Understanding Unix File Permissions and chmod Explained

The Three Permission Groups

Every file and directory has three permission groups: Owner (the file creator), Group (users in the file's group), and Others (everyone else). Each group gets three permissions.

The Three Permission Types

• Read (r = 4): View file contents or list directory contents
• Write (w = 2): Modify file or create/delete files in directory
• Execute (x = 1): Run file as program or enter directory with cd

Reading Permission Strings

When you run ls -la, you see strings like -rwxr-xr--. Break it into groups of three:

• rwx → Owner: read + write + execute
• r-x → Group: read + execute (no write)
• r-- → Others: read only

Octal (Numeric) Notation

Add the values: r(4) + w(2) + x(1). So rwxr-xr-- becomes 754:

• Owner: 4+2+1 = 7
• Group: 4+0+1 = 5
• Others: 4+0+0 = 4

Common Permission Patterns

• 755 — Standard for executables and directories (owner full, others read+execute)
• 644 — Standard for regular files (owner read+write, others read)
• 700 — Private to owner only (SSH keys, secrets)
• 777 — Full access for everyone (almost never correct — security risk)
• 600 — Owner read+write only (private config files)

Using chmod

Numeric mode: chmod 755 myfile. Symbolic mode: chmod u+x myfile (add execute for user). Both work — numeric is faster for setting all permissions at once.

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