12 KiB
| title | TARGET DECK | FILE TAGS | tags | |
|---|---|---|---|---|
| GAWK | Obsidian::STEM | linux::cli gawk |
|
Overview
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Basic
How was the name awk derived?
Back: By taking the first initials of the original three creators.
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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).
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 }
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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
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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
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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
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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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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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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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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
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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
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%%ANKI Basic By default, fields are separated by what? Back: A 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
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Basic
How does awk define whitespace?
Back: As only spaces, tabs, and newlines.
Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf
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Basic
How are fields referenced?
Back: Via the $$ sign.
Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf
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Basic
What is $0 a placeholder for?
Back: An entire record.
Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf
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Basic
What is $1 a placeholder for?
Back: The first field of a record.
Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf
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Basic
How can you remove trailing fields of $0?
Back: Assign a smaller value to $NF.
Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf
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Basic
How do you typically recompute the value of $0?
Back: $1 = $1
Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf
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%%ANKI Basic Why does the following not output what you want?
$ ls -l | awk '{ OFS=":"; print $0 }'
Back: $0 wasn't recomputed so it maintains the previous OFS value.
Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf
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%%ANKI Basic How can you update the following to behave correctly?
$ ls -l | awk '{ OFS=":"; print $0 }'
Back:
$ ls -l | awk '{ OFS=":"; $1 = $1; print $0 }'
Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf
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Basic
When is the behavior of the field reference operator (i.e. $$) undefined?
Back: When given a negative number.
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
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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
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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.
%%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
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%%ANKI Basic What is the output of the following?
$ echo ' abc' | awk '{ print }'
Back: abc (with leading whitespace)
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 the output of the following?
$ echo ' abc' | awk ' { $1 = $1; print }'
Back: abc (without leading whitespace)
Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf
END%%
%%ANKI
Basic
How is $$0 rebuilt after assignment $1 = $1?
Back: By intercalating OFS between values of $1 through NF.
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