What is chmod octal notation?

Chmod octal notation uses three digits (e.g., 755) to represent file permissions. Each digit is the sum of permission values: read (4), write (2), and execute (1). The first digit is the owner, the second is the group, and the third is others. So 755 means the owner can read, write, and execute (7), while group and others can read and execute (5).

# Chmod octal values:  4=read  2=write  1=execute
# Sum per group: owner | group | others

chmod 755 script.sh   # rwxr-xr-x — executable scripts
chmod 644 index.html  # rw-r--r-- — regular files
chmod 600 .env        # rw------- — secrets (owner only)
chmod 700 .ssh/       # rwx------ — SSH directory
chmod 664 shared.txt  # rw-rw-r-- — group-writable
chmod 777 /tmp/test   # rwxrwxrwx — avoid in production!

# Calculate: 7 = 4+2+1, 5 = 4+1, 4 = 4
# 755 → owner: rwx, group: r-x, others: r-x

Common use cases

The most common octal values are 644 (owner read-write, everyone else read-only — typical for files), 755 (owner full, everyone else read-execute — typical for directories and scripts), and 600 (owner read-write only — typical for SSH keys and secrets). Knowing these by heart saves time in daily server administration.