$ cat inventory
Item    Amount  Cost    Value   Description
----    ------  ----    -----   -----------
1       4       50      150     rubber gloves
2       100     5       500     test tubes
3       5       80      400     clamps
4       23      19      437     plates

First we use seek to locate the record and return the offset and size.

$ LOCATION=`echo 3 | seek -mb inventory Item`
$ echo $LOCATION
118 136 0 9

This means that the record is 118 bytes into the file and that it ends at byte 136. $$ is an automatic shell variable which provides the users' process ID. With this information we can unlock the record:

$ unlock inventory $$ $LOCATION
$ cat /tmp/Linventory
cat: cannot open /tmp/Linventory

unlock removes the whole file if the last lock line has been removed.