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Joshua Potter 2024-02-02 07:48:14 -07:00
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---
title: Prompts
TARGET DECK: Obsidian::STEM
FILE TAGS: bash
tags:
- bash
---
## Overview
According to Robbins a POSIX-compliant shell (like Bash) generally has the primary and secondary prompts denoted with `$` and `>` respectively.
%%ANKI
Basic
What symbol is usually used to denote the primary prompt?
Back: `$$`
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)
<!--ID: 1706882670149-->
END%%
%%ANKI
Basic
What symbol is usually used to denote the secondary prompt?
Back: `>`
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)
<!--ID: 1706882670158-->
END%%
Paths supplied to commands are typically "sanitized" by prefixing the path name with `./`. This is mentioned in a few different places:
* `find -execdir` performs this prefixing automatically on all found files.
* `awk` ambiguously interprets a file named e.g. `count=1` as variable assignment. Should write `$ awk -f program.awk ./count=1`.
%%ANKI
Basic
What methodology is commonly used to "sanitize" paths supplied as command-line arguments?
Back: Prefixing the paths with `./`.
Reference: Cooper, Mendel. “Advanced Bash-Scripting Guide,” n.d., 916.
<!--ID: 1706885111460-->
END%%
## References
* Cooper, Mendel. “Advanced Bash-Scripting Guide,” n.d., 916.
* 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)

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## Overview ## Overview
The shebang (also writting shabang or sha-bang) is a magic character at the start of a script indicating what command should be run when invoking the script directly. It always begins with ASCII characters `#!`.[^mendel] The shebang (also writting shabang or sha-bang) is a magic character at the start of a script indicating what command should be run when invoking the script directly. It always begins with ASCII characters `#!`.
%%ANKI %%ANKI
Basic Basic

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Robbins suggests executing command `set +H` on [[bash]] startup to disable [[C]] shell-style command history. Robbins suggests executing command `set +H` on [[bash]] startup to disable [[C]] shell-style command history.
## Usage ## 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**. `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**.
@ -88,7 +88,7 @@ Basic
Write the `awk` command that searches file `mail-list` for string `li`. Write the `awk` command that searches file `mail-list` for string `li`.
Back: Back:
```bash ```bash
$ awk '/li/ { print }' mail-list $ awk '/li/' 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) 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)
<!--ID: 1706819150999--> <!--ID: 1706819150999-->
@ -141,7 +141,7 @@ END%%
%%ANKI %%ANKI
Basic Basic
What is the `BEGIN` pattern? How is the `BEGIN` pattern interpreted?
Back: Code associated with it executes before any input is read. 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) 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)
<!--ID: 1706823790236--> <!--ID: 1706823790236-->
@ -149,7 +149,7 @@ END%%
%%ANKI %%ANKI
Basic Basic
What is the `END` pattern? How is the `END` pattern interpreted?
Back: Code associated with it executes after all input has been read. 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) 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)
<!--ID: 1706823790239--> <!--ID: 1706823790239-->
@ -188,6 +188,49 @@ Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 202
<!--ID: 1706824091128--> <!--ID: 1706824091128-->
END%% 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](https://www.gnu.org/software/gawk/manual/gawk.pdf)
<!--ID: 1706883732944-->
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](https://www.gnu.org/software/gawk/manual/gawk.pdf)
<!--ID: 1706885111450-->
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](https://www.gnu.org/software/gawk/manual/gawk.pdf)
<!--ID: 1706885111454-->
END%%
%%ANKI
Basic
Describe what the following command does in in a single sentence:
```bash
$ 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](https://www.gnu.org/software/gawk/manual/gawk.pdf)
<!--ID: 1706885111457-->
END%%
## References ## 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) * 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)

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---
title: "2024-02-02"
---
- [x] Anki Flashcards
- [x] KoL
- [x] Sheet Music (10 min.)
- [x] OGS (1 Life & Death Problem)
- [x] Korean (Read 1 Story)
- [ ] Interview Prep (1 Practice Problem)
- [ ] Log Work Hours (Max 3 hours)
* Read 효자 호랑이 (The Filial Tiger).
* Read through [Project #1 - Buffer Pool](https://15445.courses.cs.cmu.edu/fall2022/project1/) in anticipation of my call with Kevin later.