Autosave: 2023-01-31 13:55:42

Programming_Languages/Shell/Expansions_and_substitutions.md

@@ -0,0 +1,74 @@
+---
+categories:
+  - Programming Languages
+tags:
+  - shell
+---
+
+# Expansions and substitutions
+
+Bash is weird in that parentheses, braces and brackets are used not just as markers for different code blocks but as the designators of commands in their own right. The type of bracket you use effects how your input is interpreted.
+
+Below are the main forms of expansion and substitution:
+
+| Representation | Name                 |
+| -------------- | -------------------- |
+| `~` .          | Tilde expansion      |
+| `{...}`        | Brace expansion      |
+| `${...}`       | Parameter expansion  |
+| `$(...)`       | Command substitution |
+| `$((...))`     | Arithmetic expansion |
+
+## Brace expansion
+
+Brace expansion is for changing a smaller part of a greater whole programmatically. This is best understood by looking at examples:
+
+```
+echo c{a,o,u}t
+cat cot cut
+```
+
+```
+echo /tmp/{1..3}/file.txt
+/tmp/1/file.txt  /tmp/2/file.txt /tmp/3/file.txt
+```
+
+```
+echo {1..5}
+1 2 3 4 5
+
+echo {5..1}
+5 4 3 2 1
+
+echo {a...c}
+a b c
+```
+
+We can also set sequences. If we wanted to count to twenty in intervals of two:
+
+```
+echo {1..20..2}
+1 3 5 7 9 11 13 15 17 19
+```
+
+> Note that we type _two_ dots **not** an elipsis
+
+## Example use case
+
+We might use brace expansion to generate sequential file names, eg.
+
+```
+touch file_{01..12}{a..d}
+ls
+
+file_01a
+file_01b
+file_01c
+file_01d
+file_02a
+file_02b
+...
+file_12d
+
+
+```

Programming_Languages/Shell/Lists_and_arrays.md

@@ -22,24 +22,6 @@ for ele in $A_STR_LIST; do
 done
 ```
 
-## Brace expansion for listed variables
-
-With a sequence of variables that follow a pattern, for example the natural numbers (1, 2, 3, 4, ...) we can represent them in a condensed format using something called **brace expansion**. For instance to represent the natural numbers from 1 through 10:
-
-```bash
-{1..10}
-```
-
-Here the **two dots** stand for the intervening values.
-
-We can iterate through brace expanded variables just the same:
-
-```bash
-for num in {1..4}; do
-    echo $num
-done
-```
-
 ## Arrays
 
 We define an array as follows:

Programming_Languages/Shell/Redirect_to_dev_null.md

@@ -11,10 +11,9 @@ You'll see the following a lot when reading shell scripts:
 
 ```bash
 [some_command] > /dev/null 2>&1
-
 ```
 
-This is a redirection statement. It is redirecting data to the `null` device on Unix systems. Basically to a black hole or shredder where you can't access it because you don't want it to be output to stout.
+This is a redirection statement. It is redirecting data to the `null` device on Unix systems. Basically to a black hole or shredder where you can't access it because you don't want it to be output to `stdout`.
 
 The `2>&1` argument is the content: any errors that the program may generate and try to show in stout. Notice we are using the [file descriptors](/Programming_Languages/Shell/File_descriptors_and_redirection.md) `1` and `2`.
 

Programming_Languages/Shell/Redirection.md

@@ -15,7 +15,7 @@ The symbol `>` is called the **redirection operator** because it redirects `stdo
 ls | grep d* > result.txt
 ```
 
-### Combining redirection with file descriptors
+### Combining redirection with file escriptors
 
 It is common practice to combine redirection with the [file descriptors](/Programming_Languages/Shell/File_descriptors.md) to redirect the output of `stdout` and `stderr`. A common case is to [redirect error output to `/dev/null`](/Programming_Languages/Shell/Redirect_to_dev_null.md).