chmod Numeric Mode

You've seen permissions like 755 and 644. Let's understand where those numbers come from.

The Number System

Each permission has a value:

r (read) = 4
w (write) = 2
x (execute) = 1

Add them up for each user class:

rwx = 4+2+1 = 7
rw- = 4+2+0 = 6
r-x = 4+0+1 = 5
r-- = 4+0+0 = 4

Building Permission Numbers

Three digits: owner, group, others.

755 = rwxr-xr-x
│││
││└── Others: 5 = r-x
│└─── Group:  5 = r-x
└──── Owner:  7 = rwx

Common Permission Numbers

Number	Symbolic	Use Case
`644`	`-rw-r--r--`	Regular files
`755`	`-rwxr-xr-x`	Executables, directories
`600`	`-rw-------`	Private files (SSH keys)
`700`	`-rwx------`	Private directories
`666`	`-rw-rw-rw-`	Rarely used
`777`	`-rwxrwxrwx`	Avoid - too open

Terminal

$chmod 644 document.txt

$chmod 755 script.sh

$chmod 700 ~/.ssh

$chmod 600 ~/.ssh/id_rsa

Numeric vs Symbolic

Same result, different syntax:

Numeric	Symbolic
`chmod 755 file`	`chmod u=rwx,go=rx file`
`chmod 644 file`	`chmod u=rw,go=r file`
`chmod 600 file`	`chmod u=rw,go= file`

Numeric is shorter. Symbolic is more readable.

When to Use Which

Numeric: When you know exactly what you want (755, 644)
Symbolic: When you're adding/removing specific permissions (u+x, g-w)

Memorize These

Most of the time, you only need these:

Number	Memory Aid
`755`	Standard directories, executables
`644`	Standard files
`600`	Private sensitive files
`700`	Private directories

The Calculation

Let's work through chmod 754:

7 = 4+2+1 = rwx (owner)
5 = 4+0+1 = r-x (group)
4 = 4+0+0 = r-- (others)

Result: rwxr-xr--

Going the other way:

rw-r--r-- = ?

Owner:  rw- = 4+2+0 = 6
Group:  r-- = 4+0+0 = 4
Others: r-- = 4+0+0 = 4

Result: 644

Real-World Usage

Web Files

Terminal

$chmod 644 index.html style.css

$chmod 755 uploads/

SSH Keys

Terminal

$chmod 600 ~/.ssh/id_rsa

$chmod 644 ~/.ssh/id_rsa.pub

$chmod 700 ~/.ssh/

SSH is picky about permissions. Too open = won't work.

Scripts

Terminal

$chmod 755 deploy.sh

Knowledge Check

What permission number gives owner full access and everyone else read-only?

Quick Reference

Permission	Number
`---`	0
`--x`	1
`-w-`	2
`-wx`	3
`r--`	4
`r-x`	5
`rw-`	6
`rwx`	7

Key Takeaways

r=4, w=2, x=1 - add them for each class
Three digits: owner-group-others
755 for executables/directories
644 for regular files
600/700 for private files/directories
Numeric is faster, symbolic is clearer

Next: changing file ownership with chown and chgrp.

chmod Numeric Mode

You've seen permissions like 755 and 644. Let's understand where those numbers come from.

The Number System

Each permission has a value:

r (read) = 4
w (write) = 2
x (execute) = 1

Add them up for each user class:

rwx = 4+2+1 = 7
rw- = 4+2+0 = 6
r-x = 4+0+1 = 5
r-- = 4+0+0 = 4

Building Permission Numbers

Three digits: owner, group, others.

755 = rwxr-xr-x
│││
││└── Others: 5 = r-x
│└─── Group:  5 = r-x
└──── Owner:  7 = rwx

Common Permission Numbers

Number	Symbolic	Use Case
`644`	`-rw-r--r--`	Regular files
`755`	`-rwxr-xr-x`	Executables, directories
`600`	`-rw-------`	Private files (SSH keys)
`700`	`-rwx------`	Private directories
`666`	`-rw-rw-rw-`	Rarely used
`777`	`-rwxrwxrwx`	Avoid - too open

Terminal

$chmod 644 document.txt

$chmod 755 script.sh

$chmod 700 ~/.ssh

$chmod 600 ~/.ssh/id_rsa

Numeric vs Symbolic

Same result, different syntax:

Numeric	Symbolic
`chmod 755 file`	`chmod u=rwx,go=rx file`
`chmod 644 file`	`chmod u=rw,go=r file`
`chmod 600 file`	`chmod u=rw,go= file`

Numeric is shorter. Symbolic is more readable.

When to Use Which

Numeric: When you know exactly what you want (755, 644)
Symbolic: When you're adding/removing specific permissions (u+x, g-w)

Memorize These

Most of the time, you only need these:

Number	Memory Aid
`755`	Standard directories, executables
`644`	Standard files
`600`	Private sensitive files
`700`	Private directories

The Calculation

Let's work through chmod 754:

7 = 4+2+1 = rwx (owner)
5 = 4+0+1 = r-x (group)
4 = 4+0+0 = r-- (others)

Result: rwxr-xr--

Going the other way:

rw-r--r-- = ?

Owner:  rw- = 4+2+0 = 6
Group:  r-- = 4+0+0 = 4
Others: r-- = 4+0+0 = 4

Result: 644

Real-World Usage

Web Files

Terminal

$chmod 644 index.html style.css

$chmod 755 uploads/

SSH Keys

Terminal

$chmod 600 ~/.ssh/id_rsa

$chmod 644 ~/.ssh/id_rsa.pub

$chmod 700 ~/.ssh/

SSH is picky about permissions. Too open = won't work.

Scripts

Terminal

$chmod 755 deploy.sh

Knowledge Check

What permission number gives owner full access and everyone else read-only?

Quick Reference

Permission	Number
`---`	0
`--x`	1
`-w-`	2
`-wx`	3
`r--`	4
`r-x`	5
`rw-`	6
`rwx`	7

Key Takeaways

r=4, w=2, x=1 - add them for each class
Three digits: owner-group-others
755 for executables/directories
644 for regular files
600/700 for private files/directories
Numeric is faster, symbolic is clearer

Next: changing file ownership with chown and chgrp.