crystal:tldr:581ed

The command "crystal ${filename-cr}" is using the Crystal programming language to execute a program file.

Here's a breakdown of the different parts of the command:

1. "crystal": This is the command used to execute the Crystal compiler or interpreter, allowing you to run Crystal programs.

2. "${filename-cr}": This is a placeholder syntax called "variable substitution" or "parameter expansion". It is commonly used in command-line interfaces to substitute the value of a variable or command substitution. In this case, "filename" is the variable being substituted.

The "-cr" part in "${filename-cr}" specifies a default value for the "filename" variable if it is not set or empty.

The command is essentially taking the value of the "filename" variable (or using the default value) and using it as an argument for the "crystal" command, informing it which program file to execute.

For example, if the "filename" variable is set to "my_program.cr", the command would be executed as: "crystal my_program.cr", which would execute the Crystal program file named "my_program.cr". If the "filename" variable is not set, the "-cr" default value would be used instead.

Note that the command may need to be adjusted based on the specific context or scripting environment in which it is being used.