Running commands inside a docker container is easier than you think.
A docker container is an isolated environment that usually contains a single application with all required dependencies. Many times we need to run some commands inside a docker container. There are several ways in which we can execute a command inside a container and get the required output.
Let’s see how we can do it.
Using Interactive Shell
We can directly access the shell of a container and execute our commands as with a normal Linux terminal. To get an interactive shell of a stopped (not in running state) container, you can use:
$ docker run -it ubuntu bash [email protected]:/#
As you can see, we landed directly inside a new Ubuntu container where we can run our commands. If a container is already running, you can use exec command as below.
First, let’s find out the container ID.
$ docker ps CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES c2d969adde7a nginx “/docker-entrypoint.…” 2 hours ago Up 2 hours 127.0.0.1:80->80/tcp nginx 0960560bc24b mariadb “docker-entrypoint.s…” 2 hours ago Up 2 hours 127.0.0.1:3306->3306/tcp mariadb
And, then get inside container ID c2d969adde7a
$ docker exec -it c2d969adde7a bash [email protected]:/#
In the above output, you can observe that we started a bash session of nginx container which was in running state. Here we can execute any supported command and get the output.
Note – your container may not have bash and if so, you can use sh.
docker exec -it c2d969adde7a sh
Often we just need the output of one or two commands and do not require a full-blown interactive session for our task. You can run the required command inside a container and get its output directly without opening a new shell session using exec command without -it flag. Its syntax would be:
$ docker exec
Here’s an example:
$ docker ps CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES c2d969adde7a nginx “/docker-entrypoint.…” 2 hours ago Up 2 hours 127.0.0.1:80->80/tcp nginx 0960560bc24b mariadb “docker-entrypoint.s…” 2 hours ago Up 2 hours 127.0.0.1:3306->3306/tcp mariadb $ docker exec 0960560bc24b ps -ef | grep mysql mysql 1 0 0 13:35 ? 00:00:02 mysqld $
We executed ps -ef | grep mysql command inside the running MariaDB container and got the output directly.
This is not the exact way you can run commands inside a docker container though it may be helpful in development situations or for initial deployment debugging etc. We can use RUN command inside a Dockerfile. Here’s our sample Dockerfile:
FROM nginx:latest RUN nginx -V
It simply pulls the latest nginx image from the registry and then runs the nginx -V command to display the Nginx version when you build the image.
$ docker build -t nginx-test . Sending build context to Docker daemon 2.048kB Step 1/2 : FROM nginx:latest —> 7ce4f91ef623 Step 2/2 : RUN nginx -V —> Running in 43918bbbeaa5 nginx version: nginx/1.19.9 built by gcc 8.3.0 (Debian 8.3.0-6) built with OpenSSL 1.1.1d 10 Sep 2019 TLS SNI support enabled configure arguments: –prefix=/etc/nginx –sbin-path=/usr/sbin/nginx –modules-path=/usr/lib/nginx/modules –conf-path=/etc/nginx/nginx.conf –error-log-path=/var/log/nginx/error.log –http-log-path=/var/log/nginx/access.log –pid-path=/var/run/nginx.pid –lock-path=/var/run/nginx.lock –http-client-body-temp-path=/var/cache/nginx/client_temp –http-proxy-temp-path=/var/cache/nginx/proxy_temp –http-fastcgi-temp-path=/var/cache/nginx/fastcgi_temp –http-uwsgi-temp-path=/var/cache/nginx/uwsgi_temp –http-scgi-temp-path=/var/cache/nginx/scgi_temp –user=nginx –group=nginx –with-compat –with-file-aio –with-threads –with-http_addition_module –with-http_auth_request_module –with-http_dav_module –with-http_flv_module –with-http_gunzip_module –with-http_gzip_static_module –with-http_mp4_module –with-http_random_index_module –with-http_realip_module –with-http_secure_link_module –with-http_slice_module –with-http_ssl_module –with-http_stub_status_module –with-http_sub_module –with-http_v2_module –with-mail –with-mail_ssl_module –with-stream –with-stream_realip_module –with-stream_ssl_module –with-stream_ssl_preread_module –with-cc-opt=’-g -O2 -fdebug-prefix-map=/data/builder/debuild/nginx-1.19.9/debian/debuild-base/nginx-1.19.9=. -fstack-protector-strong -Wformat -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2 -fPIC’ –with-ld-opt=’-Wl,-z,relro -Wl,-z,now -Wl,–as-needed -pie’ Removing intermediate container 43918bbbeaa5 —> d682277f2e50 Successfully built d682277f2e50 Successfully tagged nginx-test:latest $
Observe the output of RUN command in the above snippet. The above snippet will be only displayed on the first build and consecutive builds will no-repeat the RUN command output. As a workaround, you can try –no-cache flag:
$ docker build -t nginx-test . –no-cache
The last method is not the best way but can be sometimes helpful for debugging etc.
Running command inside a docker container is simple and there are a few methods available to do it. It is a regular task of a Docker Administrator, so knowing these commands is useful.
Next, learn some of the popular Docker commands.