284 lines
11 KiB
Markdown
284 lines
11 KiB
Markdown
---
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title: GAWK
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TARGET DECK: Obsidian::STEM
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FILE TAGS: linux::cli gawk
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tags:
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- gawk
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---
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## Overview
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%%ANKI
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Basic
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How was the name `awk` derived?
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Back: By taking the first initials of the original three creators.
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<!--ID: 1706726911434-->
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END%%
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%%ANKI
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Basic
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What does the term `awk` refer to?
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Back: Both the `awk` program and the `awk` language.
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Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. [https://www.gnu.org/software/gawk/manual/gawk.pdf](https://www.gnu.org/software/gawk/manual/gawk.pdf)
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<!--ID: 1706726911439-->
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END%%
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> Dark corners are basically fractal - no matter how much you illuminate, there's always a smaller but darker one. #quote
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The above quote is attributed to Brian Kernighan (one of the authors of the [[C]] K&R book).
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%%ANKI
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Cloze
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Dark corners are basically {1:fractal} - {1:no matter how much you illuminate, there's always a smaller but darker one.} - Brian Kernighan
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Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. [https://www.gnu.org/software/gawk/manual/gawk.pdf](https://www.gnu.org/software/gawk/manual/gawk.pdf)
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<!--ID: 1706726911442-->
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END%%
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## Setup
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Robbins suggests executing command `set +H` on [[bash]] startup to disable [[C]] shell-style command history.
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## Structure
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`awk` applies actions to lines matching specified patterns. In this way `awk` is said to be data-driven - we specify the lines `awk` should act on and `awk` is responsible for finding and acting on them. Instructions are provided via a **program**.
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%%ANKI
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Basic
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What is the basic function of `awk`?
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Back: To apply actions to lines matching specified patterns.
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Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. [https://www.gnu.org/software/gawk/manual/gawk.pdf](https://www.gnu.org/software/gawk/manual/gawk.pdf)
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<!--ID: 1706726911445-->
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END%%
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An `awk` program consists of **rules**, each made up by a **pattern** and **action**. For example:
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```awk
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BEGIN { print "hello world" }
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pattern { action }
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```
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%%ANKI
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Basic
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An `awk` program consists of a series of what?
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Back: Rules.
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Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. [https://www.gnu.org/software/gawk/manual/gawk.pdf](https://www.gnu.org/software/gawk/manual/gawk.pdf)
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<!--ID: 1706726911448-->
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END%%
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%%ANKI
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Basic
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A rule found in an `awk` program consists of what two parts?
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Back: A pattern and an action.
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Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. [https://www.gnu.org/software/gawk/manual/gawk.pdf](https://www.gnu.org/software/gawk/manual/gawk.pdf)
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<!--ID: 1706726911451-->
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END%%
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%%ANKI
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Basic
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A standalone `awk` program usually has what shebang?
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Back: `#!/bin/awk -f`
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Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. [https://www.gnu.org/software/gawk/manual/gawk.pdf](https://www.gnu.org/software/gawk/manual/gawk.pdf)
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<!--ID: 1706726911454-->
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END%%
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%%ANKI
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Basic
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Write the `awk` command that searches file `mail-list` for string `li`.
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Back:
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```bash
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$ awk '/li/' mail-list
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```
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Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. [https://www.gnu.org/software/gawk/manual/gawk.pdf](https://www.gnu.org/software/gawk/manual/gawk.pdf)
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<!--ID: 1706819150999-->
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END%%
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%%ANKI
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Basic
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How is an `awk` rule without a pattern interpreted?
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Back: As applying the specified action for every input line.
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Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. [https://www.gnu.org/software/gawk/manual/gawk.pdf](https://www.gnu.org/software/gawk/manual/gawk.pdf)
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<!--ID: 1706822165633-->
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END%%
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%%ANKI
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Basic
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How is an `awk` rule without an action interpreted?
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Back: As printing every line matching the specified pattern.
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Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. [https://www.gnu.org/software/gawk/manual/gawk.pdf](https://www.gnu.org/software/gawk/manual/gawk.pdf)
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<!--ID: 1706822165637-->
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END%%
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%%ANKI
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Basic
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Describe what the following command does in in a single sentence:
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```bash
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$ awk 'length($0) > 80' data
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```
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Back: Prints every line of `data` that is longer than `80` characters.
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Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. [https://www.gnu.org/software/gawk/manual/gawk.pdf](https://www.gnu.org/software/gawk/manual/gawk.pdf)
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<!--ID: 1706822165641-->
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END%%
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`awk` reads in files in units called **records**. Each record is automatically broken up into chunks called **fields**. By default, a record corresponds to a single line. `$0` would then refer to the entire line and `$1` would refer to the first field of this line.
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%%ANKI
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Basic
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In `awk`, what does a "record" refer to?
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Back: The unit that input is read in.
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Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. [https://www.gnu.org/software/gawk/manual/gawk.pdf](https://www.gnu.org/software/gawk/manual/gawk.pdf)
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<!--ID: 1707310981378-->
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END%%
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%%ANKI
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Basic
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What is the default record separator?
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Back: The newline character.
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Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. [https://www.gnu.org/software/gawk/manual/gawk.pdf](https://www.gnu.org/software/gawk/manual/gawk.pdf)
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<!--ID: 1707310981380-->
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END%%
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%%ANKI
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Cloze
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The {`RS`} variable is used to change the {record separator}.
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Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. [https://www.gnu.org/software/gawk/manual/gawk.pdf](https://www.gnu.org/software/gawk/manual/gawk.pdf)
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<!--ID: 1707310981382-->
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END%%
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%%ANKI
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Cloze
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If `RS` is a string with {more than one character}, it is treated as a {regexp}.
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Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. [https://www.gnu.org/software/gawk/manual/gawk.pdf](https://www.gnu.org/software/gawk/manual/gawk.pdf)
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<!--ID: 1707310981384-->
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END%%
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%%ANKI
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Cloze
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The {`RT`} variable matches the {input characters that matched `RS`}.
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Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. [https://www.gnu.org/software/gawk/manual/gawk.pdf](https://www.gnu.org/software/gawk/manual/gawk.pdf)
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<!--ID: 1707310981386-->
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END%%
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%%ANKI
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Basic
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Barring the final record, when is `RT` always equal to `RS`?
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Back: When `RS` is a string containing a single character.
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Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. [https://www.gnu.org/software/gawk/manual/gawk.pdf](https://www.gnu.org/software/gawk/manual/gawk.pdf)
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<!--ID: 1707310981388-->
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END%%
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%%ANKI
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Basic
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What value of `RS` may `gawk` not process correctly?
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Back: A regexp with optional trailing part, e.g. `AB(XYZ)?`.
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Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. [https://www.gnu.org/software/gawk/manual/gawk.pdf](https://www.gnu.org/software/gawk/manual/gawk.pdf)
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<!--ID: 1707310981390-->
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END%%
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%%ANKI
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Basic
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What implementation detail inspires avoiding `RS = "\0"`?
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Back: Most `awk` implementations store strings internally as C-style strings?
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Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. [https://www.gnu.org/software/gawk/manual/gawk.pdf](https://www.gnu.org/software/gawk/manual/gawk.pdf)
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<!--ID: 1707310981392-->
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END%%
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%%ANKI
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Basic
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What equivalent assignment do most `awk` implementations interpret `RS = "\0"` as?
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Back: `RS = ""`
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Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. [https://www.gnu.org/software/gawk/manual/gawk.pdf](https://www.gnu.org/software/gawk/manual/gawk.pdf)
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<!--ID: 1707310981394-->
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END%%
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%%ANKI
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Basic
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In `awk`, what does a "field" refer to?
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Back: A specific part of a record.
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Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. [https://www.gnu.org/software/gawk/manual/gawk.pdf](https://www.gnu.org/software/gawk/manual/gawk.pdf)
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<!--ID: 1707310981395-->
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END%%
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%%ANKI
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Basic
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What is `$0` a placeholder for?
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Back: An entire record.
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Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. [https://www.gnu.org/software/gawk/manual/gawk.pdf](https://www.gnu.org/software/gawk/manual/gawk.pdf)
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<!--ID: 1706823790230-->
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END%%
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%%ANKI
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Basic
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What is `$1` a placeholder for?
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Back: The first field of a record.
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Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. [https://www.gnu.org/software/gawk/manual/gawk.pdf](https://www.gnu.org/software/gawk/manual/gawk.pdf)
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<!--ID: 1706823790233-->
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END%%
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%%ANKI
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Basic
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How is the `BEGIN` pattern interpreted?
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Back: Code associated with it executes before any input is read.
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Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. [https://www.gnu.org/software/gawk/manual/gawk.pdf](https://www.gnu.org/software/gawk/manual/gawk.pdf)
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<!--ID: 1706823790236-->
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END%%
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%%ANKI
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Basic
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How is the `END` pattern interpreted?
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Back: Code associated with it executes after all input has been read.
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Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. [https://www.gnu.org/software/gawk/manual/gawk.pdf](https://www.gnu.org/software/gawk/manual/gawk.pdf)
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<!--ID: 1706823790239-->
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END%%
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%%ANKI
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Basic
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Describe what the following command does in in a single sentence:
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```bash
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$ awk 'NF > 0' data
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```
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Back: Prints every record in `data` with at least one field.
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Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. [https://www.gnu.org/software/gawk/manual/gawk.pdf](https://www.gnu.org/software/gawk/manual/gawk.pdf)
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<!--ID: 1706823927382-->
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END%%
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%%ANKI
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Basic
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Describe what the following command does in in a single sentence:
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```bash
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$ awk 'END { print NR }' data
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```
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Back: Prints the number of records in `data`.
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Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. [https://www.gnu.org/software/gawk/manual/gawk.pdf](https://www.gnu.org/software/gawk/manual/gawk.pdf)
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<!--ID: 1706824091124-->
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END%%
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%%ANKI
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Basic
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Describe what the following command does in in a single sentence:
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```bash
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$ awk 'NR % 2 == 0' data
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```
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Back: Prints every even-numbered record in `data`.
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Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. [https://www.gnu.org/software/gawk/manual/gawk.pdf](https://www.gnu.org/software/gawk/manual/gawk.pdf)
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<!--ID: 1706824091128-->
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END%%
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`awk` is said to be a "line-oriented" language. Every rule's action must begin on the same line as the pattern.
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%%ANKI
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Basic
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When can a rule's pattern and action exist on different lines?
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Back: Only when using backslash continuation.
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Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. [https://www.gnu.org/software/gawk/manual/gawk.pdf](https://www.gnu.org/software/gawk/manual/gawk.pdf)
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<!--ID: 1706883732944-->
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END%%
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## Exit Status
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On success, `gawk` exits with status code `EXIT_SUCCESS`. On failure, with status code `EXIT_FAILURE`. On fatal error, `gawk` exists with status code `2`. #c
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You can specify a custom exit status by using the `exit` statement from within the `awk` program.
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## References
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* Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. [https://www.gnu.org/software/gawk/manual/gawk.pdf](https://www.gnu.org/software/gawk/manual/gawk.pdf) |