`ps` - report a snapshot of the current processes

ps [aux] [-ef] [-eo FORMAT] [--sort KEY]

Options you will use

aux: BSD syntax: every process, with user and TTY-less ones included.
-ef: UNIX syntax: every process, full format (shows PPID).
-e: Select all processes.
-o FORMAT: Pick your own columns: pid,ppid,user,rss,vsz,stat,etime,comm,args. A trailing = drops the header.
-p PID: Only these PIDs.
--sort KEY: Sort by a column; prefix with - for descending (--sort=-rss).
-f, --forest: Show the process hierarchy.
--no-headers: Print no header line.
--ppid PID: Only the children of this parent PID.
-a: All processes with a terminal, except session leaders.
-C NAME: Only processes whose command name is NAME.
-L: Show threads, one line each (LWP column).
-u USER: Only processes of this (effective) user.

$ ps -eo pid,user,rss,comm --sort=-rss | head

biggest memory consumers

$ ps -p $$ -o ppid=

what launched your shell

$ ps -p 1 -o comm=

systemd

STAT: R running/runnable, S interruptible sleep, D uninterruptible sleep (cannot be killed, usually blocked I/O), Z zombie (already dead, parent has not reaped it), T stopped. Modifiers: s session leader, l multi-threaded, + foreground, < high priority, N low priority.

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