What is Bash used for?

The Bash shell is both a command interpreter and a programming language.

• As a command interpreter, the shell provides the user interface to the rich set of utilities.
• The programming language features allow these utilities to be combined.

The discussion of the shell internals may feel abstract at first. Return to it later after going through practical exercises. Did it help understanding the shell commands and scripts?

How does the shell work?

At its base, a shell is simply a macro processor that executes commands. It parses the command line (or a line of a script), makes variable expansions, etc, and finally, tries to execute the first word. The remaining words are arguments to the first (command).

We’ll return to etc in more detail later.

Command search: Have you ever seen ...: command not found error? How does the shell search for commands?!?

What is a command?

A “command” can be a shell builtin, a shell keyword, an executable binary, an executable script, a shell function or a shell alias. These all behave almost the same.

Command types: Which commands can you use to find what a “command” more precisely is, and where it comes from? What is the precedence of different command types? For example, if you have a shell function and an executable binary with the same name, which one gets executed?

Exit status

Every command returns an exit status value (0-255) to the calling shell. Value zero indicates success, and non-zero value indicates failure. It is up to the calling shell (script) to decide what to do with the exit status.

Special parameter $? expands to the status of the most recently executed foregorund command (pipeline).

Command inputs and outputs

Six standard “channels”:

IN: 1) command arguments, 2) standard input stream (stdin), 3) environment variables

OUT: 1) standard output stream (stdout), 2) standard error stream (stderr), 3) command exit status

Command inputs and outputs: What are the conventions of using different input and output channels, what type of data is communicated through different channels? Give examples for using each.

Variables

As a working assumption, shell has two variable types, strings and arrays of strings (and hash arrays, for Bash 4+). String variables may be exported to the shell’s environment (see ENVIRONMENT chapter in man bash).

• only the environment variables are visible to the commands that the shell starts
• variables are a “sub-class” of PARAMETERS, see man bash…

Environment: How do you promote a variable to an environment variable?

Sub-shells

• Shell spawns sub-shells when explicitly asked to, and…
• Sub-shells inherit exported variables from their parent. No variables or other shell environment changes propagate from the sub-shell back to the parent.

TIP: When playing around, start a new subshell with command bash, and when you are done, exit, and you are back in the original, unaltered environment.

Subshells: How do you run a command (or compound command!) in a sub-shell? Can you name situations when the shell creates a sub-shell?

Interactive or non-interactive?

Shells can be started in interactive or in non-interactive mode. In interactive mode, they accept input typed from the keyboard. When executing non-interactively, shells execute commands read from a file.

Shell initialization: Which shell initialization files are read (in your machine) when interactive/non-interactive shell is started? What initializations should go to which initialization file and why?

Simple command expansion — I press enter and…?

The shell performs the following actions, from left to right:

1. Variable assignments and redirections are stripped away and saved for later processing.
2. Any words left are expanded (next slide).
3. Redirections are performed.
4. Variable assignments undergo expansions and quote removal.
5. Command is executed.

Command expansion: Explain why a=works echo $a does not work, but both a=works bash -c 'echo $a' and a=works; echo $a work?

Expansion types

Expansion is a process of exchanging a word with another word(s), with certain rules. Bash performs following expansions (in order):

1. Brace expansion {...}
2. Tilde expansion ~...
3. Parameter and variable expansion $..., ${...}
4. Arithmetic expansion $((...))
5. Command substitution $(...)
6. Word splitting, see man bash and IFS
7. Pathname expansion (filename wildcards) *, ?, [...]

Tip: Remember the set -x trick!

Quoting — Take it literally

Quoting is used to control expansions, and word splitting

Quoting: …is pretty important. Can you guess what you get with file="file name"; touch $file? Spaces in filenames is evil, the correct solution is to use snake_case or CamelCase. Using double quotes is a good default when expanding variables, unless you intentionally wish to expand a string variable as separate words/arguments!

Signals — there’s a trap

Signals are asynchronous notifications that are sent to processes when certain events occur. Command trap allows you to catch signals and execute specified code when a process receives a signal. Option -l lists all available signals, and option -p lists all signals currently trapped.

Signals: Write a script that uses a temporary file (really, much better to avoid temporary files altogether), and uses trap to clean it after the script exits, or even when the script is interrupted before finishing!

Common pitfalls

Common pitfalls is basically everything above. Let’s try to hit as many as possible.

Good luck! :)