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  • Jack Orenstein

Pipelines and Variables

Most interaction with marcel involves the execution of a pipeline. For example:

ls -fr | select (f: now() - f.mtime < days(1))

ls explores the current directory recursively (-r), returning only files (-f). The resulting File objects are piped to the select operator, which keeps only those that have changed in the past day.


You write a command like this, hit return, get your result. But you can also store a pipeline in a variable for later execution, much as you would when writing a function in any programming language.


To store the above command in a variable, use conventional assignment notation, putting the pipeline inside brackets:

recent = [ls -fr | select (f: now() - f.mtime < days(1))

You can run this command at any time by just using the variable as if it were an operator:

recent

You can also parameterize a pipeline. For example, the following pipeline explores the current directory recursively, and locates files with a given extension, (e.g. .py in util.py):


ext = [e: ls -fr | select (f: f.suffix == e)]

You can now run this pipeline, providing the extension of interest either as an argument:

ext .py

Or using flag notation, where the flag name is determined by the parameter's name:

exp -e .py

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