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title TARGET DECK FILE TAGS tags
Bash Obsidian::STEM bash
bash

Overview

Bash, the Bourne-Again Shell, is a shell program and command language.

%%ANKI Basic How do you escape a ' within a single-quote string? Back: This is impossible. Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

%%ANKI Basic What does the null string refer to? Back: The empty string, i.e. "". Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

%%ANKI Basic When does Bash remove null strings from a command? Back: When they occur as part of a non-null command-line argument. Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

%%ANKI Basic Why does the following not work correctly?

$ # -F specifies the field separator
$ awk -F"" 'program' files

Back: Bash removes the null string before executing the command. Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

Prompts

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.

%%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

END%%

%%ANKI Basic What environment variable controls Bash's primary prompt? Back: $$PS1 Reference: Cooper, Mendel. “Advanced Bash-Scripting Guide,” n.d., 916.

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

END%%

%%ANKI Basic What environment variable controls Bash's secondary prompt? Back: $$PS2 Reference: Cooper, Mendel. “Advanced Bash-Scripting Guide,” n.d., 916.

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.

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Shebang

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 Basic What ASCII characters do shebangs start with? Back: #! Reference: Cooper, Mendel. “Advanced Bash-Scripting Guide,” n.d., 916.

END%%

%%ANKI Basic What class of programs can be specified in a shebang? Back: Interpreters Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

%%ANKI Basic The OS will parse a shebang into what three parts? Back: #!, the interpreter, and the rest of the line as a single argument. Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

%%ANKI Basic What gotcha does Robbins highlight with shebang parsing? Back: The remainder of the line following the interpreter is passed as a single argument. Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

Some systems limit the length of interpreters to just 32 characters. A simple workaround when encountering this limitation is to introduce a symbolic link.

%%ANKI Basic What workaround is used when shebang interpreter names are too long? Back: Introduce a symbolic link. Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

Robustness

An interesting point Robbins discusses in his introduction to posix/awk/index is this idea of command robustness. He states that:

A self-contained shell script is more reliable because there are no other files to misplace.

%%ANKI Basic What is a self-contained shell script? Back: A shell script that does not rely on the presence of other files. Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

%%ANKI Basic What makes a self-contained shell script more reliable? Back: There are no other files to misplace. Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

He argues that the first command below is more robust than the second since the command is more loosely coupled to its environment:

$ awk 'program' input-file1 input-file2 ...
$ awk -f program-file input-file1 input-file2 ...

It's interesting to think what else can be used as a measure of a command's robustness:

  • Required environment variables or environment variables formatted in a certain way
    • $PATH needs to point to a specific location
  • Whether the invoked program is present by default in a distribution or must be installed
  • The ability to run with or without an associated TTY
  • The ability to run with or without associated standard streams
    • stdout, stderr, stdin
  • How backwards compatible the invoked program is
    • Version mismatch may silently cause the same invocation to fail
  • Expected permissions
    • EUID, read permissions on an input file, etc.
  • Determinism of the program itself
    • Does output rely entirely on input or can it make nondeterministic choices
  • The amount of resources dedicated to the program
    • Failure may occur if not enough memory is provided to the command
  • Whether a program acts idempotently
    • What happens if I run the command twice in a row?
  • Whether a program acts atomically
    • Is it possible intermediate files are left that affect subsequent runs?
  • The presence of timeouts
    • Perhaps a program waits a specified amount of time before input is available. The command's success is now externally determined.
  • Locale-aware functionality

The above scenarios are what makes something like nix so compelling.

Bibliography