Have you ever worked in latest versions of fedora ?
If yes, you might have observed some directories “/run/user/some-number” when doing “df -h”. Lets see the command output.
[saket@localhost ~]$ df -h Filesystem Size Used Avail Use% Mounted on /dev/mapper/fedora-root 18G 4.4G 12G 27% / devtmpfs 492M 0 492M 0% /dev tmpfs 502M 80K 502M 1% /dev/shm tmpfs 502M 904K 501M 1% /run tmpfs 502M 0 502M 0% /sys/fs/cgroup tmpfs 502M 16K 502M 1% /tmp /dev/sda1 477M 95M 353M 22% /boot tmpfs 101M 12K 101M 1% /run/user/42 tmpfs 101M 0 101M 0% /run/user/1000
Understanding /run/user directories
1. Identifying users who own these directories.
In the above output, we can see two directories /run/user/42 and /run/user/1000.
Lets try to understand which users are these.
[saket@localhost ~]$ id -a 42 uid=42(gdm) gid=42(gdm) groups=42(gdm) [saket@localhost ~]$ id -a 1000 uid=1000(saket) gid=1000(saket) groups=1000(saket),10(wheel) [saket@localhost ~]$ [saket@localhost ~]$ ls -l /run/user/ total 0 drwx------. 3 saket saket 60 Sep 21 20:23 1000 drwx------. 11 gdm gdm 220 Sep 21 20:23 42 [saket@localhost ~]$
Hence as we can see in above output, one directory is owned by user saket having UID 1000, and another one is system user gdm with UID 42.
This new concept is deployed by systemd daemon. pam_systemd daemon creates /run/user/$uid directory for every logged in user which is used for storing running processes’ files for that user. It includes various things such as your keyring daemon, pulseaudio, etc.
Earlier these running processes’ files were stored in /tmp and now with systemd its simplified and now stored in /run/user/$uid directory.
/home/$uid can not be sued as it is sometimes mounted on network locations and hence /tmp is the only location other than home directory where all users and read and write.
However storing files in /tmp can be problematic as /tmp can be modified by anyone.
Hence systemd came up with a new directory structure /run/user/$uid.
This directory is local to each system and can be only accessed by the target user. It means applications dont have to worry about access control now.
It even helped to keep things in an organized fashion. On user logout, when no active session exist, pam_systemd will clean /run/user/$uid directory which was very difficult to achieve when using /tmp as files are scattered in same directory.