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

We use seek to locate the record and return the offset and size.

$ LOCATION=`echo 2 | seek -mb inventory Item`
$ echo $LOCATION
95     118     0     9

The numbers mean that the record is offset 95 bytes into the file and that it is 23 bytes long, ending at byte 118. The 0 and 9 mean that there is no secondary index file. Now we can call lock to lock the record:

$ lock inventory $$ $LOCATION
$ cat /tmp/Linventory
1812    95     118     0     9

The $$ invokes the shell variable which provides our process id (1812).