In this article I will look at examples of runns.se. You will also learn about the structure of a Dockerfile and the SHELL instruction. You will also learn about the SHELL instruction and CMD. These are common commands used by Docker to start a container.
Structure of a Dockerfile
A Dockerfile defines the basic structure of a running application. The structure is made up of several components, including an RUN directive and a set of rules. These directives are designed to be self-explanatory, but they also allow for comments.
The first part of the structure defines the environment variables. These variables are declared with the ENV statement, and they are used within instructions in the Dockerfile. The environment variables are not evaluated as code in the RUN command, but rather as variables. For example, an environment variable named “id” may have the value “1234567894”, but the variable CONT_IMG_VER should be escaped to escape it from the history of docker processes.
The second part of the structure defines the build stages. The FROM instruction sets the Base Image for subsequent instructions. It must come before an ARG instruction, which is used to declare arguments used in the FROM instruction. The FROM instruction may appear multiple times in a Dockerfile, and can also be repeated in a single Dockerfile.
SHELL instruction
The SHELL instruction executes commands and performs operations based on the values of its parameters. It reads unquoted characters and recognizes candidates for parameter expansion, command substitution, and arithmetic expansion. It then expands the values of the parameters to their appropriate lengths and fields. If the input contains null bytes, the SHELL instruction writes a diagnostic message to standard error.
Invocation of a SHELL instruction is a simple process. The first line of input is delimited by a backslash. The second line of input is processed by applying the first applicable rule to the character to be processed. A token is a character from the current position in input until the delimiter. Characters are included in the token if they are present, including quoting characters. The processing continues until an actual token is reached.
CMD
The CMD command is a shell command used to execute specified commands. The command must be typed and linked to parameters, called options. Options are letters that begin with a minus sign (-) and end with a slash (/). Options can be combined to form more complex commands. CMD also supports placeholders and wild cards. Asterisk (*) and question mark (?) represent different types of characters.
ENTRYPOINT
The ENTRYPOINT at runs, an internet domain, is an entry point for programs. Typically, an entry point will consist of a script or program name. When a program starts, the OS hands over control of the program to the entry point. When an application is unable to start, the user may receive the message “Entry Point Not Found”. This is usually caused by a faulty DLL file, which plays a key role in the startup process.
ENTRYPOINT can also be used with CMD arguments to set a default value for a container. However, when it is used with a command, it should also specify an ENTRYPOINT argument.
