204 lines
8.1 KiB
Markdown
204 lines
8.1 KiB
Markdown
---
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title: Bash
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TARGET DECK: Obsidian::STEM
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FILE TAGS: bash
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tags:
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- bash
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---
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## Overview
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Bash, the Bourne-Again Shell, is a shell program and command language.
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%%ANKI
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Basic
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How do you escape a `'` within a single-quote string?
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Back: This is impossible.
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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)
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<!--ID: 1706816752230-->
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END%%
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%%ANKI
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Basic
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What does the null string refer to?
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Back: The empty string, i.e. `""`.
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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)
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<!--ID: 1706816752237-->
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END%%
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%%ANKI
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Basic
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When does Bash remove null strings from a command?
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Back: When they occur as part of a non-null command-line argument.
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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)
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<!--ID: 1706816752241-->
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END%%
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%%ANKI
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Basic
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Why does the following not work correctly?
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```bash
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$ # -F specifies the field separator
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$ awk -F"" 'program' files
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```
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Back: Bash removes the null string before executing the command.
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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)
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<!--ID: 1706816764555-->
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END%%
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## Prompts
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According to Robbins a POSIX-compliant shell (like Bash) generally has the primary and secondary prompts denoted with `$` and `>` respectively. Adjust these values using environment variables `$PS1` and `PS2` respectively.
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%%ANKI
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Basic
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What symbol is usually used to denote the primary prompt?
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Back: `$$`
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Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. [https://www.gnu.org/software/gawk/manual/gawk.pdf](https://www.gnu.org/software/gawk/manual/gawk.pdf)
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<!--ID: 1706882670149-->
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END%%
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%%ANKI
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Basic
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What environment variable controls Bash's primary prompt?
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Back: `$$PS1`
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Reference: Cooper, Mendel. “Advanced Bash-Scripting Guide,” n.d., 916.
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<!--ID: 1706973587222-->
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END%%
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%%ANKI
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Basic
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What symbol is usually used to denote the secondary prompt?
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Back: `>`
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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)
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<!--ID: 1706882670158-->
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END%%
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%%ANKI
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Basic
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What environment variable controls Bash's secondary prompt?
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Back: `$$PS2`
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Reference: Cooper, Mendel. “Advanced Bash-Scripting Guide,” n.d., 916.
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<!--ID: 1706973587232-->
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END%%
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Paths supplied to commands are typically "sanitized" by prefixing the path name with `./`. This is mentioned in a few different places:
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* `find -execdir` performs this prefixing automatically on all found files.
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* `awk` ambiguously interprets a file named e.g. `count=1` as variable assignment. Should write `$ awk -f program.awk ./count=1`.
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%%ANKI
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Basic
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What methodology is commonly used to "sanitize" paths supplied as command-line arguments?
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Back: Prefixing the paths with `./`.
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Reference: Cooper, Mendel. “Advanced Bash-Scripting Guide,” n.d., 916.
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<!--ID: 1706885111460-->
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END%%
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## Shebang
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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 `#!`.
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%%ANKI
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Basic
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What ASCII characters do shebangs start with?
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Back: `#!`
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Reference: Cooper, Mendel. “Advanced Bash-Scripting Guide,” n.d., 916.
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<!--ID: 1706726911458-->
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END%%
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%%ANKI
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Basic
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What class of programs can be specified in a shebang?
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Back: Interpreters
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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)
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<!--ID: 1706726911461-->
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END%%
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%%ANKI
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Basic
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The OS will parse a shebang into what three parts?
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Back: `#!`, the interpreter, and the rest of the line as a single argument.
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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)
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<!--ID: 1706726911464-->
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END%%
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%%ANKI
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Basic
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What gotcha does Robbins highlight with shebang parsing?
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Back: The remainder of the line following the interpreter is passed as a *single* argument.
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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)
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<!--ID: 1706726911467-->
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END%%
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Some systems limit the length of interpreters to just 32 characters. A simple workaround when encountering this limitation is to introduce a symbolic link.
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%%ANKI
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Basic
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What workaround is used when shebang interpreter names are too long?
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Back: Introduce a symbolic link.
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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)
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<!--ID: 1706726911470-->
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END%%
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## Robustness
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An interesting point Robbins discusses in his introduction to [[posix/awk/index|gawk]] is this idea of command robustness. He states that:
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> A self-contained shell script is more reliable because there are no other files to misplace.
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%%ANKI
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Basic
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What is a self-contained shell script?
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Back: A shell script that does not rely on the presence of other files.
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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)
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<!--ID: 1706726911473-->
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END%%
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%%ANKI
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Basic
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What makes a self-contained shell script more reliable?
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Back: There are no other files to misplace.
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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)
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<!--ID: 1706726911475-->
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END%%
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He argues that the first command below is more robust than the second since the command is more loosely coupled to its environment:
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```bash
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$ awk 'program' input-file1 input-file2 ...
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$ awk -f program-file input-file1 input-file2 ...
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```
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It's interesting to think what else can be used as a measure of a command's robustness:
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* Required environment variables or environment variables formatted in a certain way
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* `$PATH` needs to point to a specific location
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* Whether the invoked program is present by default in a distribution or must be installed
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* The ability to run with or without an associated TTY
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* The ability to run with or without associated standard streams
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* `stdout`, `stderr`, `stdin`
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* How backwards compatible the invoked program is
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* Version mismatch may silently cause the same invocation to fail
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* Expected permissions
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* EUID, read permissions on an input file, etc.
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* Determinism of the program itself
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* Does output rely entirely on input or can it make nondeterministic choices
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* The amount of resources dedicated to the program
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* Failure may occur if not enough memory is provided to the command
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* Whether a program acts idempotently
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* What happens if I run the command twice in a row?
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* Whether a program acts atomically
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* Is it possible intermediate files are left that affect subsequent runs?
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* The presence of timeouts
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* Perhaps a program waits a specified amount of time before input is available. The command's success is now externally determined.
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* Locale-aware functionality
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* Consider for instance [[posix/awk/index|gawk]]'s `\u` [[escape-sequences|sequence]] which targets characters in the current locale's character set as opposed to specifically Unicode.
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* POSIX standard [[regexp#Character Classes|character classes]] serve as another example.
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The above scenarios are what makes something like nix so compelling.
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## Bibliography
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* Cooper, Mendel. “Advanced Bash-Scripting Guide,” n.d., 916.
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* 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) |