Update 'posix_shell.md'

posix_shell.md

@@ -1,11 +1,13 @@
 # Shell Cheatsheet
 
-I am only going over POSIX shell things here as there are a bunch
-of extensions found in some shells / on some systems but not others.
+## Introduction
+
+I am only going over POSIX shell things here as there are a bunch of extensions
+found in some shells / on some systems but not others.
 The sections below should apply to any UNIX-like system.
-This cheatsheet will by no means be a comprehensive guide to UNIX
-(it’s a cheatsheet, not a handbook) but you can always find out
-more information about any given command right there on the shell.
+This cheatsheet is by no means a comprehensive guide to UNIX shell scripting
+(it’s a cheatsheet, not a handbook, and I don’t know everything) but you can usually
+find out more information about any given command right there on the shell.
 
 Many systems come with manuals, try `man COMMAND` or
 `info COMMAND` to find out more about most utilities
@@ -14,6 +16,7 @@ Alternatively, if no manual is available,
 passing the flag `-h` or `--help` will usually print some information.
 I find myself consulting these constantly because nobody is expected
 to remember everything beyond the basics and it’s usually faster than googling it.
+That said, googling for a solution is generally also a viable option.
 
 If you are trying to learn how to use the shell, I recommend you start
 with the examples section at the end and work your way backwards from there
@@ -22,7 +25,7 @@ I also highly recommend you check the manuals or help information for
 any command you want to use - especially if it was given to you
 by a stranger on the internet.
 
-## Shell features:
+## Shell Features
 
 The shell’s main tasks are providing a text based user interface to an
 operating system as well as running scripts.
@@ -42,14 +45,14 @@ Relative paths are just the equivalent of an absolute path with the working dire
 In many cases, just the name of a file or subdirectory inside the working directory is used.
 
 You can change to another working directory using `cd` (change directory) like so: `cd PATH_TO_DIRECTORY`.
-When no path is specified, `cd` will take you to the current user’s home directory (usually...).
+When no path is specified, `cd` takes you to the current user’s home directory (usually...).
 
 **Streams**
 
 Normally, there are two output streams and one input stream for a running program:
 Standard output (stdout), standard error (stderr), and standard input (stdin).
 By default, they are passed to the stdout, stderr, and stdin of the shell,
-which will end up in your terminal when you run a shell interactively.
+which end up in your terminal when you run a shell interactively.
 There are two notable exceptions to this which you can specify:
 
 **Redirects**: You can send the output of a command to a file instead of
@@ -82,58 +85,146 @@ They can be added to the environment of subsequent commands using `export VARIAB
 To alter the environment for just one command, prepend it with the variable like so: `VARIABLE=VALUE COMMAND`.
 Variables can be unset using `unset VARIABLE`.
 
-exit code
-if, esle
-while, for
-pipes, redirects
-ctrl-c
+Unknown or unset variables are ignored.
 
-*Builtin commands:*
-- `exit` exit (optionally with an exit code though that feature is more useful for scripts)
-- `jobs` show programs that are currently running in the background
-- `fg` get a program from the background in the foreground
-wait
+**Control Flow**
 
-__Basic commands:__
-- `sh`
-- `export`
-- `unset`
-- `cd` [change directory] (see "Working Directory" section)
-- `pwd` [print working directory] self-explanatory
-- `ls` [list] Get a listing of the specified directory. If none given, the current directory is used.
-- `cat` [concatenate] Originally created to concatenate files, it is most commonly used to print the contents of a file to the standard output. It takes file paths as arguments.
-- `mv` [move] Move or rename a file or directory. It takes two arguments: Origin and destination.
-- `cp` [copy] Copy a file or directoy. It takes two arguments: Origin and destination.
-- `touch`
+When a command finishes running, it produces an exit code that gives some rudimentary status information
+Usually, an exit code of 0 indicates success and anything else an error of some kind.
+
+This can be used in control structures (while loops, conditions) or directly accessed using the special
+shell variable `$?`.
+
+If statements are built as follows:
+```sh
+if COMMAND; then
+    OTHER
+    COMMANDS
+    HERE
+else
+    MORE
+    COMMANDS
+    HERE
+fi
+```
+There are also two operators that can be used as short forms of if and else: `&&` and `||`.
+They can be combined and chained as needed.
+Commands that are chained using `&&` only run if the previous command exited with exit code 0.
+Commands that are chained using `||` run if the previous command *didn’t* exit with exit code 0.
+In many cases, this is used as follows: `COMMAND && THEN_COMMAND || ELSE_COMMAND`
+
+
+While loops are built as follows:
+```sh
+while COMMAND; do
+    OTHER
+    COMMANDS
+    HERE
+done
+```
+The `[ ... ]` seen in many if statements and while loops is not actually part of the shell syntax
+but an alias to the `test` command (see below).
+
+For loops are different in that they act as "for each" instead of checking a condition.
+```sh
+for VARNAME in STRING; do
+    COMMANDS
+    USING $VARNAME
+    HERE
+done
+```
+You can break out of a loop using `break`.
+
+You can end the shell session or script using `exit`. It optionally takes an exit code as argument.
+
+**Job Control**
+
+You can send a process to the background by adding an `&` at the end of the command.
+The `jobs` command shows programs that are currently running in the background.
+You can get a process from the background into the foreground using `fg`.
+Alternatively, if you just want to wait for all jobs to finish, `wait` does that.
+
+**Miscellaneous**
+
+Spaces separate arguments and newlines separate commands. To override this, use quoted strings.
+Single-quoted strings are taken verbatim, variables in double-quoted strings get replaced.
+
+The POSIX shell is case-sensitive but it’s not whitespace-sensitive meaning you can put
+multiple whitespace characters and they count as one or get ignored.
+You can put multiple commands on the same line using semicolons.
+This is commonly used for formatting as it makes scripts more readable.
+
+The built-in `read` command can be used in scripts to read input into a variable.
+
+Command line arguments given to a script end up in the variables `$0` through `$9`.
+They can be moved to the left using `shift`. `$0` starts out being the name of the script itself.
+
+You can send Ctrl+c to the terminal to send an interrupt to the process running inside it.
+Note that this is a feature of the terminal, not of the shell itself.
+Usually, this is used to stop whatever child process of the shell is running in the foreground.
+
+## Commands
+
+This section gives you a list of commonly used UNIX utilities.
+This does not include shell-builtins as I have covered them above already.
+Some shells have built-in versions of some of these commands but they should mostly work
+the same as their dedicated counterparts.
+
+- `sh` (shell) The shell itself, can also be used to invoke scripts
 - `echo` output text to standard output
-- `test` [more commonly known as `[ ... ]`] used to check whether a condition is true or false, result is passed back using exit code
-- `df` [disk free] show available disk space
-- `du` [disk usage] show how much space a given file or directoy is using
-- `su` [switch user] self-explanatory
-- `rm` [remove] remove a file or directoy
-- `mkdir` [make directory] self-explanatory
-- `find` find a file or directoy by specified criteria
+- `pwd` (print working directory)
+- `ls` (list) Get a listing of the specified directory. If none given, the current directory is used.
+- `cat` (concatenate) Originally created to concatenate files, it is most commonly used to print
+  the contents of a file to the standard output.
+- `mkdir` (make directory) self-explanatory
+- `touch` Change the access and modification timestamp of a file. Also commonly used to create empty files.
+- `mv` (move) files or directories
+- `cp` (copy) files or directories
+- `rm` (remove) files or directories
+- `test` (more commonly known as `[ ... ]`) used to check conditions, result is passed back using exit code
+- `true` returns exit code 0
+- `false` returns non-zero exit code
 - `grep` search for regular expressions in text
-- `ps`
-- `id`
-- `uname` [UNIX name] get information about the operating system - usually name, architecture, and build information
-- `chmod` [change mode] change file permissions
-- `chown` [change owner] change file ownership
-- `sed` [stream editor] edit text on the fly
-- `vi` unholy abommination of a text editor :)
+- `tr` (text replace) replace all occurrences of a character in text
+- `sed` (stream editor) edit text on the fly
+- `uname` (UNIX name) get information about the operating system
+- `id` (identity) get information about users
+- `su` (switch user)
+- `ps` (process status) get information about running processes
 - `kill` kill a given process
-- `less` or `more` print only a screen worth of text at a time and wait so you have the time to read it
+- `find` find a file or directoy by specified criteria
+- `chmod` (change mode) change file permissions
+- `chown` (change owner) change file ownership
+- `df` (disk free) show available disk space
+- `du` (disk usage) show how much space a given file or directoy is using
+- `vi` unholy abommination of a text editor :)
+- `more` print only a screen worth of text at a time and wait so you have the time to read it
 
-__Commonly used special files__
-. ..
-dev null
-dev urandom
+## Commonly Used Special Files
+
+- `.` The working directory
+- `..` Parent directory of the working directory
+- `/dev/null` Your personal trash can to redirect all the output you don’t care about to.
+Also known as a black hole or the indefinite void. :)
+- `/dev/urandom` Source of infinite random data
+- `/dev/zero` Source of infinite null bytes
+
+## Honorable Mentions
 
-__Honorable mentions:__
 The following things won’t be available on all systems, but are really handy when they are:
-- `free` shows information about used/free RAM on Linux systems
+
+- `less` better version of `more`
 - `nano` nice user-friendly terminal text editor available on many systems
 - `htop` nice user-friendly terminal task manager
 - `which` easy way to find out where the binary for a given command is located
+- `sudo` become root (or any other user) by authenticating the current user
+instead of the target user given the current user is permitted to do so - useful when root login is disabled
+- `free` shows information about used/free RAM on Linux systems
+- `/dev/stdin`, `/dev/stdout`, and `/dev/stderr` special files for you guessed what
 - `sl` a great way to infuriate anyone who happens to type too quickly
-- `sudo` become root (or any other user) by authenticating the current user instead of the target user given the current user is permitted to do so - useful when root login is disabled
+
+# Examples
+
+Below are some examples that you can use to better understand above material.
+
+... to be finished another time ...