Permissions

Core Idea

Unix permissions say who may read, write, and execute a path.

They are filesystem metadata. Changing permissions does not edit file content; it changes who can use that path and how.

Long Listing Shape

-rw-r--r-- 1 username username 12 Aug 1 10:00 hi.txt
| |  |  |
| |  |  other: read only
| |  group: read only
| owner: read and write
regular file

The first character is the file type. The next nine characters are three permission triplets: owner, group, other.

Bits

Bit	File Meaning	Directory Meaning
`r`	Read file content.	List directory names.
`w`	Write file content.	Add, remove, or rename entries.
`x`	Execute the file as a program or script.	Traverse through the directory.

Directory execute permission is easy to miss. Without it, a user may know a directory exists but cannot enter it or reach paths below it.

Symbolic Changes

chmod u+x script.sh
chmod go-r private-notes.txt
chmod a+r public-page.html

u: owner.
g: group.
o: other.
a: all three classes.
+: add a bit.
-: remove a bit.
=: set exactly these bits.

Octal Modes

Octal modes are the same permission bits written as numbers.

Each permission has a value:

Bit	Value
`r`	4
`w`	2
`x`	1

Add the values in each triplet:

rwx is 4 + 2 + 1 = 7.
rw- is 4 + 2 = 6.
r-- is 4.
--- is 0.

The three digits are owner, group, and other:

Mode	Text Form	Meaning
`644`	`rw-r--r--`	Owner can edit; everyone can read. Common for public files.
`755`	`rwxr-xr-x`	Owner can edit and execute; everyone can read and execute. Common for scripts and public directories.
`600`	`rw-------`	Only owner can read and write. Common for private files.
`700`	`rwx------`	Only owner can enter or execute. Common for private directories.

Use symbolic modes when you are changing one bit deliberately, such as chmod u+x script.sh. Use octal modes when you want to set the full permission shape at once, such as chmod 644 page.html or chmod 755 script.sh.

Directories need x to be entered or traversed, so public web directories usually need 755, not just read permission.

Practice Alone

Use ls -l before and after chmod so the permission text becomes visible.

touch ~/playground/permission-test.txt
ls -l ~/playground/permission-test.txt
chmod go-r ~/playground/permission-test.txt
ls -l ~/playground/permission-test.txt
chmod u+x ~/playground/permission-test.txt
ls -l ~/playground/permission-test.txt

Do this only in scratch files you own.

Common Failures

Permission denied: read the path, owner, group, and bits with ls -l or ls -ld.
Script still fails after chmod +x: executable permission only permits execution; it does not fix syntax or shebang problems.
Directory is visible but unusable: check execute permission on every parent directory.
Public file is still unreachable: check the file and the directories above it.

Done When

You can read owner, group, and other permission triplets.

Docs Pointers

Run man chmod, man ls, and man stat.
Read Filesystem, File, Directory, and Script Permissions.
Read Number Bases: Decimal, Hexadecimal, Octal before using numeric modes such as 644 or 755.