7.8 KiB
| title | TARGET DECK | FILE TAGS | tags | |
|---|---|---|---|---|
| GAWK | Obsidian::STEM | linux::cli gawk |
|
Overview
%%ANKI
Basic
How was the name awk derived?
Back: By taking the first initials of the original three creators.
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%%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
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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
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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.
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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
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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
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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
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%%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
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%%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
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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
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%%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
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%%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%%
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
- Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf