[Linux: system data locations]

data.md

System Data Locations

Processes and Hardware Info

• /proc and /sys are pseudo filesystems; they're kept in memory by the computer the whole time
• /proc contains info abt active processes, system hardware, and kernel configuration
• /dev is how to access hardware devices
• /sys contains info abt the hardware devices in /dev

Viewing Running Processes

• pstree: shows parent/child processes' relationship
• ps: shows current processes running in the current shell
  • --forest: shows a tree view
  • aux/-ef: shows all running processes
  • -u: show process by user (ps -u root)
• top: show live system processes feedback
  • K: terminate the process (9 == SIGKILL)
  • R: adjust priority of process (-20 to 19)
• uptime//proc/loadavg: cpu load over past (1, 5, 15) minutes
  • load avgs are proportional to number of CPU cores --> 1.12 on a 4-core is 28% loaded

Memory

• kernel space: kernel code is stored and executed in a "protected" range of memory addresses and remains isolated from other processes with lower privileges
• user space: available to all users and programs and communicate with the kernel through "system call" API
• seperating these memory address like so provides protection, giving the Linux kernel stability -- hence the saying linux is "safer"

Logging and System Messaging

• /var/log: location of log files
• syslog: generic meaning whichever daemon is in place
• daemons: syslogd w/ klogd, rsyslogd, and journald/journalctl
• binary files
  • file: describes the data type (text vs. data (binary))
  • binary files have their own tools to open: journalctl, lastb, and last

Filesystem Hierarchy Standard (FHS)

• /: base/root (first hierarchy)

• /bin: essential binaries

• /boot: files necessary to boot the system, such as kernel, etc.

• /dev: files that represent hardware devices and other special files (/dev/null or /dev/random)

• /etc: host config files

• /home: user home directories

• /lib: libraries for /bin//sbin

• /lib64: essential libraries built for a specific architecture

• /media: automatically mounted removable media

• /mnt: manually mounted temporary filesystems

• /opt: optional third party software install location

• /proc: virtual filesystem for the kernel to report process information

• /root: home directory of the root user

• /sbin: essential binaries for the root user

• /sys: virtual filesystem for information about connected hardware

• /srv: location where site-specific services may be hosted

• /tmp: temporary files that should be cleared at boot time (accessible by all users; often isnt actually cleared)

• /usr: non-essential files for shared user use (second hierarchy)

• /usr/local: files for software not originating from distribution (third hierarchy) (often contains user compiled source code)

• /var: files that change over time (fourth hierachy)

• /var/cache: files used for caching application data (fifth hierarchy)

• /var/log: most log files

• /var/lock: lock files for shared resources

• /var/spool: spool files for printing and mail

• /var/tmp: temporary files to be preserved between boots

• sub-hierarchies repeat the pattern of many of the key directories found under the first hierarchy