[Day 1] Linux Basics - File Permissions, Vim, Shell Script

1. Basic Commands

Command	Description
pwd	Print current working directory
ls -la	List all files including hidden ones with permissions
cd	Change directory
mkdir	Create directories (multiple at once: mkdir a b c)
touch	Create an empty file
cat	Display file contents

2. File Permissions (chmod)

-rwxr-xr-- → file type / owner / group / others

• r(read) = 4, w(write) = 2, x(execute) = 1 → binary bit values

• Numeric mode: chmod 755 file → owner(rwx) group(r-x) others(r-x)

• Symbolic mode: chmod u+x file → add execute permission to owner

Common Permission Patterns

Number	Meaning	Use Case
755	rwxr-xr-x	Executable files, scripts
644	rw-r--r--	Regular files
600	rw-------	Private files (SSH keys, etc.)
700	rwx------	Private executables

3. Owner / Group / Others

• Owner (user): The person who created the file

• Group: Users belonging to the same group (e.g., docker group)

• Others: Everyone else — not the owner, not in the group

How the OS checks permissions: Is this the owner? → Same group? → Others

groups frog    # Check which groups frog belongs to

4. Vim Basics

Key	Action
i	Enter insert mode
Esc	Return to normal mode
:wq	Save and quit
dd	Delete current line
o	Open new line below + enter insert mode
u	Undo
A	Jump to end of line + enter insert mode
$	Move to end of line
0	Move to beginning of line
w / b	Move forward / backward by word

5. Shell Script

#!/bin/bash          ← shebang: tells the OS to run this with bash
echo "Hello"         ← print to screen
$(whoami)            ← command substitution (returns command output as string)
$(date +%Y-%m-%d)    ← format date output
echo "text" > file   ← redirection: write output to a file

bash myfirst.sh vs ./myfirst.sh

• bash myfirst.sh → bash reads the file as text and executes it. Only needs read(r) permission.

• ./myfirst.sh → OS treats the file itself as an executable. Requires execute(x) permission.

6. Other Useful Commands

• sudo → Run as administrator (root)

• apt update → Refresh package list

• apt install → Install a package

• && → Run next command only if the previous one succeeds

• -y → Auto-confirm installation prompts

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Tomorrow: find, grep, pipe (|), redirection (>, >>)