Autosave: 2023-02-10 07:37:15

Programming_Languages/Shell/Awk.md

@@ -26,7 +26,33 @@ We can also obviously pipe to it.
 
 ## Syntactic structure
 
-An `awk` program consists in a sequence of pattern-action statements and optional functional definitions. `awk` is line-oriented.
+`awk` is a line-oriented language.
+
+An `awk` program consists in a sequence of **pattern: action** statements and optional functional definitions.
+
+For most of the examples we will use this list as the input:
+
+```
+cloud
+existence
+ministerial
+falcon
+town
+sky
+top
+bookworm
+bookcase
+war
+Peter 89
+Lucia 95
+Thomas 76
+Marta 67
+Joe 92
+Alex 78
+Sophia 90
+Alfred 65
+Kate 46
+```
 
 ### Patterns and actions
 
@@ -38,16 +64,18 @@ pattern {action}
 
 A **pattern** is what you want to match against. It can be a literal string or a regex. The **action** is what process you want to execute against the lines in the input that match the pattern.
 
-The following script prints each line of input that contains the word "error":
+The following script prints the line that matches `Joe`:
 
 ```bash
-/error/ {print} file.tx
+awk '/Joe/ {print}' list.txt
 ```
 
-`/error/` is the patttern and `{print}` is the action.
+`/Joe/` is the patttern and `{print}` is the action.
 
 ### Lines, records, fields
 
+![](/_img/awk-outline.png)
+
 When `awk` receives a file it divides the lines into **records**.
 
 Each line `awk` receives is broken up into a sequence of **fields**.
@@ -56,18 +84,66 @@ The fields are accessed by special variables:
 
 - `$1` reads the first field, `$2` reads the second field and so on.
 
-- The variable `$0` refers to the whole recordk
+- The variable `$0` refers to the whole record
 
-### Patterns and actions
+So, in the picture `cloud existence ministerial` corresponse to `$1` `$2` `$3`
 
-### Variables
+## Basic examples
 
-Variables are denoted with a leading `$`. The fields of input are denoted sequentially with `$1, $2, $3...`
+**_Match a pattern_**
-
-This script prints the first and third fields of each line of input:
 
 ```bash
-{ print $1, $2 }
+awk '/book/ { print }' list.txt
+# bookworm
+# bookcase
 ```
 
+**_Print all words that are longer that five characters_**
+
+```bash
+awk 'length($1) > 5 { print $0 }' list.txt
+```
+
+For the first field of every line (we only have one field per line), if it is greater than 5 characters print it. The "every line" part is provided for via the all fields variable - `$0`.
+
+We actually don't need to include the `{ print $0 }` action, as this is the default behaviour. We could have just put `length($1) > 5  list.txt`
+
+**_Print all words that do not have three characters_**
+
+```bash
+awk '!(length($1) == 3)' list.txt
+```
+
+Here we negate by prepending the pattern with `!` and wrapping it in parentheses.
+
+**_Return words that are either three characters or four characters in length_**
+
+```
+awk '(length($1) == 3) || (length($1) == 4)' list.txt
+```
+
+Here we use the logical OR to match against more than one pattern. Notice that whenever we use a Boolean operator such as NOT or OR, we wrap our pattern in parentheses.
+
+**_Match and string-interpolate the output_**
+
+```bash
+awk 'length($1) > 0  {print $1, "has", length($1), "chars"}' list.txt
+
+# storeroom has 9 chars
+# tree has 4 chars
+# cup has 3 chars
+```
+
+**_Match against a numerical property_**
+
+```bash
+awk '$2 >= 90 { print $0 }' scores.txt
+
+# Lucia 95
+# Joe 92
+# Sophia 90
+```
+
+This returns the records where there is a secondary numerical field that is greater than 90.
+
 https://zetcode.com/lang/awk/

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@@ -0,0 +1 @@
+{}