Shell – inotifywait – get old and new file name when renaming

inotifylinuxshell-script

I'm looking for a reliable way to detect renaming of files and get both old and new file names. This is what I have so far:

COUNTER=0;
inotifywait -m --format '%f' -e moved_from,moved_to ./ | while read FILE
do
if [ $COUNTER -eq 0 ]; then
    FROM=$FILE;
    COUNTER=1;
else
    TO=$FILE;
    COUNTER=0;
    echo "sed -i 's/\/$FROM)/\/$TO)/g' /home/a/b/c/post/*.md"
    sed -i 's/\/'$FROM')/\/'$TO')/g' /home/a/b/c/post/*.md
fi
done

It works, but it assumes you will never move files into or out of the watched folder. It also assumes that events come in pairs, first moved_from, then moved_to. I don't know if this is always true (works so far).

I read inotify uses a cookie to link events. Is the cookie accessible somehow?
Lacking the cookie, I thought about using timestamps to link events together. Any tips on getting FROM and TO in a more reliable way?

Full script gist.

Best Answer

I think your approach is correct, and tracking the cookie is a robust way of doing this. However, the only place in the source of inotify-tools (3.14) that cookie is referenced is in the header defining the struct to match the kernel API.

If you like living on the edge, this patch (issue #72) applies cleanly to 3.14 and adds a %c format specifier for the event cookie in hex:

--- libinotifytools/src/inotifytools.c.orig     2014-10-23 18:05:24.000000000 +0100
+++ libinotifytools/src/inotifytools.c  2014-10-23 18:15:47.000000000 +0100
@@ -1881,6 +1881,12 @@
                        continue;
                }

+               if ( ch1 == 'c' ) {
+                       ind += snprintf( &out[ind], size-ind, "%x", event->cookie);
+                       ++i;
+                       continue;
+               }
+
                if ( ch1 == 'e' ) {
                        eventstr = inotifytools_event_to_str( event->mask );
                        strncpy( &out[ind], eventstr, size - ind );

This change modifies libinotifytools.so, not the inotifywait binary. To test before installation:

LD_PRELOAD=./libinotifytools/src/.libs/libinotifytools.so.0.4.1 \
  inotifywait  --format="%c %e %f" -m -e move /tmp/test
Setting up watches.
Watches established.
40ff8 MOVED_FROM b
40ff8 MOVED_TO a

Assuming that MOVED_FROM always occurs before MOVED_TO (it does, see fsnotify_move(), and it's an ordered queue, though independent moves might get interleaved) in your script you cache the details when you see a MOVED_FROM line (perhaps in an associative array indexed by ID), and run your processing when you see a MOVED_TO with the matching half of the information.

declare -A cache
inotifywait  --format="%c %e %f" -m -e move /tmp/test |
while read id event file; do
    if [ "$event" = "MOVED_FROM" ]; then
        cache[$id]=$file
    fi
    if [ "$event" = "MOVED_TO" ]; then
        if [ "${cache[$id]}" ]; then
            echo "processing ..."
            unset cache[$id]
        else
            echo "mismatch for $id"
        fi
    fi
done

(With three threads running to shuffle a pair of files each 10,000 times, I never saw a single out of order event, or event interleaving. It may depend on filesystem and other conditions of course.)

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