Why does diff fail when invoked from a Makefile

diff()makepatch

Narrow-down of a patch problem I am trying to troubleshoot:
Only two files, each with a size of 1 byte:

file a (containing 'a')
file b (containing 'b') and

Goal is to make and then apply a patch that will change the value of 'b' to 'a'. Contents of Makefile are:

patch:
        diff -u b a > b2a.patch
        patch -o b.corrected b < b2a.patch
        diff a b.corrected

clean:
        rm -f b2a.patch b.corrected

Given the above, make fails with the following output:

$ make
diff -u b a > b2a.patch
make: *** [patch] Error 1

If, however, I execute the commands in the Makefile one after the other on the bash shell, there is no problem at all.

Best Answer

Make assumes that an exit code of 0 means success, anything else means failure. This is the standard convention used by almost all command-line tools.

Unfortunately, diff is not one of those. Checking the GNU diff info page, and also the Single Unix Specification "diff" entry, 0 means no differences found, 1 means differences found, and ≥2 means error.

You can tell Make to ignore the exit status entirely by prefixing the command with a hyphen, as you did in your comment, but this will ignore actual errors—probably not what you want. Instead, you can:

patch:
        diff -u b a > b2a.patch; [ $$? -eq 1 ]
        patch -o b.corrected b < b2a.patch
        diff a b.corrected; [ $$? -eq 1 ]

Note the ; [ $$? -eq 1 ] bit I've added to the end of the two diff lines. You can use ; test $$? -eq 1 as well, of course. The $? shell variable is $$? due to normal Makefile escaping conventions. Note this also rejects exit status 0 (no differences), which is probably what you want.

BTW: it appears this really ought to be:

patch: b.corrected
        diff …
b.corrected: b2a.patch
        patch …
b2a.patch: a b
        diff …

so that modifications of a and b get picked up, and the files regenerated correctly.

Related Solutions

Forcing GNU make to run commands in order

Actually, you don't have a problem with make, but with your command:

tex dummy.tex &> /dev/null;

Runs 'tex' in the background. You don't need to remove '>/dev/null', but '&' is sending 'tex' to the background.

Try this, it must be fine for you:

tex dummy.tex > /dev/null;

or run everything in the same subshell, like this:

(tex dummy.tex > /dev/null;rm *.log)

or less sane, this:

if test 1 = 1; then tex dummy.tex > /dev/null;rm *.log; fi

PD: &> is an extension provided by some shells (including bash) to redirect both stdout and stderr to the same destination, but it's not portable, you should use '>/dev/null 2>&1' instead. (Thanks @Gilles)

Cheers

How to use patch and diff to merge two files and automatically resolve conflicts

You don't need patch for this; it's for extracting changes and sending them on without the unchanged part of the file.

The tool for merging two versions of a file is merge, but as @vonbrand wrote, you need the "base" file from which your two versions diverged. To do a merge without it, use diff like this:

diff -DVERSION1 file1.xml file2.xml > merged.xml

It will enclose each set of changes in C-style #ifdef/#ifndef "preprocessor" commands, like this:

#ifdef VERSION1
<stuff added to file1.xml>
#endif
...
#ifndef VERSION1
<stuff added to file2.xml>
#endif

If a line or region differs between the two files, you'll get a "conflict", which looks like this:

#ifndef VERSION1
<version 1>
#else /* VERSION1 */
<version 2>
#endif /* VERSION1 */

So save the output in a file, and open it in an editor. Search for any places where #else comes up, and resolve them manually. Then save the file and run it through grep -v to get rid of the remaining #if(n)def and #endif lines:

grep -v '^#if' merged.xml | grep -v '^#endif' > clean.xml

In the future, save the original version of the file. merge can give you much better results with the help of the extra information. (But be careful: merge edits one of the files in-place, unless you use -p. Read the manual).

Best Answer

Related Solutions

Forcing GNU make to run commands in order

How to use patch and diff to merge two files and automatically resolve conflicts

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