-bash: pstree: command not found

root@localhost:~# pstree
-bash: pstree: command not found

root@localhost:~# apt install psmisc
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following NEW packages will be installed:
  psmisc
0 upgraded, 1 newly installed, 0 to remove and 5 not upgraded.
Need to get 123 kB of archives.
After this operation, 603 kB of additional disk space will be used.
Get:1 http://cloudfront.debian.net/debian stretch/main amd64 psmisc amd64 22.21-2.1+b2 [123 kB]
Fetched 123 kB in 1s (68.4 kB/s)
Selecting previously unselected package psmisc.
(Reading database ... 45434 files and directories currently installed.)
Preparing to unpack .../psmisc_22.21-2.1+b2_amd64.deb ...
Unpacking psmisc (22.21-2.1+b2) ...
Setting up psmisc (22.21-2.1+b2) ...