Intro to Bash

Next: Compound command Job control Parameters Patterns Arrays IO Sourcing Practices

Notes on The Bash Guide: A quality driven guide through the shell’s many features

Bash resources

Bash FAQs

Key new terms (all chapters)

• append mode - file’s original content is retained and new stream’s bytes are added to it. E.g. echo World >>~/world
• builtin - small operations built into bash
• byte - unit of digital info; eight bits
• compound command - several basic commands grouped into a larger logical block (e.g. an if command)
• control operator - tells bash how preceding command should be executed. (Comes after command in list of commands.) (e.g. ; - start a new line, || - run subsequent command only if preceding command failed, && - logical AND
• duplicating file descriptors - copying one FD’s stream connection to another FD. Use the >& operator, preceded by FD to change and followed by FD whose stream to copy
• file descriptor (FD) - “plug” to connect processes to files, devices or other processes. They are identified by numbers. The first three have standard names:

0 - standard input (e.g. keyboard).
1 - standard output (e.g. window within display). Redirecting FD1 is done with > operator.
2 - standard error (where errors & info messages are sent to - e.g. also display)\

• array - parameter that can hold a list of strings
• kernel - the core part of the computer’s operating system, and has control over everything else in the system
• PATH - an environment variable that contains a set of directories that should be searched for programs. The PATH variable can be updated.
• pipeline - connect two commands by the linking the first process’ standard output to the second’s standard input
• positional parameters - parameters with a number - e.g. $1 - where the number is the argument number
• redirections - operations that change the source or destination of a file descriptor (e.g. write the result of a command to a file instead of to the terminal display). Redirecting standard output (FD1) to a file is most common redirection. Redirect any FD by prefixing the > operator with the number of the FD (e.g. 2> to redirect standard error). Redirections are evaluated from L to R.
• shell internal variables - shell variables created automatically by the bash shell. They are in ALL CAPS.
• simple commands - specifies name of command, with optional arguments, environment variables and file descriptor redirections
• streams - flows of info (bytes) between files, devices & processes
• terminal multiplexer - program that can run several login sessions in the same terminal window

Important commands (all chapters)

• bash - start bash in most shells/terminals (if it’s not already running)
• break - jump out of loop & continue with script after it
• case - evaluates a parameter’s value against several given patterns/choices`
• cat [filename] - show contents of file
• cp - copy a file
• chmod +x [filename] - marks a file as executable
• continue - skips to next iteration of loop
• coproc - allows you to run a command asynchronously (‘in the background’)
• CTRL-C - stops running process
• echo "$BASH_VERSION - check what version of bash currently running
• expansions - e.g $variable or $(command) - TO BE COMPLETED
• fi - closes an if statement and starts the command
• grep - checks input for patterns
• rev - reverses a string/line
• select - offers the user a choice of several options & executes a code block with the user’s choice as the parameter. Menu repeats until break command is executed.
• test ([ ]) - tests things & returns an exit status based on results.
• type \[name] - returns the location of the specified program [name]

Important switches (all chapters)

List of swiches

• -a - show all the possibilities
• -c - copy from a string

What to follow up on (all chapters)

• More practice with adding/removing items from the PATH
• More practice with file descriptor redirections
• Here documents - better understand (something about making FD0 Standard Input read from document between two delimiters)
• Here string - understand use of (making FD0 Standard Input read from string between two delimiters)

Ch. 1: Inception - What is bash and where does it live?

• Each shell program has its own language
• A binary is an executable program that contains binary code executed by the system’s kernel
• Windows doesn’t come with bash installed
• Terminals programs are actually “terminal emulators”, and there are many different ones available
• Bash is only one of many programs that can run in the terminal
• When a program is run, the system creates a running process for it. Processes have plugs (file descriptors) that allow them to connect streams that lead to files, devices or other programs

Ch.2 Commands & Arguments

• Most command execution in bash is synchronous - one command must finish executing before it will execute the next command
• The hashbang line at the top of a bash script /usr/bin/env bash commands the env program to find the bash program and use it for interpreting the language in the script
• Bash is not a strict interpreter. As a result, many bash scripts are buggy.
• Variable assignments go before a command name and apply to the enviroment of the one command only
• There are different kinds of commands (e.g. simple & compound)
• Braces ([ ]) join everything within them into a single compound command
• For delaring functions in bash, the function name is followed by empty parens - e.g. exists()
• Bash only does a PATH search on command names that don’t contain a / character
• Two ways to make characters literal: quotation marks (preferred) and escaping
• Use double-quotes in args with expansions, single quotes everywhere else
• QUOTING ISSUES ARE THE CAUSE OF MANY BASH PROBLEMS. IF THERE IS WHITESPACE OR A SYMBOL IN AN ARG, USE QUOTES AROUND IT!
• To discard unwanted error messages, stream them to the null device - 2>/dev/null
• To send both output and error bytes to a file, need to make sure they’re on the same stream by duplicating file descriptors

Ch. 3 - Variables & Expansions

• Lost notes from the first half of the chapter :(
• Shell variables are stored in the process environment
• Environment varibales are more general than shell variables
• Users can modify/add to both shell & environment variables
• All processes share environment varaibles. Some programs only use some of them.
• Use single quotes to wrap strings (not double)
• Use \ to resume on a new line
• Give your own shell variables lowercase names & environment variables uppercase
• Important to include space in front of negative start values

Tests & Conditionals

• Exit codes are between 0 - 255 inclusive. 0 indicates success; all other numbers indicate failures of some sort.
• A branch is only executed if its exit code 0 (success).
• In conditional command lists, the data gathering command can either precede the if statement or be integrated with it. Both options come with pros and cons.
• Syntax for equality test using [[ ]] is [[ arg = arg ]]. The spacing between commands and args are important!
• OK to string together && operators, but otherwise best not to string together multiple control operators.
• Group statements together with curly braces { ;} to avoid failures of logic. (NEED SEMICOLON/NEWLINE before closing curly brace!)
• Can use grouping for purposes besides conditonal operators (e.g. redirecting input to a group of statements, specifying multiple steps in error handling)
• if not needed to run test/ [] command
• [[ ]] is a new style of conditional testing that has several advantages to [ ]. For example, it supports pattern matching.
• Be aware of when to use [[ and when to use [
• Never use -a and -o tests with [
• Use for loop when you have a list of things & want to run through them sequentiall
• Use while loop when you want to keep going until you find what you’re looking for
• When then are multiple ways to write something, compare the flexibility & simplicity of the resulting code
• until loop not used often - more common to use while !
• choice and select are used for building an application logic based on the content of a variable in a more streamlined way than using a seris of if statements

Loops & Functions

Iterating commands using for loops, while and until loops, select statements and grouping them in functions. (Ch. 9 in PDF - compound commands)

Asynchronous Commands

About jobs, asynchronous commands, job control, process identifiers and signals, process management, inter-process communications. (Ch 11 in PDF - job control)

Colors & Terminal Commands

Terminals and terminal sequences, terminal identifiers, terminfo and terminal capabilities, outputting colors, moving the cursor and querying the terminal’s state. (Not in PDF)

Customizing the Prompt

Prompting, prompt commands and the DEBUG signal, readline, bind and input modes and hotkeys, programmatic command completion.

Advanced Topics

About syntax sugar, specific use cases and shell tricks.

Recommendations & Pitfalls

How to do things well and how to do things very, very badly.

Noteworthy External Tools

Bash is limited, but augmented by a powerful toolset.

General notes

• To run a bash script, looks like one can use bash [file] [args] or [file] [args]
• To exit man page, enter q