Shell

High-level

• A shell program is typically an executable binary that takes commands that you type and (once you hit return), translates those commands into (ultimately) system calls to the Operating System API.
  • a shell language is a language designed to ‘glue’ together other programs.
• a shell is simply a macro processor that executes commands.
  • The term macro processor means functionality where text and symbols are expanded to create larger expressions.

Since other shells are also programs, they can be run from within one another

"Although most users think of the shell as an interactive command interpreter, it is really a programming language in which each statement runs a command. Because it must satisfy both the interactive and programming aspects of command execution, it is a strange language, shaped as much by history as by design."

• GNU specifies the shell as both a programming language AND a command interpreter. The command interpreter part provides the user interface to the GNU utilities, while the programming language part allows these utilities to be combined.

Piping

Shell languages are exceptional at one thing: piping

• The |, & and ; operators, plus () and ``` form a tidy little language for describing pipelines.

  • a & b is concurrent
  • a ; b is sequential
  • a | b is a pipeline where a feeds b

• Think of ( a & b & c ) | tee capture | analysis as the kind of thing that's hard to express in Ruby (or Python).

• environment variables exist in the same environment as the code that is executed

Precedence of Synonyms

1. Alias
2. Keyword (ie. syntax of the shell)
3. Function
4. Builtin
   • commands that are built into the shell. These are executed directly, as opposed to the shell having to load and execute the executable
     • ex. pwd, cd

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interactive means that input is accepted from the command line, while non-interactive means that input is accepted from a file.

if the first word of a given command does not correspond to a built-in shell command, then the shell assumed that it is the name of an executable file.

when we run bash -c, a script, or other executable in the shell, it becomes a child of the current environment.

• ex. bash -c 'echo "child [$var]"; will only have access to $var if it was exported beforehand from the shell that the command is run in.

In bash utilities, the -- signifies the end of command options, after which only positional parameters are accepted.

• often, -- can signify the separation from the command options, and the following Regex

Status code 127

Think of it as "Error: the program you tried to use was not found"

• it's returned by your shell /bin/bash when any given command within your bash script or on bash command line is not found in any of the paths defined by PATH system environment variable.
• to fix, make sure the command we are using is discoverable through $PATH

Subshell: $()

when we execute commands within $(...), they are executed in a subshell

UE Resources

GNU High-quality documentation

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Children

1. CLI
2. Cmds
3. Cook
4. Direnv
5. Expansion
6. Glob
7. Lang
8. Pipeline
9. Redirection
10. Script
11. Special Parameters
12. Strings
13. Subshell
14. Test
15. Variables