为特定日期之前的提交更改git commit消息

Question

I need to update commit messages for commits, that were made before certain date. My idea for the code:

git filter-branch -f --msg-filter '
    if [ "$GIT_AUTHOR_DATE" < "2012-11-01T00:00:00" ];
        then
            cat && echo "#imported"
        else
            cat &
    fi' 
HEAD

Can the 'if' statement compare dates with greater than / less than at all?

Answer 1

The test program ¹ usually can compare strings like this, but it does lexicographic comparisons. The ability to do such comparisons is an extension , but is available on Linux , Mac OS X , and most if not all BSDs .

There are a number of issues with doing this:

• The < or > must be escaped from the shell. [ "abc" < "def" ] runs /bin/[ with arguments "abc" and ] , with standard input being redirected from file def , assuming file def exists (otherwise you get an error).

• As already noted, the comparison is lexicographic. This means that to compare dates, they must be in a form in which such a comparison produces the correct result. Using ISO 8601 format will actually work, since the fields are fixed size and are in big-endian order. However, filter-branch does not export the date in ISO 8601 form. What you get instead looks like 1460884741 -0700 , for instance. The first number is a time stamp in seconds since 1970, and the second is a time zone offset (which you should largely ignore and simply work directly in UTC).

Besides these problems, your message-filter needs to avoid backgrounding the cat command. Hence, what you want is:

• First, convert 2012-11-01T00:00:00 into a UTC time stamp. This is a bit tricky. We can use python and its time.mktime but this insists on doing local time correction, so we must un-do the local time correction, which depends in part on whether DST is in force. My local time zone offset is -08:00 except during DST (such as now) when it is -07:00. This may thus be off by an hour (it was in my first paste, and I didn't even notice; edited now to fix):

   $ python -c 'import time; print(int(time.mktime((2012,11,1,-8,0,0,0,0,0))))' 1351728000 

  To verify that this is the correct time stamp:

   $ env TZ=UTC date -r 1351728000 Thu Nov 1 00:00:00 UTC 2012 

• Then, use this value in your --msg-filter with a numeric comparison on the first (non-offset) field in $GIT_AUTHOR_DATE or $GIT_COMMITTER_DATE as desired:

   ... --msg-filter 'cat; set $GIT_AUTHOR_DATE; \\ test $1 -lt 1351728000 && echo "#imported"' ... 

All the other usual caveats about using git filter-branch apply, of course.

---

¹ /bin/[ is a link to /bin/test . When test is invoked as [ it looks for a closing ] but is otherwise the same program. It is also built in to many shells for speed, but still implements the same semantics. ²

² Or at least, is supposed to do so.

Answer 2

Thanks to ideas from torek, I updated my script:

git filter-branch -f --msg-filter '
        if [ "$GIT_AUTHOR_DATE" \< "@1351728000" ];
        then
                cat && echo "#imported"
        else
                cat &
        fi' HEAD

Notable changes:

• < is replaced with \\<, which is equivalent to -lt
• Timestamp uses integer format and has a prefix "@"