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CMD vs ENTRYPOINT

· 2 min read
Xiaohai Huang

The ENTRYPOINT specifies a command that will always be executed when the container starts.

The CMD specifies arguments that will be fed to the ENTRYPOINT.

  • ENTRYPOINT can also be overridden using --entrypoint flag
  • CMD will be overridden when running the container with alternative arguments (e.g. bash).

ping example​

Dockerfile
FROM debian:wheezy
ENTRYPOINT ["/bin/ping"]
CMD ["localhost"]

Running the image without any argument will ping the localhost

$ docker run -it ping-image
PING localhost (127.0.0.1): 48 data bytes
56 bytes from 127.0.0.1: icmp_seq=0 ttl=64 time=0.096 ms
56 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.088 ms
56 bytes from 127.0.0.1: icmp_seq=2 ttl=64 time=0.088 ms
^C--- localhost ping statistics ---
3 packets transmitted, 3 packets received, 0% packet loss
round-trip min/avg/max/stddev = 0.088/0.091/0.096/0.000 ms

Running the image with an argument will ping the argument

$ docker run -it test google.com
PING google.com (173.194.45.70): 48 data bytes
56 bytes from 173.194.45.70: icmp_seq=0 ttl=55 time=32.583 ms
56 bytes from 173.194.45.70: icmp_seq=2 ttl=55 time=30.327 ms
56 bytes from 173.194.45.70: icmp_seq=4 ttl=55 time=46.379 ms
^C--- google.com ping statistics ---
5 packets transmitted, 3 packets received, 40% packet loss
round-trip min/avg/max/stddev = 30.327/36.430/46.379/7.095 ms
info

Docker has a default entrypoint which is /bin/sh -c

If the Dockerfile is:

Dockerfile
FROM debian:wheezy
CMD ["/bin/ping", "localhost"]

Running the image without any argument will ping localhost

$ docker run -it ping-image
PING localhost (127.0.0.1): 48 data bytes
56 bytes from 127.0.0.1: icmp_seq=0 ttl=64 time=0.096 ms
56 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.088 ms
56 bytes from 127.0.0.1: icmp_seq=2 ttl=64 time=0.088 ms
^C--- localhost ping statistics ---
3 packets transmitted, 3 packets received, 0% packet loss
round-trip min/avg/max/stddev = 0.088/0.091/0.096/0.000 ms

Running the image with argument will run the argument, the

docker run -it test bash
root@e8bb7249b843:/#

References: