How to view the process tree in Linux

In Linux, each running process is spawned by a parent process, except for the initial init process, which is started by the kernel at boot. This results in a hierarchical structure where processes are organized in a tree format, with parent processes at the top and their child processes branching out below them.

To view this tree-like structure, Linux provides tools like ps and pstree. These commands show how processes are grouped, with child processes listed under their corresponding parent processes. This is helpful for understanding the relationships between processes and for monitoring system performance.

The ps command lists running processes and can display them in a hierarchical view. The pstree command goes further by visually representing the process tree in a clearer and more organized format. Both tools are useful for managing processes and troubleshooting system issues.

1. Open the terminal in Linux.
2. Use the ps command to display running processes.

   $ ps -x
      PID TTY      STAT   TIME COMMAND
     1080 ?        Ss     0:00 /lib/systemd/systemd --user
     1081 ?        S      0:00 (sd-pam)
     1092 tty1     S+     0:00 -bash
     1175 ?        S      0:00 sshd: user@pts/0
     1176 pts/0    Ss     0:00 -bash
     1424 pts/0    R+     0:00 ps -x

3. Use the –forest option with the ps command to display processes in a tree structure.

   $ ps -x --forest
      PID TTY      STAT   TIME COMMAND
     1175 ?        S      0:00 sshd: user@pts/0
     1176 pts/0    Ss     0:00  \_ -bash
     1436 pts/0    R+     0:00      \_ ps -x --forest
     1092 tty1     S+     0:00 -bash
     1080 ?        Ss     0:00 /lib/systemd/systemd --user
     1081 ?        S      0:00  \_ (sd-pam)

   You can use the ps -aux –forest command to see more detailed information about all processes.

   Manual: ps manual

4. Install pstree if it is not available on your system.

   $ sudo apt update && sudo apt install --assume-yes psmisc # Ubuntu and Debian
   $ sudo yum install --assumeyes pstree # CentOS and Red Hat

5. Use the pstree command to view the process tree.

   $ pstree
   systemd─┬─VGAuthService
           ├─accounts-daemon───2*[{accounts-daemon}]
           ├─atd
           ├─cron
           ├─dbus-daemon
           ├─login───bash
           ├─multipathd───6*[{multipathd}]
           ├─networkd-dispat
           ├─packagekitd───2*[{packagekitd}]
           ├─polkitd───2*[{polkitd}]
           ├─rsyslogd───3*[{rsyslogd}]
           ├─snapd───8*[{snapd}]
           ├─sshd───sshd───sshd───bash───pstree
           ├─systemd───(sd-pam)
           ├─systemd-journal
           ├─systemd-logind
           ├─systemd-network
           ├─systemd-resolve
           ├─systemd-timesyn───{systemd-timesyn}
           ├─systemd-udevd
           ├─unattended-upgr───{unattended-upgr}
           └─vmtoolsd───{vmtoolsd}

   You can use the pstree -p option to show process IDs along with the tree structure.

   More options for pstree:

   Usage: pstree [-acglpsStuZ] [ -h | -H PID ] [ -n | -N type ]
                 [ -A | -G | -U ] [ PID | USER ]
          pstree -V
   Display a tree of processes.
   
     -a, --arguments     show command line arguments
     -A, --ascii         use ASCII line drawing characters
     -c, --compact       don't compact identical subtrees
     -h, --highlight-all highlight current process and its ancestors
     -H PID,
     --highlight-pid=PID highlight this process and its ancestors
     -g, --show-pgids    show process group ids; implies -c
     -G, --vt100         use VT100 line drawing characters
     -l, --long          don't truncate long lines
     -n, --numeric-sort  sort output by PID
     -N type,
     --ns-sort=type      sort by namespace type (cgroup, ipc, mnt, net, pid,
                                                 user, uts)
     -p, --show-pids     show PIDs; implies -c
     -s, --show-parents  show parents of the selected process
     -S, --ns-changes    show namespace transitions
     -t, --thread-names  show full thread names
     -T, --hide-threads  hide threads, show only processes
     -u, --uid-changes   show uid transitions
     -U, --unicode       use UTF-8 (Unicode) line drawing characters
     -V, --version       display version information
     -Z, --security-context
                         show SELinux security contexts
     PID    start at this PID; default is 1 (init)
     USER   show only trees rooted at processes of this user

   Manual: pstree - display a tree of processes