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64 lines
2.6 KiB
Markdown
64 lines
2.6 KiB
Markdown
# Miller command structure
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## Overview
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The outline of an invocation of Miller is:
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* The program name `mlr`.
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* Options controlling input/output formatting, etc. (See [I/O options](reference-main-io-options.md)).
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* One or more verbs -- such as `cut`, `sort`, etc. (see [Verbs Reference](reference-verbs.md)) -- chained together using [then](reference-main-then-chaining.md). You use these to transform your data.
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* Zero or more filenames, with input taken from standard input if there are no filenames present. (You can place the filenames up front using `--from` or `--mfrom` as described on the [keystroke-savers page](keystroke-savers.md#file-names-up-front-including-from).)
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For example, reading from a file:
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GENMD_RUN_COMMAND
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mlr --icsv --opprint head -n 2 then sort -f shape example.csv
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GENMD_EOF
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GENMD_RUN_COMMAND
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mlr --from example.csv --icsv --opprint head -n 2 then sort -f shape
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GENMD_EOF
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Reading from standard input:
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GENMD_RUN_COMMAND
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cat example.csv | mlr --icsv --opprint head -n 2 then sort -f shape
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GENMD_EOF
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The rest of this reference section gives you full information on each of these parts of the command line.
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See also the [Glossary](glossary.md) for more about terms such as
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[record](glossary.md#record), [field](glossary.md#field),
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[key](glossary.md#value), [streaming](glossary.md#streaming), and more.
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## Verbs vs DSL
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When you type `mlr {something} myfile.dat`, the `{something}` part is called a **verb**. It specifies how you want to transform your data. Most of the verbs are counterparts of built-in system tools like `cut` and `sort` -- but with file-format awareness, and giving you the ability to refer to fields by name.
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The verbs `put` and `filter` are special in that they have a rich expression language (domain-specific language, or "DSL"). More information about them can be found at on the [Intro to Miller's programming language page](programming-language.md); see also [DSL reference](reference-dsl.md) for more details.
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Here's a comparison of verbs and `put`/`filter` DSL expressions:
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Example of using a verb for data processing:
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GENMD_RUN_COMMAND
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mlr stats1 -a sum -f x -g a data/small
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GENMD_EOF
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* Verbs are coded in Go
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* They run a bit faster
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* They take fewer keystrokes
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* There's less to learn
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* Their customization is limited to each verb's options
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Example of doing the same thing using a DSL expression:
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GENMD_RUN_COMMAND
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mlr put -q '@x_sum[$a] += $x; end{emit @x_sum, "a"}' data/small
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GENMD_EOF
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* You get to write your own expressions in Miller's programming language
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* They run a bit slower
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* They take more keystrokes
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* There's more to learn
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* They're highly customizable
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