Bash

Line-by-line reading:

while IFS= read -r line; do
    echo "$line"
done < file.txt

IFS= prevents trimming whitespace; -r prevents backslash escaping; Each iteration gets one complete line

AWK - Pattern Scanning & Processing

How AWK reads: Reads file line by line automatically; Splits each line into fields by whitespace (default); $0 = entire line, $1 = first field, $2 = second field, etc.

Essential AWK patterns:

# Print specific column
awk '{print $2}' file.txt

# Print with custom delimiter
awk -F',' '{print $1, $3}' file.csv

# Pattern matching + action
awk '/error/ {print $0}' log.txt

# Conditional printing
awk '$3 > 100 {print $1, $3}' data.txt

# NR is line number, NF is field count
awk 'NR==5 {print}' file.txt           # 5th line
awk '{print NF, $NF}' file.txt         # field count and last field

awk 'condition {action}' file

SED - Stream Editor

How SED reads: Processes file line by line; Applies pattern/command to each line; By default, prints all lines

# Substitute (first occurrence)
sed 's/old/new/' file.txt

# Substitute (all occurrences on each line)
sed 's/old/new/g' file.txt

# Delete lines matching pattern
sed '/pattern/d' file.txt

# Print only matching lines
sed -n '/pattern/p' file.txt

# Replace line 3
sed '3s/.*/new line/' file.txt

# Delete lines 2-5
sed '2,5d' file.txt

# Edit file in place
sed -i 's/old/new/g' file.txt

sed 's/find/replace/g' file (think: substitute)

---

Python

readline() reads until newline character (\n); readlines() splits file at each \n into a list; Iterating over file object yields one line at a time

Reading Files

# Method 1: Auto-closes file
with open('file.txt', 'r') as f:
    content = f.read()              # entire file as string

# Method 2: Line by line (memory efficient)
with open('file.txt', 'r') as f:
    for line in f:
        print(line.strip())         # strip() removes \n

# Method 3: All lines as list
with open('file.txt', 'r') as f:
    lines = f.readlines()           # ['line1\n', 'line2\n', ...]

Writing Files

with open('file.txt', 'w') as f:    # 'w' = overwrite, 'a' = append
    f.write('Hello\n')
    f.writelines(['line1\n', 'line2\n'])

Split line into fields:

line = "name,age,city"
fields = line.split(',')            # ['name', 'age', 'city']

---

C++

getline() reads until newline (\n), excludes the \n; >> operator reads until whitespace (space, tab, newline); istringstream splits a string by whitespace

Reading Files

#include <fstream>
#include <sstream>
#include <string>
using namespace std;

// Line by line
ifstream file("file.txt");
string line;
while (getline(file, line)) {
    cout << line << endl;
}
file.close();

// Split line into words
string text = "hello world example";
istringstream iss(text);
string word;
while (iss >> word) {               // reads until whitespace
    cout << word << endl;
}

Writing Files

ofstream file("output.txt");        // creates/overwrites
file << "Hello" << endl;
file.close();

// Append mode
ofstream file("output.txt", ios::app);

Split by custom delimiter:

#include <sstream>
string line = "a,b,c";
istringstream ss(line);
string token;
while (getline(ss, token, ',')) {   // split by comma
    cout << token << endl;
}

---

Java

BufferedReader.readLine() reads until \n, excludes it; Scanner.nextLine() reads until \n, excludes it; String.split() divides string by delimiter

Reading Files

import java.io.*;
import java.util.*;

// Method 1: BufferedReader (efficient)
BufferedReader br = new BufferedReader(new FileReader("file.txt"));
String line;
while ((line = br.readLine()) != null) {
    System.out.println(line);
}
br.close();

// Method 2: Scanner (simple)
Scanner sc = new Scanner(new File("file.txt"));
while (sc.hasNextLine()) {
    String line = sc.nextLine();
    System.out.println(line);
}
sc.close();

Writing Files

BufferedWriter bw = new BufferedWriter(new FileWriter("output.txt"));
bw.write("Hello\n");
bw.close();

Split line into fields:

String line = "a,b,c";
String[] parts = line.split(",");   // ["a", "b", "c"]

// Split by whitespace
String text = "hello world";
String[] words = text.split("\\s+"); // \\s+ = one or more spaces

---

Go

bufio.Scanner splits input by newlines (default); Scan() advances to next token; Text() returns current token without newline

Reading Files

package main
import (
    "bufio"
    "fmt"
    "os"
    "strings"
)

func main() {
    // Line by line
    file, _ := os.Open("file.txt")
    defer file.Close()
    
    scanner := bufio.NewScanner(file)
    for scanner.Scan() {                    // reads until \n
        line := scanner.Text()              // excludes \n
        fmt.Println(line)
    }
}

Writing Files

// Write entire content
content := []byte("Hello\nWorld\n")
os.WriteFile("output.txt", content, 0644)

// Buffered writing
file, _ := os.Create("output.txt")
defer file.Close()

writer := bufio.NewWriter(file)
writer.WriteString("Hello\n")
writer.Flush()                              // don't forget!

Split line into fields:

line := "a,b,c"
fields := strings.Split(line, ",")          // []string{"a", "b", "c"}

// Split by whitespace
text := "hello world"
words := strings.Fields(text)               // []string{"hello", "world"}

---

Quick Reference Table

Language	Read Line	Split by Space	Split by Delimiter
Bash	read -r line	$1 $2 $3	IFS=','
Python	line = f.readline()	line.split()	line.split(',')
C++	getline(file, line)	iss >> word	getline(ss, token, ',')
Java	br.readLine()	line.split("\\s+")	line.split(",")
Go	scanner.Text()	strings.Fields()	strings.Split(s, ",")

---

NOTE

• Python: Use with to avoid forgetting .close()
• C++: getline() for lines, >> for words
• Java: BufferedReader is faster than Scanner
• Go: Always defer file.Close() and Flush() writers
• Bash AWK: Fields are $1, $2, $3..., line is $0
• Bash SED: s/find/replace/g (substitute globally)