Classic Shell Scripting

Summary

In this chapter, we have re-created and solved a "real-world" problem: merging the password files of two separate computers so that their files can be shared via NFS.

Careful study of the password files of both systems allows us to classify users into different categories: those only on the first system, those only on the second, and those with accounts on both. The problem is to ensure that when we're done, each user has an identical unique UID number on both systems, and that each user's files belong only to that user.

Solving the problem requires finding new unused UID numbers to use when there are UID conflicts, and careful ordering of the commands that change the ownership of the files. Furthermore, the entirety of both systems must be searched to be sure that every file's owner is updated correctly.

Other issues would need to be solved in a similar fashion; most notably, the merging of the group files, and assigning owners to any unowned files. For safety, the systems should be quiet while these operations are in progress, and we also outlined a different solution when efficiency is an issue.

The solution involved careful filtering of the original password files, with awk, sort, uniq, and while read ... loops being used heavily to process the data and prepare the commands to change the ownership of user files. find, xargs, and chown (of course) do the work.

The total solution represents less than 170 lines of code, including comments! A program in C that solved the same ...