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

Targets are specified as $n where n is a placeholder for the nth whitespace-separated fields of the input line. For example, $1 refers to the first field of the input line. $0 is a special target referring to the entire list of arguments, i.e. the entire line.

%%ANKI Basic What is $0 a placeholder for? Back: The entire input line. 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 the 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 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 line of 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 lines 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 line 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%%

Variables

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

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

END%%

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

$ 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

END%%

References