--- title: GAWK TARGET DECK: Obsidian::STEM FILE TAGS: linux::cli gawk tags: - 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](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](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. ## Usage `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](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: ```awk 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](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](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](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: ```bash $ awk '/li/ { print }' mail-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) 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](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](https://www.gnu.org/software/gawk/manual/gawk.pdf) END%% %%ANKI Basic Describe what the following command does in in a single sentence: ```bash $ awk 'length($0) > 80' data ``` Back: Prints every line of `data` that is longer than `80` characters. Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. [https://www.gnu.org/software/gawk/manual/gawk.pdf](https://www.gnu.org/software/gawk/manual/gawk.pdf) END%% **Targets** are specified as `$n` where `n` is a placeholder for the `n`th whitespace-separated **field**s 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](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](https://www.gnu.org/software/gawk/manual/gawk.pdf) END%% %%ANKI Basic What is the `BEGIN` pattern? 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](https://www.gnu.org/software/gawk/manual/gawk.pdf) END%% %%ANKI Basic What is the `END` pattern? 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](https://www.gnu.org/software/gawk/manual/gawk.pdf) END%% %%ANKI Basic Describe what the following command does in in a single sentence: ```bash $ 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](https://www.gnu.org/software/gawk/manual/gawk.pdf) END%% %%ANKI Basic Describe what the following command does in in a single sentence: ```bash $ 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](https://www.gnu.org/software/gawk/manual/gawk.pdf) END%% %%ANKI Basic Describe what the following command does in in a single sentence: ```bash $ 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](https://www.gnu.org/software/gawk/manual/gawk.pdf) 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)