WYNTK: UNIX System Admininistrator

Sometimes several users have to work on several sets of files at once.

At our office, we produce books. At the final stage of a book’s production, several people might be editing the files at the same time: one is entering index entries, another is fixing page breaks, and a third is entering last-minute changes from the author.

When things are going smoothly, we all know what each other is doing, and we stay out of each other’s files. But occasionally there are conflicts.

The initial permissions of a file are determined by the user who creates it

A file initially belongs to the person who created it

The permissions for a file depends on whoever originally created the file:

• The person who owns the file is the person who created it.

• The group that the file belongs to is the user’s default group.

• The initial permissions on the file are set by the user’s umask value.

Each user has a default group

Each user has a default group. This is the group that a file belongs to when you first create it.

Your default group is set by your system administrator in your passwd entry. See Chapter 2 for information on how to see your passwd entry. The default group ID appears as the fourth field in the passwd entry—for example, my entry reads:

    lmui:ctA1223xqXp/Q:183:100:Linda Mui:/home/lmui:/bin/tcsh

In this case, my default group ID is 100. To find out what group 100 corresponds to, look in the system’s group database. If your system supports NIS (aka Yellow Pages ), then you can see the group database by typing:

    % ypcat
 group

Otherwise, just look in the file /etc/group. In our group database, you’ll see that group 100 corresponds to the group called ora:

    ora:*:100:root,sybase

If I create a new file, I’ll find that it belongs to group ora.

The initial permissions for new files are set by your umask value

Every user has a value called a umask. The best way to think about your umask is to imagine that, left to their own devices, all new files are created read/ write to everyone—that is, with the permissions -rw-rw-rw-, or permissions 666.

Making a file read/write to everyone isn’t always the best idea. So the umask command can be used to “mask out” parts of those permissions, based on chmod’s bit syntax.

Like chmod’s bit syntax, umask values are given as a 3-digit string, which is subtracted from the default 666 permissions. For example, a umask of 002 means that the default permissions will be 666-002 = 664, or read/write for the owner and group and read-only for everyone else. All new files are now created with permissions -rw-rw-r--.

By default, the umask value is set to 022—so all files you create are read/write to you and read-only to everyone else (i.e. permissions 644, or -rw-r—r--).

umask is generally run in your .cshrc or .profile file. For example:

    umask 066

This means that all newly created files will have permissions 600, i.e. read/write for owner and no access for anyone else.