In linux there are lots of processes running, there are a number of commands available to find and view these processes. To get a full list, we use the ps command:
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$ ps aux USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND root 1 0.0 0.4 59744 4848 ? Ss May01 0:05 /usr/lib/systemd/systemd --switched-root --system --deserialize 24 root 2 0.0 0.0 0 0 ? S May01 0:00 [kthreadd] root 3 0.0 0.0 0 0 ? S May01 0:00 [ksoftirqd/0] root 5 0.0 0.0 0 0 ? S< May01 0:00 [kworker/0:0H] . . ...etc
This lists (a)ll processes for every (u)ser and it lifts the “must have tty” restri(x)ion so to also allowing to display processes owned by system accounts (e.g. processes owned by the sshd service).
If you see the man page for ps, you'll find that the "aux" is bsd syntax, which isn't recommended. Instead we should be using the "standard syntax" alternative for specifying flags:
$ ps -ef UID PID PPID C STIME TTY TIME CMD root 1 0 0 May01 ? 00:00:05 /usr/lib/systemd/systemd --switched-root --system --deserialize 24 root 2 0 0 May01 ? 00:00:00 [kthreadd] . . .
This lists the same processes, but gives different column info when compared to the aux, you can fix this by manually specifying what column you want:
$ ps -eo user,pid,pcpu,pmem,vsize,rss,tname,stat,start_time,bsdtime,command | head USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND root 1 0.0 0.4 59744 4848 ? Ss May01 0:05 /usr/lib/systemd/systemd --switched-root --system --deserialize 24 root 2 0.0 0.0 0 0 ? S May01 0:00 [kthreadd] root 3 0.0 0.0 0 0 ? S May01 0:00 [ksoftirqd/0] root 5 0.0 0.0 0 0 ? S< May01 0:00 [kworker/0:0H] root 6 0.0 0.0 0 0 ? S May01 0:00 [kworker/u2:0] root 7 0.0 0.0 0 0 ? S May01 0:00 [migration/0] root 8 0.0 0.0 0 0 ? S May01 0:00 [rcu_bh] root 9 0.0 0.0 0 0 ? S May01 0:00 [rcuob/0] root 10 0.0 0.0 0 0 ? R May01 0:04 [rcu_sched]
You can find info about all the columns in the ps command's man page. Here's some one liner descriptions for some of the more less obvious columns:
VSZ - This is the amount of ram space that this process has reserved.
RSS - This is the amount of ram that this process is actually using.
TTY - A "?" indicates that this is process is not a shell job, i.e. it is a background process that has been triggered by a service, or parent processes
STAT - this means status, where "S" means it is asleep, and "R" means it is running.
Parent and Child process
Processes can trigger other child processes. You can find the parent process id for a given process by displaying the parent process id "ppid" column:
[root@localhost /]# ps -eo user,pid,ppid,pcpu,pmem,vsize,rss,tname,stat,start_time,bsdtime,command | head USER PID PPID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND root 1 0 0.0 0.4 59744 4848 ? Ss May01 0:05 /usr/lib/systemd/systemd --switched-root --system --deserialize 24 root 2 0 0.0 0.0 0 0 ? S May01 0:00 [kthreadd] root 3 2 0.0 0.0 0 0 ? S May01 0:00 [ksoftirqd/0] root 5 2 0.0 0.0 0 0 ? S< May01 0:00 [kworker/0:0H] root 6 2 0.0 0.0 0 0 ? S May01 0:00 [kworker/u2:0] root 7 2 0.0 0.0 0 0 ? S May01 0:00 [migration/0] root 8 2 0.0 0.0 0 0 ? S May01 0:00 [rcu_bh] root 9 2 0.0 0.0 0 0 ? S May01 0:00 [rcuob/0] root 10 2 0.0 0.0 0 0 ? R May01 0:04 [rcu_sched]
However this isn't that easy to following. A better command to use is the pstree, which shows the relationships more visually:
$ pstree -ap
systemd,1 --switched-root --system --deserialize 24
├─ModemManager,626
│ ├─{ModemManager},635
│ └─{ModemManager},691
├─NetworkManager,2112 --no-daemon
│ ├─dhclient,11814 -d -q -sf /usr/libexec/nm-dhcp-helper -pf /var/run/dhclient-enp0s3.pid -
├─at-spi-bus-laun,3794
│ ├─dbus-daemon,3798 --config-file=/etc/at-spi2/accessibility.conf --nofork --print-address 3
│ │ └─{dbus-daemon},3800
│ ├─{at-spi-bus-laun},3795
│ ├─{at-spi-bus-laun},3797
│ └─{at-spi-bus-laun},3799
├─at-spi2-registr,3803 --use-gnome-session
│ └─{at-spi2-registr},3806
├─atd,1332 -f
├─gdm,1334
│ ├─gdm-simple-slav,1355 --display-id /org/gnome/DisplayManager/Displays/_0
│ │ ├─Xorg,1367 :0 -background none -verbose -auth /run/gdm/auth-for-gdm-AS4ueL/database -seat seat0 -nolisten tcp vt1
│ │ ├─gdm-session-wor,3593
│ │ │ ├─gnome-session,3626 --session gnome-classic
│ │ │ │ ├─abrt-applet,4015
│ │ │ │ │ └─{abrt-applet},4035
│ │ │ │ ├─gnome-settings-,3816
│ │ │ │ │ ├─{gnome-settings-},3824
│ │ │ │ │ ├─{gnome-settings-},3829
.
.
...etc
You can also view the process visually using ps, like this:
$ ps -ef f UID PID PPID C STIME TTY STAT TIME CMD root 2 0 0 12:09 ? S 0:00 [kthreadd] root 3 2 0 12:09 ? S 0:00 \_ [ksoftirqd/0] root 5 2 0 12:09 ? S< 0:00 \_ [kworker/0:0H] root 7 2 0 12:09 ? S 0:00 \_ [migration/0] root 8 2 0 12:09 ? S 0:00 \_ [rcu_bh] root 9 2 0 12:09 ? S 0:00 \_ [rcu_sched] root 10 2 0 12:09 ? S 0:00 \_ [watchdog/0] . . . . ...etc
Note if a child process is note responding for any reason, then a quick way to fix this is by killing the parent process.
One of the things you will often find yourself doing is locating a particular process. This can be done by piping the output of "ps -ef" to a grep command. However you can also use the pgrep command. For example to list all processes called something like "http" and are running under the user "apache", we do:
$ pgrep -lu apache http 20428 httpd 20429 httpd 20430 httpd 20431 httpd 20432 httpd
[post-content post_name=rhsca-quiz]
$ ps -ef
# this list's (e)very process in (f)ull format
$ pstree -ap
# this will show all (p)id values and command line (a)rguements
$ pgrep -lu apache httpd
# this will (l)ist pids along with process names that are running by the given (u)ser
$ pgrep -lu root
# this will (l)ist pids along with process names that are running by the given (u)ser