Set the command line in a container instance to override the default command line operation

Grein
08/29/2024

When you create a container instance, optionally specify a command to override the default command line instruction baked into the container image. This behavior is similar to the --entrypoint command-line argument to docker run.

Like setting environment variables for container instances, specifying a starting command line is useful for batch jobs where you need to prepare each container dynamically with task-specific configuration.

Command line guidelines

By default, the command line specifies a single process that starts without a shell in the container. For example, the command line might run a Python script or executable file. The process can specify additional parameters or arguments.
To execute multiple commands, begin your command line by setting a shell environment that is supported in the container operating system. Examples:

Operating system Default shell

Ubuntu /bin/bash

Alpine /bin/sh

Windows cmd

Follow the conventions of the shell to combine multiple commands to run in sequence.
Depending on the container configuration, you might need to set a full path to the command line executable or arguments.
Set an appropriate restart policy for the container instance, depending on whether the command-line specifies a long-running task or a run-once task. For example, a restart policy of Never or OnFailure is recommended for a run-once task.
If you need information about the default entrypoint set in a container image, use the docker image inspect command.

Operating system	Default shell
Ubuntu	`/bin/bash`
Alpine	`/bin/sh`
Windows	`cmd`

Command line syntax

The command line syntax varies depending on the Azure API or tool used to create the instances. If you specify a shell environment, also observe the command syntax conventions of the shell.

az container create command: Pass a string with the --command-line parameter. Example: --command-line "python myscript.py arg1 arg2").
New-AzureRmContainerGroup Azure PowerShell cmdlet: Pass a string with the -Command parameter. Example: -Command "echo hello".
Azure portal: In the Command override property of the container configuration, provide a comma-separated list of strings, without quotes. Example: python, myscript.py, arg1, arg2).
Resource Manager template or YAML file, or one of the Azure SDKs: Specify the command line property as an array of strings. Example: the JSON array ["python", "myscript.py", "arg1", "arg2"] in a Resource Manager template.

If you're familiar with Dockerfile syntax, this format is similar to the exec form of the CMD instruction.

Examples

	Azure CLI	Portal	Template
Single command	`--command-line "python myscript.py arg1 arg2"`	Command override: `python, myscript.py, arg1, arg2`	`"command": ["python", "myscript.py", "arg1", "arg2"]`
Multiple commands	`--command-line "/bin/bash -c 'mkdir test; touch test/myfile; tail -f /dev/null'"`	Command override: `/bin/bash, -c, mkdir test; touch test/myfile; tail -f /dev/null`	`"command": ["/bin/bash", "-c", "mkdir test; touch test/myfile; tail -f /dev/null"]`

Azure CLI example

As an example, modify the behavior of the microsoft/aci-wordcount container image, which analyzes text in Shakespeare's Hamlet to find the most frequently occurring words. Instead of analyzing Hamlet, you could set a command line that points to a different text source.

To see the output of the microsoft/aci-wordcount container when it analyzes the default text, run it with the following az container create command. No start command line is specified, so the default container command runs. For illustration purposes, this example sets environment variables to find the top three words that are at least five letters long:

az container create \
    --resource-group myResourceGroup \
    --name mycontainer1 \
    --image mcr.microsoft.com/azuredocs/aci-wordcount:latest \
    --environment-variables NumWords=3 MinLength=5 \
    --restart-policy OnFailure

Once the container's state shows as Terminated (use az container show to check state), display the log with az container logs to see the output.

az container logs --resource-group myResourceGroup --name mycontainer1

[('HAMLET', 386), ('HORATIO', 127), ('CLAUDIUS', 120)]

Now set up a second example container to analyze different text by specifying a different command line. The Python script executed by the container, wordcount.py, accepts a URL as an argument, and processes that page's content instead of the default.

For example, to determine the top three words that are at least five letters long in Romeo and Juliet:

az container create \
    --resource-group myResourceGroup \
    --name mycontainer2 \
    --image mcr.microsoft.com/azuredocs/aci-wordcount:latest \
    --restart-policy OnFailure \
    --environment-variables NumWords=3 MinLength=5 \
    --command-line "python wordcount.py http://shakespeare.mit.edu/romeo_juliet/full.html"

Again, once the container is Terminated, view the output by showing the container's logs:

az container logs --resource-group myResourceGroup --name mycontainer2

[('ROMEO', 177), ('JULIET', 134), ('CAPULET', 119)]

Next steps

Task-based scenarios, such as batch processing a large dataset with several containers, can benefit from custom command lines at runtime. For more information about running task-based containers, see Run containerized tasks with restart policies.

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