---
tags:
  - AI
  - notes
  - coding-notes
  - docs
  - public
---
### 1. Basic Syntax

The standard structure of an `awk` command:

Bash

```
awk [options] 'BEGIN { initialization } pattern { action } END { finalization }' file
```
commands
- **`BEGIN { ... }`**: Runs _once_ before the first line is read. Great for setting variables or printing headers.
    
- **`pattern`**: A condition (regex or math) that must be met for the action to run.
    
- **`{ action }`**: The command(s) executed on each line that matches the pattern.
    
- **`END { ... }`**: Runs _once_ after the last line is processed. Great for printing totals or summaries.
    

---

### 2. Common Command-Line Flags

|**Flag**|**Description**|**Example**|
|---|---|---|
|`-F`|Defines the field separator (default is space/tab)|`awk -F"," '{print $1}' file.csv`|
|`-v`|Assigns an external variable to use inside AWK|`awk -v max=100 '$1 > max' file`|
|`-f`|Reads AWK commands from a script file|`awk -f script.awk input.txt`|
|`-i inplace`|Edits the file in place (requires GNU awk)|`gawk -i inplace '{print $1}' file`|

---

### 3. Built-in Variables

|**Variable**|**Meaning**|**Example Use Case**|
|---|---|---|
|`$0`|The entire current line|`print $0`|
|`$1, $n`|The 1st, 2nd... _n_th field (column)|`print $2, $4`|
|`NR`|Number of Records (Current line number)|`NR > 1` (Skip headers)|
|`NF`|Number of Fields (Total columns in current line)|`print $NF` (Print the last column)|
|`FS`|Input Field Separator (Same as `-F`)|`BEGIN { FS=":" }`|
|`OFS`|Output Field Separator|`BEGIN { OFS="," }`|
|`RS`|Input Record Separator (Default: newline)|`BEGIN { RS="" }` (Paragraph mode)|
|`ORS`|Output Record Separator (Default: newline)|`BEGIN { ORS="\n\n" }` (Double space)|

---

### 4. Operators & Conditionals

**Relational & Logical Operators:**

- `==`, `!=`, `<`, `>`, `<=`, `>=` (Standard math comparisons)
    
- `~` (Matches regular expression)
    
- `!~` (Does not match regular expression)
    
- `&&` (AND), `||` (OR), `!` (NOT)
    

**Control Structures (used inside action blocks):**

```
# If/Else Statement
{ if ($1 > 50) print "Pass"; else print "Fail" }

# For Loop
{ for (i=1; i<=NF; i++) print $i }

# While Loop
{ i=1; while (i<=NF) { print $i; i++ } }
```

---

### 5. Essential Built-in Functions

|**Function**|**Description**|**Example**|
|---|---|---|
|`length(str)`|Returns the length of a string|`length($0) > 80`|
|`tolower(str)`|Converts string to lowercase|`print tolower($1)`|
|`toupper(str)`|Converts string to uppercase|`print toupper($1)`|
|`substr(s,p,l)`|Extracts substring from _s_ starting at _p_ with length _l_|`print substr($1, 1, 3)`|
|`gsub(r,s,t)`|Globally replaces regex _r_ with string _s_ in target _t_|`gsub(/apple/, "orange", $0)`|

---

### 6. Handy Everyday One-Liners

**Printing & Filtering:**

- **Print the last column of every line:**
    
    Bash
    
    ```
    awk '{ print $NF }' file.txt
    ```
    
- **Print lines longer than 80 characters:**
    
    Bash
    
    ```
    awk 'length($0) > 80' file.txt
    ```
    
- **Print lines matching "Error" or "Warning":**
    
    Bash
    
    ```
    awk '/Error|Warning/' file.log
    ```
    

**Math & Summaries:**

- **Sum the values in the first column:**
    
    Bash
    
    ```
    awk '{ sum += $1 } END { print sum }' numbers.txt
    ```
    
- **Calculate the average of the first column:**
    
    Bash
    
    ```
    awk '{ sum += $1 } END { print sum / NR }' numbers.txt
    ```
    
- **Count the number of empty lines:**
    
    Bash
    
    ```
    awk '/^$/ { count++ } END { print count }' file.txt
    ```
    

**Text Manipulation:**

- **Remove duplicate consecutive lines (simulates `uniq`):**
    
    Bash
    
    ```
    awk 'a != $0; { a = $0 }' file.txt
    ```
    
- **Number each line (simulates `cat -n`):**
    
    Bash
    
    ```
    awk '{ print NR, $0 }' file.txt
    ```
    

---