Debian – What’s the difference between ‘useradd’ and ‘adduser’

Attempting to expand on @Zoredache's answer, as I give this a go myself:

• Create a new group (www-pub) and add the users to that group

  groupadd www-pub

  usermod -a -G www-pub usera ## must use -a to append to existing groups

  usermod -a -G www-pub userb

  groups usera ## display groups for user

• Change the ownership of everything under /var/www to root:www-pub

  chown -R root:www-pub /var/www ## -R for recursive

• Change the permissions of all the folders to 2775

  chmod 2775 /var/www ## 2=set group id, 7=rwx for owner (root), 7=rwx for group (www-pub), 5=rx for world (including apache www-data user)

  Set group ID (SETGID) bit (2) causes the group (www-pub) to be copied to all new files/folders created in that folder. Other options are SETUID (4) to copy the user id, and STICKY (1) which I think lets only the owner delete files.

  There's a -R recursive option, but that won't discriminate between files and folders, so you have to use find, like so:

  find /var/www -type d -exec chmod 2775 {} +

• Change all the files to 0664

  find /var/www -type f -exec chmod 0664 {} +

• Change the umask for your users to 0002

  The umask controls the default file creation permissions, 0002 means files will have 664 and directories 775. Setting this (by editing the umask line at the bottom of /etc/profile in my case) means files created by one user will be writable by other users in the www-group without needing to chmod them.

Test all this by creating a file and directory and verifying the owner, group and permissions with ls -l.

Note: You'll need to logout/in for changes to your groups to take effect!

aptitude is a wrapper for dpkg just like apt-get/apt-cache, but it is a one-stop-shop tool for searching/installing/removing/querying. A few examples that apt might not supply:

$ aptitude why libc6
i   w64codecs Depends libc6 (>= 2.3.2)
$ aptitude why-not libc6
Unable to find a reason to remove libc6.

$ aptitude show libc6
Package: libc6
State: installed
Automatically installed: no
Version: 2.9-4ubuntu6
Priority: required
Section: libs
Maintainer: Ubuntu Core developers <ubuntu-devel-discuss@lists.ubuntu.com>
Uncompressed Size: 12.1M
Depends: libgcc1, findutils (>= 4.4.0-2ubuntu2)
Suggests: locales, glibc-doc
Conflicts: libterm-readline-gnu-perl (< 1.15-2), 
tzdata (< 2007k-1), tzdata-etch, nscd (< 2.9)
Replaces: belocs-locales-bin
Provides: glibc-2.9-1
Description: GNU C Library: Shared libraries
 Contains the standard libraries that are used by nearly all programs 
 on the system. This package includes shared versions of the standard 
 C library and the standard math library, as well as many others.