Linux – What does a + mean at the end of the permissions from ls -l

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!

Virtual memory isn't even necessarily memory. For example, if a process memory-maps a large file, the file is actually stored on disk, but it still takes up "address space" in the process.

Address space (ie. virtual memory in the process list) doesn't cost anything; it's not real. What's real is the RSS (RES) column, which is resident memory. That's how much of your actual memory a process is occupying.

But even that isn't the whole answer. If a process calls fork(), it splits into two parts, and both of them initially share all their RSS. So even if RSS was initially 1 GB, the result after forking would be two processes, each with an RSS of 1 GB, but you'd still only be using 1 GB of memory.

Confused yet? Here's what you really need to know: use the free command and check the results before and after starting your program (on the +/- buffers/cache line). That difference is how much new memory your newly-started program used.