You can run the top command from the Docker host machine using the following command:

docker top [OPTIONS] CONTAINER [ps OPTIONS]  

This gives a list of the running processes of a container without logging in to the container, as follows:

$ sudo docker top  a245253db38b
UID             PID            PPID            C
STIME           TTY            TIME            CMD
root            5232           3585            0
Mar22           ?              00:00:53        python  app.py  
$ sudo docker top  a245253db38b  -aef
UID             PID            PPID            C
STIME           TTY            TIME            CMD
root            5232           3585            0
Mar22           ?              00:00:53        python  app.py  

The Docker top command provides information about the CPU, memory, and swap usage if you run it inside a Docker container:

root@a245253db38b:/opt/webapp# top
top - 19:35:03 up 25 days, 15:50,  0 users,  load average: 0.00,  0.01, 0.05
Tasks:   3 total,   1 running,   2 sleeping,   0 stopped,   0 zombie
%Cpu(s):  0.0%us,  0.0%sy,  0.0%ni, 99.9%id,  0.0%wa,  0.0%hi,   0.0%si,  0.0%st
Mem:   1016292k total,   789812k used,   226480k free,    83280k  buffers
Swap:        0k total,        0k used,        0k free,   521972k  cached
PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    
TIME+  COMMAND
  1 root      20   0 44780  10m 1280 S  0.0  1.1   0:53.69 python
  62 root      20  0 18040 1944 1492 S  0.0  0.2   0:00.01 bash
  77 root      20  0 17208 1164  948 R  0.0  0.1   0:00.00 top  

In case you get the error - TERM environment variable not set error while running the top command inside the container, perform the following steps to resolve it:

Run the echo $TERM command. You will get the result as dumb. Then, run the following command:

$ export TERM=dumb 

This will resolve the error.