Concept: variables (#700)

* variables concept
* review feedback
* add links
* feedback for the introduction; add Isaac as a contributor; add the about doc
* Blurbs

concepts/commands-and-arguments/.meta/config.json

@@ -3,7 +3,8 @@
     "glennj"
   ],
   "contributors": [
-    "IsaacG"
+    "IsaacG",
+    "kotp"
   ],
-  "blurb": "Types of commands; argument splitting; writing scripts."
+  "blurb": "Bash scripts are written as sequences of commands. Learn about command and arguments, types of commands, and the essentials about writing scripts."
 }

concepts/commands-and-arguments/about.md

@@ -1,4 +1,4 @@
-# Introduction
+# About
 
 ## Shells
 

concepts/commands-and-arguments/links.json

@@ -1,10 +1,10 @@
 [
   {
-    "description": "Commands and Arguments in the Bash Guide",
+    "description": "\"Commands and Arguments\" in the Bash Guide",
     "url": "https://mywiki.wooledge.org/BashGuide/CommandsAndArguments"
   },
   {
-    "description": "Shell Operation in the bash manual",
+    "description": "\"Shell Operation\" in the bash manual",
     "url": "https://www.gnu.org/software/bash/manual/bash.html#Shell-Operation"
   }
 ]

concepts/variables/.meta/config.json

@@ -0,0 +1,9 @@
+{
+  "authors": [
+    "glennj"
+  ],
+  "contributors": [
+    "IsaacG"
+  ],
+  "blurb": "Learn about variables in bash, and some of the variable expansions."
+}

concepts/variables/about.md

@@ -0,0 +1,164 @@
+# About
+
+## Variables
+
+_Variables_ are a place to hold data.
+Bash has two kinds of data: strings and arrays.
+We'll talk about arrays later.
+
+You assign a variable like this
+```bash
+name="value"
+```
+
+~~~~exercism/note
+**Important** -- there must be _no spaces_ around the equal sign!
+
+When there are spaces, like in this example,
+
+```bash
+x = 10
+```
+
+bash is going to call the **command** `x` with the **two arguments** `=` and `10`!
+~~~~
+
+Variables can also be called _parameters_.
+The two terms will be used interchangably in this document.
+
+Some important parameters used by or set by bash include:
+
+* `PATH` -- a colon-separated list of directories used to find external commands.
+* `IFS` -- the "internal field separator".
+   We'll see how it's used below.
+* `PWD` -- your current working directory.
+   It will be the same value as the output of the `pwd` command.
+* `RANDOM` -- a pseudo-random integer between 0 and 32767.
+* `BASH_VERSION` -- the version string of the running instance of bash.
+
+## Variable Names
+
+A variable name consists of _uppercase letters, lowercase letters, numbers and underscore_.
+It cannot start with a number.
+
+~~~~exercism/caution
+It is best practice to **avoid using ALLCAPS variable names**.
+The shell uses this convention for its parameters, and you don't want to accidentally overwrite shell parameters that have special meaning.
+For example: [1][1], [2][2], [3][3]
+
+[1]: https://stackoverflow.com/q/27555060/7552
+[2]: https://stackoverflow.com/q/28310594/7552
+[3]: https://unix.stackexchange.com/q/114596/4667
+~~~~
+
+## Parameter Expansion
+
+You get the value of a parameter with the `$varname` syntax.
+
+```bash
+x=10
+echo "The value of x is $x"
+```
+
+To prevent the variable name from being confused with surrounding text, you can enclose the variable name in braces.
+For example to print the string `10x10`
+
+```bash
+echo "${x}x$x"
+# or
+echo "${x}x${x}"
+```
+
+Some [style guides][google-style-guide] recommend using braces in most cases for readability and/or consistency.
+
+## Positional Parameters
+
+As we discussed in the [Commands and Arguments][cmds-args] lesson, commands can take arguments.
+Bash scripts are no exception.
+The arguments to the current instance of bash are called _positional parameters_.
+They can be retrieved with these special parameters:
+
+* `$1` -- the first positional parameter
+* `$2` ... `$9` -- subsequent parameters
+* `${10}` ... -- double digits require braces.
+* `$0` -- the name of the current script.
+
+Arguments to a shell function are also positional parameters, handled just like this.
+We'll see more about functions in a later lesson.
+
+### Special Positional Parameters
+
+There are some _special parameters_ that help you work with the positional parameters.
+
+* `$#` expands to the _number_ of positional parameters.
+* `"$@"` expands to the list of all of the positional parameters, each as a separate word.
+* `"$*"` expands to a single string of all the positional parameters joined together by a character.
+  * The join character is _the first character_ of the `$IFS` variable, which is a space by default.
+
+The `"$@"` is the safest way to pass positional parameters to another command while keeping them safely quoted.
+It is also what you will use to loop over the positional parameters:
+
+```bash
+for arg in "$@"; do
+    do_something_with "$arg"
+done
+```
+
+In practice, `"$@"` is used more often than `"$*"`.
+
+## Command Substitution
+
+A very frequent operation you will do in bash scripts is to capture the output of a command and store it in a variable.
+To do this, you will use _command substitution_.
+
+```bash
+var=$(command)
+```
+
+For example, to upper-case a string, you can do this:
+
+```bash
+text="Hello world!"
+uppercase=$(echo "$text" | tr '[:lower:]' '[:upper:]')
+echo "$uppercase"
+# => HELLO WORLD!
+```
+
+## More on Parameter Expansion
+
+Bash has many builtin facilities to manipulate variables and strings so that you don't need to call out to external commands.
+
+* Uppercasing text.
+  In the previous section I showed how to use `tr` to upper case some text.
+  To do it in bash:
+
+  ```bash
+  echo "${text^^}"
+  ```
+
+  Similarly, `${text,,}` lowercases the value.
+
+* Provide a default value if the variable is empty.
+
+  ```bash
+  default="nothing here"
+  result=""
+  echo "${result:-$default}"
+  # => nothing here
+  ```
+
+* Search and replace.
+
+  ```bash
+  text="Hello world!"
+  echo "${text//[aeiou]/X}"
+  # => HXllX wXrld!"
+  ```
+
+There are many other interesting parameter expansions: extract a substring, get the string length, _assign_ a default value, remove text from the start or end of the value, and more.
+Read about them [in the manual][param-exp].
+
+
+[google-style-guide]: https://google.github.io/styleguide/shellguide.html
+[cmds-args]: https://exercism.org/tracks/bash/concepts/commands-and-arguments
+[param-exp]: https://www.gnu.org/software/bash/manual/bash.html#Shell-Parameter-Expansion

concepts/variables/introduction.md

@@ -0,0 +1,164 @@
+# Introduction
+
+## Variables
+
+_Variables_ are a place to hold data.
+Bash has two kinds of data: strings and arrays.
+We'll talk about arrays later.
+
+You assign a variable like this
+```bash
+name="value"
+```
+
+~~~~exercism/note
+**Important** -- there must be _no spaces_ around the equal sign!
+
+When there are spaces, like in this example,
+
+```bash
+x = 10
+```
+
+bash is going to call the **command** `x` with the **two arguments** `=` and `10`!
+~~~~
+
+Variables can also be called _parameters_.
+The two terms will be used interchangably in this document.
+
+Some important parameters used by or set by bash include:
+
+* `PATH` -- a colon-separated list of directories used to find external commands.
+* `IFS` -- the "internal field separator".
+   We'll see how it's used below.
+* `PWD` -- your current working directory.
+   It will be the same value as the output of the `pwd` command.
+* `RANDOM` -- a pseudo-random integer between 0 and 32767.
+* `BASH_VERSION` -- the version string of the running instance of bash.
+
+## Variable Names
+
+A variable name consists of _uppercase letters, lowercase letters, numbers and underscore_.
+It cannot start with a number.
+
+~~~~exercism/caution
+It is best practice to **avoid using ALLCAPS variable names**.
+The shell uses this convention for its parameters, and you don't want to accidentally overwrite shell parameters that have special meaning.
+For example: [1][1], [2][2], [3][3]
+
+[1]: https://stackoverflow.com/q/27555060/7552
+[2]: https://stackoverflow.com/q/28310594/7552
+[3]: https://unix.stackexchange.com/q/114596/4667
+~~~~
+
+## Parameter Expansion
+
+You get the value of a parameter with the `$varname` syntax.
+
+```bash
+x=10
+echo "The value of x is $x"
+```
+
+To prevent the variable name from being confused with surrounding text, you can enclose the variable name in braces.
+For example to print the string `10x10`
+
+```bash
+echo "${x}x$x"
+# or
+echo "${x}x${x}"
+```
+
+Some [style guides][google-style-guide] recommend using braces in most cases for readability and/or consistency.
+
+## Positional Parameters
+
+As we discussed in the [Commands and Arguments][cmds-args] lesson, commands can take arguments.
+Bash scripts are no exception.
+The arguments to the current instance of bash are called _positional parameters_.
+They can be retrieved with these special parameters:
+
+* `$1` -- the first positional parameter
+* `$2` ... `$9` -- subsequent parameters
+* `${10}` ... -- double digits require braces.
+* `$0` -- the name of the current script.
+
+Arguments to a shell function are also positional parameters, handled just like this.
+We'll see more about functions in a later lesson.
+
+### Special Positional Parameters
+
+There are some _special parameters_ that help you work with the positional parameters.
+
+* `$#` expands to the _number_ of positional parameters.
+* `"$@"` expands to the list of all of the positional parameters, each as a separate word.
+* `"$*"` expands to a single string of all the positional parameters joined together by a character.
+  * The join character is _the first character_ of the `$IFS` variable, which is a space by default.
+
+The `"$@"` is the safest way to pass positional parameters to another command while keeping them safely quoted.
+It is also what you will use to loop over the positional parameters:
+
+```bash
+for arg in "$@"; do
+    do_something_with "$arg"
+done
+```
+
+In practice, `"$@"` is used more often than `"$*"`.
+
+## Command Substitution
+
+A very frequent operation you will do in bash scripts is to capture the output of a command and store it in a variable.
+To do this, you will use _command substitution_.
+
+```bash
+var=$(command)
+```
+
+For example, to upper-case a string, you can do this:
+
+```bash
+text="Hello world!"
+uppercase=$(echo "$text" | tr '[:lower:]' '[:upper:]')
+echo "$uppercase"
+# => HELLO WORLD!
+```
+
+## More on Parameter Expansion
+
+Bash has many builtin facilities to manipulate variables and strings so that you don't need to call out to external commands.
+
+* Uppercasing text.
+  In the previous section I showed how to use `tr` to upper case some text.
+  To do it in bash:
+
+  ```bash
+  echo "${text^^}"
+  ```
+
+  Similarly, `${text,,}` lowercases the value.
+
+* Provide a default value if the variable is empty.
+
+  ```bash
+  default="nothing here"
+  result=""
+  echo "${result:-$default}"
+  # => nothing here
+  ```
+
+* Search and replace.
+
+  ```bash
+  text="Hello world!"
+  echo "${text//[aeiou]/X}"
+  # => HXllX wXrld!"
+  ```
+
+There are many other interesting parameter expansions: extract a substring, get the string length, _assign_ a default value, remove text from the start or end of the value, and more.
+Read about them [in the manual][param-exp].
+
+
+[google-style-guide]: https://google.github.io/styleguide/shellguide.html
+[cmds-args]: https://exercism.org/tracks/bash/concepts/commands-and-arguments
+[param-exp]: https://www.gnu.org/software/bash/manual/bash.html#Shell-Parameter-Expansion

concepts/variables/links.json

@@ -0,0 +1,10 @@
+[
+  {
+    "description": "\"Parameters\" in the Bash Guide",
+    "url": "https://mywiki.wooledge.org/BashGuide/Parameters"
+  },
+  {
+    "description": "\"Shell Parameters\" in the bash manual",
+    "url": "https://www.gnu.org/software/bash/manual/bash.html#Shell-Parameters"
+  }
+]

config.json

@@ -1223,6 +1223,11 @@
       "uuid": "07e48a7d-7dd0-477c-96e1-23d2558d507d",
       "slug": "commands-and-arguments",
       "name": "Commands and Arguments"
+    },
+    {
+      "uuid": "e29f24ae-3c3b-479a-9333-d733c1ff8fb4",
+      "slug": "variables",
+      "name": "Variables"
     }
   ],
   "key_features": [