13.8. 群組
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## 13.8. 群組

A group is simply a list of users. Groups are identified by their group name and GID (Group ID). In FreeBSD (and most other UNIX® like systems), the two factors the kernel uses to decide whether a process is allowed to do something is its user ID and list of groups it belongs to. Unlike a user ID, a process has a list of groups associated with it. You may hear some things refer to the “group ID” of a user or process; most of the time, this just means the first group in the list.

The group name to group ID map is in `/etc/group`. This is a plain text file with four colon-delimited fields. The first field is the group name, the second is the encrypted password, the third the group ID, and the fourth the comma-delimited list of members. It can safely be edited by hand (assuming, of course, that you do not make any syntax errors!). For a more complete description of the syntax, see the group(5) manual page.

If you do not want to edit `/etc/group` manually, you can use the pw(8) command to add and edit groups. For example, to add a group called `teamtwo` and then confirm that it exists you can use:

範例 13.7. Adding a Group Using pw(8)

    # pw groupadd teamtwo
    # pw groupshow teamtwo
    teamtwo:*:1100:

The number `1100` above is the group ID of the group `teamtwo`. Right now, `teamtwo` has no members, and is thus rather useless. Let's change that by inviting `jru` to the `teamtwo` group.

範例 13.8. Adding Somebody to a Group Using pw(8)

    # pw groupmod teamtwo -M jru
    # pw groupshow teamtwo
    teamtwo:*:1100:jru

The argument to the `-M` option is a comma-delimited list of users who are members of the group. From the preceding sections, we know that the password file also contains a group for each user. The latter (the user) is automatically added to the group list by the system; the user will not show up as a member when using the `groupshow` command to pw(8), but will show up when the information is queried via id(1) or similar tool. In other words, pw(8) only manipulates the `/etc/group` file; it will never attempt to read additionally data from `/etc/passwd`.

範例 13.9. Using id(1) to Determine Group Membership

    % id jru
    uid=1001(jru) gid=1001(jru) groups=1001(jru), 1100(teamtwo)

As you can see, `jru` is a member of the groups `jru` and `teamtwo`.

For more information about pw(8), see its manual page, and for more information on the format of `/etc/group`, consult the group(5) manual page.

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