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notebook/notes/gawk/variables.md

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---
title: Variables
TARGET DECK: Obsidian::STEM
FILE TAGS: linux::cli gawk
tags:
- gawk
---
## Overview
Variables are defined like `var=val`. They can be specified in two different places:
1. Via the `-v` command line flag. Using this allows accessing the variable value from within a `BEGIN` rule.
2. In the file list. Using this allows accessing the variable value in all subsequent file processing.
%%ANKI
Basic
Where in an `awk` invocation can variables be assigned?
Back: As a `-v` argument or in the file list.
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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END%%
%%ANKI
Basic
The `-v` flag was introduced to accommodate what functionality?
Back: Accessing variables from a `BEGIN` rule.
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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END%%
%%ANKI
Basic
Describe what the following command does in in a single sentence:
```bash
$ awk 'program' pass=1 data pass=2 data
```
Back: Evaluates `'program'` against the `data` file twice with a different value of `pass` on each run.
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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END%%
%%ANKI
Basic
How is `stdin` specified in `awk`'s file list?
Back: `-`
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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END%%
## Predefined Variables
There exists a number of useful predefined variables:
* `NR` (**N**umber of **R**ecords)
* The 1-indexed number of records so far read.
* The count includes the current record.
* `FNR` (**F**ile **N**umber of **R**ecords)
* The 1-indexed number of records so far read from the current file.
* The count includes the current record.
%%ANKI
Cloze
The {`NR`} variable specifies the {number of read input records}.
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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END%%
%%ANKI
Cloze
The {`FNR`} variable specifies the {number of read input records for the current file}.
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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END%%
* `RS` (**R**ecord **S**eparator)
* The separator used to distinguish records from one another.
* `RT` (**R**ecord **T**ext)
* The matching separator used to distinguish the currently read record.
%%ANKI
Cloze
The {`RS`} variable is used to change the {record separator}.
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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END%%
%%ANKI
Cloze
If `RS` is a string with {more than one character}, it is treated as a {regexp}.
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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END%%
%%ANKI
Cloze
The {`RT`} variable matches the {input characters that matched `RS`}.
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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END%%
%%ANKI
Basic
Barring the final record, when is `RT` always equal to `RS`?
Back: When `RS` is a string containing a single character.
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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END%%
%%ANKI
Basic
What value of `RS` may `gawk` not process correctly?
Back: A regexp with optional trailing part, e.g. `AB(XYZ)?`.
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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END%%
%%ANKI
Basic
What implementation detail inspires avoiding `RS = "\0"`?
Back: Most `awk` implementations store strings internally as C-style strings.
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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END%%
%%ANKI
Basic
What equivalent assignment do most `awk` implementations interpret `RS = "\0"` as?
Back: `RS = ""`
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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END%%
* `NF` (**N**umber of **F**ields)
* The 1-indexed number of fields found in the current record.
* `FS` (**F**ield **S**eparator)
* The separator used to distinguish fields from one another.
* Defaults to `" "` which is a special character for **runs** of spaces, tabs, and newlines.
%%ANKI
Basic
What is the arithmetical value of `${NF + 1}`?
Back: `0`
Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. [https://www.gnu.org/software/gawk/manual/gawk.pdf](https://www.gnu.org/software/gawk/manual/gawk.pdf)
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END%%
%%ANKI
Basic
What is the printed value of `${NF + 1}`?
Back: `""`
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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END%%
%%ANKI
Basic
What value is `${NF + 1}` given when we run `${NF + 2} = "test"`?
Back: `""`
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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END%%
%%ANKI
Cloze
The {`NF`} variable specifies the {number of fields in the current record}.
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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END%%
%%ANKI
Cloze
The {`FS`} variable is used to change the {field separator}.
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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END%%
%%ANKI
Cloze
{`FS`} is to `awk` as {`IFS`} is to Bash.
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)
/END%%
%%ANKI
Basic
What is the default value of `FS`?
Back: `" "`
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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END%%
%%ANKI
Basic
What value of `FS` is specially handled?
Back: `" "`
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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END%%
%%ANKI
Basic
How is `FS = " "` interpreted?
Back: As a contiguous sequence of spaces, tabs, and newlines.
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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END%%
%%ANKI
Basic
What distinguishes `FS` value `" "` and `[ \t\n]+`?
Back: When set to the former, `awk` strips leading and trailing whitespace from each record.
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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END%%
%%ANKI
Cloze
Setting `FS` to {`""`} allows examining {each character of a record separately}.
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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END%%
%%ANKI
Cloze
Setting `FS` to {`"\n"`} treats the {record as the single field}.
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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END%%
%%ANKI
Basic
What value of `FS` ensures `$1 = $0`?
Back: `"\n"`
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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END%%
%%ANKI
Basic
*Why* does `awk` support a CSV mode?
Back: Because CSV fields may contain commas and newlines.
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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END%%
## References
* 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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