file2 differ message goes to stdout.

I presume that other versions of cmp do something similar, but I have not checked.

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cmp is not a GNU-only command nor did it originate there, it was already in the first version of Unix in the early 70s. The -n option is GNU specific though. – Stéphane Chazelas Jun 7 '14 at 19:26
You could do cmp file1 file2 2>&1 | grep EOF on file1 – David Z Jun 8 '14 at 1:39
@StéphaneChazelas: That is true. I didn't mean to imply that cmp was unique to GNU, just that GNU cmp was the only version I tried. I added a sentence to clarify. – Nate Eldredge Jun 8 '14 at 4:19
@DavidZ: Yes, you could, but it gets a little less robust. Imagine that you are trying to do this with two files supplied by the user, and one of them is named file1 and the other is named file12. (Or worse yet, what if the second file is named EOF on file1?) Solving this robustly using cmp is probably much more trouble than writing the obvious 5-line program in C... – Nate Eldredge Jun 8 '14 at 4:23
There may be contexts where a C program isn't practical, though. And it's not that hard to make it fairly robust, because the output of cmp is so tightly constrained. Using the -x option on grep to match the entire line will take care of all but the most exotic cases (e.g. newlines in the filename). – David Z Jun 8 '14 at 4:29

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