Process Hierarchy

  • Init Process

    • Kernel starts init after boot, always assigned PID 1.
    • System V: /sbin/init
    • systemd: /bin/systemd → usually links to /lib/system/systemd.

  • Parent and Child Processes

    • Process starting another → Parent Process
    • Process started → Child Process
    • Parent PID is labeled PPID.

  • PID Limits

    • Maximum PID value can be viewed/changed: /proc/sys/kernel/pid_max
    • After reaching maximum, PID rolls over to available values.

  • Process Tree

    • Processes can be visualized as a family tree.
    • Command: pstree Example:

    sysadmin@localhost:~$ pstree
init-+-cron
    |-login---bash---pstree
    |-named---18*[{named}]
    |-rsyslogd---2*[{rsyslogd}]
    `-sshd
    * Relationships from example:

    • init → parent of login
    • login → child of init, parent of bash
    • bash → child of login, parent of pstree
    • pstree → child of bash