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title TARGET DECK FILE TAGS tags
GAWK Obsidian::STEM linux::cli gawk
gawk

Overview

%%ANKI Basic How was the name awk derived? Back: By taking the first initials of the original three creators.

END%%

%%ANKI Basic What does the term awk refer to? Back: Both the awk program and the awk language. Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

Dark corners are basically fractal - no matter how much you illuminate, there's always a smaller but darker one. #quote

The above quote is attributed to Brian Kernighan (one of the authors of the C K&R book).

%%ANKI Cloze Dark corners are basically {1:fractal} - {1:no matter how much you illuminate, there's always a smaller but darker one.} - Brian Kernighan Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

Setup

Robbins suggests executing command set +H on bash startup to disable C shell-style command history.

Structure

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.

%%ANKI Basic What is the basic function of awk? Back: To apply actions to lines matching specified patterns. Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

An awk program consists of rules, each made up by a pattern and action. For example:

BEGIN { print "hello world" }
pattern { action }

%%ANKI Basic An awk program consists of a series of what? Back: Rules. Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

%%ANKI Basic A rule found in an awk program consists of what two parts? Back: A pattern and an action. Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

%%ANKI Basic A standalone awk program usually has what shebang? Back: #!/bin/awk -f Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

%%ANKI Basic Write the awk command that searches file mail-list for string li. Back:

$ awk '/li/' mail-list

Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

%%ANKI Basic How is an awk rule without a pattern interpreted? Back: As applying the specified action for every input line. Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

%%ANKI Basic How is an awk rule without an action interpreted? Back: As printing every line matching the specified pattern. Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

%%ANKI Basic Describe what the following command does in in a single sentence:

$ awk 'length($0) > 80' data

Back: Prints every line of data that is longer than 80 characters. Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

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.

%%ANKI Basic In awk, what does a "record" refer to? Back: The unit that input is read in. Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

%%ANKI Basic What is the default record separator? Back: The newline character. Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

%%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

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

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

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

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

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

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

END%%

%%ANKI Basic In awk, what does a "field" refer to? Back: A specific part of a record. Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

%%ANKI Basic What is $0 a placeholder for? Back: An entire record. Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

%%ANKI Basic What is $1 a placeholder for? Back: The first field of a record. Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

%%ANKI Basic How is the BEGIN pattern interpreted? Back: Code associated with it executes before any input is read. Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

%%ANKI Basic How is the END pattern interpreted? Back: Code associated with it executes after all input has been read. Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

%%ANKI Basic Describe what the following command does in in a single sentence:

$ awk 'NF > 0' data

Back: Prints every record in data with at least one field. Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

%%ANKI Basic Describe what the following command does in in a single sentence:

$ awk 'END { print NR }' data

Back: Prints the number of records in data. Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

%%ANKI Basic Describe what the following command does in in a single sentence:

$ awk 'NR % 2 == 0' data

Back: Prints every even-numbered record in data. Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

awk is said to be a "line-oriented" language. Every rule's action must begin on the same line as the pattern.

%%ANKI Basic When can a rule's pattern and action exist on different lines? Back: Only when using backslash continuation. Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

Exit Status

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

You can specify a custom exit status by using the exit statement from within the awk program.

References