Chmod Calculator

Each digit represents one of three permission groups: the first digit is the owner (user who owns the file), the second is the group (users in the file's group), and the third is others (everyone else). Each digit is calculated by adding the values of the enabled permissions: read=4, write=2, execute=1. So 755 means owner has rwx (4+2+1=7), group has r-x (4+0+1=5), others have r-x (5). The maximum value per digit is 7 (rwx).

Both are common for sensitive files, but with a key difference. 644 (rw-r--r--) means the owner can read and write, while the group and others can only read — suitable for non-sensitive config files, web server static files, and documentation. 600 (rw-------) gives read/write only to the owner with no access for anyone else — the correct permission for SSH private keys (~/.ssh/id_rsa), credentials files, and .env files with secrets. Using 644 on a private key will cause SSH to refuse to use it.

For regular files, execute (x) means the file can be run as a program or script. For directories, execute means the ability to traverse (enter) the directory — to cd into it or access files inside it. You typically need execute on every directory in a path to access a file. A common pattern is 755 for directories (owner can do everything, others can read and traverse) and 644 for files inside (owner can read/write, others can read).

Use chmod -R [mode] directory. However, be careful: -R applies the same mode to both files AND directories, which can cause issues if you set 644 recursively (directories need execute to be accessible). A safer pattern is to use find: 'find /path -type f -exec chmod 644 {} \;' for files, and 'find /path -type d -exec chmod 755 {} \;' for directories, applying appropriate permissions to each type separately.

Symbolic mode lets you modify permissions relative to their current state without knowing the full octal value. u=user/owner, g=group, o=other, a=all. +adds, -removes, =sets exactly. Examples: 'chmod u+x file' adds execute for owner only; 'chmod go-w file' removes write from group and others; 'chmod a+r file' makes a file readable by everyone; 'chmod u=rwx,go=rx file' is equivalent to chmod 755. Use symbolic mode in scripts when you want to add/remove specific permissions without overwriting others.