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[Linux: system data locations]
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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

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