Here’s a complete tutorial on the docker exec command, including its purpose, how to use it, and a comprehensive list of examples.

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What is docker exec?

docker exec is used to run a command inside a running Docker container. Unlike docker attach, which connects to the container’s primary process, docker exec allows you to execute new commands in real-time within the container without affecting the main process.

Key Features of docker exec:

• Run interactive commands (like a bash shell) in a running container.
• Useful for debugging, maintenance, and inspection.
• Supports running single commands or opening an interactive shell.

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Basic Syntax

docker exec [OPTIONS] CONTAINER COMMAND [ARG...]

Common Options:

• -i → Keep STDIN open (interactive mode).
• -t → Allocate a pseudo-TTY (useful for interactive commands like bash).
• -e → Set environment variables inside the container.
• --privileged → Run the command with elevated privileges inside the container.

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Examples of docker exec

1. Run a Simple Command in a Container

docker exec my_container ls /var/log

This command lists the contents of /var/log in my_container.

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2. Open an Interactive Shell (bash)

docker exec -it my_container bash

This opens an interactive bash shell inside my_container.

If the container doesn’t have bash installed, use sh:

docker exec -it my_container sh

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3. Run a Command with Elevated Privileges

docker exec --privileged -it my_container bash

This runs the bash shell with elevated privileges, allowing access to restricted parts of the filesystem.

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4. Execute a Command with Environment Variables

docker exec -e ENV_VAR=value my_container printenv ENV_VAR

This sets ENV_VAR inside the container and prints its value.

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5. Run a Command in a Detached Mode

You can run a command without waiting for it to finish:

docker exec -d my_container touch /tmp/newfile.txt

This creates a file /tmp/newfile.txt inside the container and immediately detaches.

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6. Inspect Running Processes

docker exec my_container ps aux

This lists all running processes in the container.

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7. Monitor Resource Usage

docker exec my_container top

This displays resource usage (CPU, memory) inside the container.

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8. Check the Container’s Hostname

docker exec my_container hostname

Returns the hostname of the container.

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9. Inspect Networking Information

docker exec my_container ifconfig

Displays the network configuration of the container.

If ifconfig isn’t available, use ip:

docker exec my_container ip a

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10. Check Disk Space

docker exec my_container df -h

Shows the container’s disk usage.

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11. Debug an Application Log

docker exec my_container tail -f /var/log/app.log

This command follows the application log inside the container.

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12. Interact with a Database in a Container

MySQL Example:

docker exec -it my_mysql mysql -u root -p

This opens a MySQL client inside the my_mysql container.

PostgreSQL Example:

docker exec -it my_postgres psql -U postgres

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13. Restart a Service Inside the Container

docker exec my_container service nginx restart

Restarts the NGINX service inside the container.

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14. Copy a File from One Directory to Another

docker exec my_container cp /path/to/source /path/to/destination

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15. Run a Health Check

If the container has a custom health check script:

docker exec my_container /usr/local/bin/health_check.sh

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Combining docker exec with Other Commands

1. docker exec + docker logs:
   Debug a service by checking its logs and running commands in the container. docker logs my_container docker exec -it my_container bash
2. docker exec + docker cp:
   Copy a file into the container and then run it. docker cp ./script.sh my_container:/tmp/script.sh docker exec my_container sh /tmp/script.sh

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List of Common docker exec Commands

Command	Description
docker exec my_container ls /var/log	List files in /var/log
docker exec -it my_container bash	Open an interactive bash shell
docker exec -it my_container sh	Open an interactive shell
docker exec my_container ps aux	List running processes
docker exec -e VAR=value my_container env	Set and print an environment variable
docker exec --privileged my_container bash	Run bash with elevated privileges
docker exec -d my_container touch /tmp/file	Run a command in detached mode
docker exec my_container tail -f /var/log/app.log	Follow the log file inside the container
docker exec my_mysql mysql -u root -p	Open MySQL client inside a MySQL container

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Best Practices for Using docker exec:

1. Use docker exec for debugging and interactive tasks.
2. Combine with docker logs to troubleshoot container issues.
3. Avoid running critical commands unless you know the container’s behavior.
4. Use -it for interactive tasks (like opening a shell).

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Common Errors and Solutions

1. “OCI runtime exec failed: exec failed: container not running”
   → Ensure the container is running by checking with docker ps. Start it with docker start CONTAINER_NAME.
2. “command not found”
   → The specified command may not be installed in the container. Use sh or bash to explore and confirm.
3. “permission denied”
   → Use --privileged or check file permissions inside the container.

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