Remove $nothing (nushell#1239)

book/dataframes.md

@@ -1110,7 +1110,7 @@ command categories.
 | is-duplicated   | Series                      | Creates mask indicating duplicated values                                  |                               |
 | is-in           | Series                      | Checks if elements from a series are contained in right series             | in                            |
 | is-not-null     | Series                      | Creates mask where value is not null                                       |                               |
-| is-null         | Series                      | Creates mask where value is null                                           | `<column_name> == $nothing`   |
+| is-null         | Series                      | Creates mask where value is null                                           | `<column_name> == null`       |
 | is-unique       | Series                      | Creates mask indicating unique values                                      |                               |
 | join            | DataFrame                   | Joins a dataframe using columns as reference                               |                               |
 | last            | DataFrame                   | Creates new dataframe with last rows                                       | last                          |

book/hooks.md

@@ -199,7 +199,7 @@ An example for PWD env change hook:
 $env.config = ($env.config | upsert hooks.env_change.PWD {|config|
     let val = ($config | get -i hooks.env_change.PWD)
 
-    if $val == $nothing {
+    if $val == null {
         $val | append {|before, after| print $"changing directory from ($before) to ($after)" }
     } else {
         [

book/types_of_data.md

@@ -480,7 +480,7 @@ print $x
 
 ## Null
 
-Finally, there is `null` (also known as `$nothing`) which is the language's "nothing" value, similar to JSON's "null". Whenever Nushell would print the `null` value (outside of a string or data structure), it prints nothing instead. Hence, most of Nushell's file system commands (like [`save`](/commands/docs/save.md) or [`cd`](/commands/docs/cd.md)) produce `null`.
+Finally, there is `null` which is the language's "nothing" value, similar to JSON's "null". Whenever Nushell would print the `null` value (outside of a string or data structure), it prints nothing instead. Hence, most of Nushell's file system commands (like [`save`](/commands/docs/save.md) or [`cd`](/commands/docs/cd.md)) produce `null`.
 
 You can place `null` at the end of a pipeline to replace the pipeline's output with it, and thus print nothing:
 

lang-guide/lang-guide.md

@@ -173,7 +173,7 @@ The second syntax is simply a list of records. This plays on the Nushell data mo
 
 ### Nothing
 
-The type or value `$nothing` is to be used to represent the absence of another value. Commands that explicitly do not return a value return `$nothing`
+The type `nothing` is to be used to represent the absence of another value. Commands that explicitly do not return a value return `null`.
 Use as a missing value indicator.
 
 ### Error
@@ -271,7 +271,7 @@ if false {
 }
 ```
 
-The `else` part of `if` is optional. If not provided, if a condition is false, the `if` expression returns `$nothing`.
+The `else` part of `if` is optional. If not provided, if a condition is false, the `if` expression returns `null`.
 
 The code that follows the `else` is an expression rather than a block, allowing any number of follow-on `if` expressions as well as other types of expressions. For example, this expression returns 100: `if false { 1 } else 100`.