Quick Reference for Linux and C Language Basics

This is a clean version of notes compiled from classroom/self-study, covering commonly used Linux commands, the minimal structure of C programs, input/output, and formatting placeholders, along with directly runnable example code.

1. Quick Reference for Common Linux Commands

• Create Directory

  mkdir lab
  

• Change Directory

  cd lab
  

• View Current Path

  pwd
  

• List Current Directory Contents

  ls
  

• Create Empty File

  touch list
  

• View File Contents

  cat temp
  

• View First 10 Lines of a File

  head -10 temp
  

• Rename / Move File

  mv temp temp1
  

• Delete File

  rm temp
  

• Change Permissions (r=4, w=2, x=1)

  chmod 744 filename
  

• Example:<span><span>744</span></span> means the owner can read, write, and execute, while the group and others can read.

• Clear Screen

  clear
  

2. Minimal Structure of a C Program

• Header Files:

  #include<stdio.h>
  

• Common standard header files are located in <span><span>/usr/include/</span></span>.

• Main Function:

  int main(void) {/* Program entry */return 0;}
  // void indicates "no parameter list"; return 0; usually indicates normal program termination.
  // \n is the newline character.
  

Example 1: Hello, World

#include<stdio.h>
int main(void) {printf("Hello, World!\n");  // \n is the newline
return 0;}

3. Standard Input / Output

Include <span><span>#include</span><span> <stdio.h></span></span>.

• Input Functions:<span><span>scanf()</span></span>, <span><span>fscanf()</span></span>, <span><span>getchar()</span></span>, <span><span>gets()</span></span>*

• Output Functions:<span><span>printf()</span></span>, <span><span>fprintf()</span></span>, <span><span>putchar()</span></span>, <span><span>puts()</span></span>

*Note:<span><span>gets()</span></span> is unsafe; it is recommended to use <span><span>fgets()</span></span> in actual programming.

Common Formatting Placeholders

• <span><span>%d</span></span> : Read/PrintInteger

  int radius;
  scanf("%d", &radius);  // & gets the variable address
  

• <span><span>%f</span></span> : Float

• <span><span>%c</span></span> : Character

• <span><span>%s</span></span> : String

4. Example Code

Example 2: Find Maximum of Two Numbers

#include<stdio.h>
int main(void) {int val1, val2, max;
scanf("%d %d", &amp;val1, &amp;val2);
if (val1 &gt; val2)        max = val1;
else        max = val2;
printf("Largest value is %d.\n", max);
return 0;}

Example 3: Sum of Two Numbers

#include<stdio.h>
int main(void) {int num1, num2, sum;
scanf("%d %d", &amp;num1, &amp;num2);    sum = num1 + num2;
printf("The sum of %d and %d is %d\n", num1, num2, sum);
return 0;}

5. Quickly Create and Edit C Files in Linux (Note Version)

• Create Source File:

  touch main.c
  

• Edit Using Terminal Editor (Example: nano):

  nano main.c
  Edit in nano, then press Ctrl+X → Y → Enter to save and exit.
  

(Optional) If you need to compile and run:

gcc main.c -o main
./main

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