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
root@c520631f652d:/#
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
root@c2d969adde7a:/#
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.
Ex:
docker exec -it c2d969adde7a sh
Direct Output
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 <container-id or name> <command>
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.
Dockerfile Way
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.
Summary
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.