Unix Permissions Calculator

Unix permissions control who can read, write, and execute a file or directory. They are divided into three principals: Owner (the user who owns the file), Group (a set of users sharing access), and Others (everyone else). Each principal gets three permission bits: Read (r), Write (w), and Execute (x). For files, execute means the file can be run as a program. For directories, read means you can list its contents, write means you can create or delete files inside it, and execute means you can enter it and access files within.

Octal notation represents Unix permissions as a 3-digit (or 4-digit for special bits) number in base 8. Each principal's three bits map to a single octal digit: Read = 4, Write = 2, Execute = 1. Add the values together: rwx = 4+2+1 = 7, rw- = 4+2+0 = 6, r-x = 4+0+1 = 5, r-- = 4+0+0 = 4. The three digits then represent Owner, Group, and Others — so 755 means Owner=rwx, Group=r-x, Others=r-x. This is the format used by chmod: chmod 755 myfile.

Symbolic notation uses letters to represent permissions. The full 9-character string shows Owner, Group, and Others in order: rwxr-xr-x means Owner has rwx, Group has r-x, Others has r-x. A dash (-) means the bit is not set. The ls -l command shows a 10-character string: the first character is the file type (- for file, d for directory, l for symlink), followed by the 9 permission bits. chmod also accepts symbolic modifiers: chmod u+x adds execute for Owner, chmod go-w removes write from Group and Others.

Unix has three special permission bits beyond the standard rwx. SetUID (SUID, 4000): when set on an executable, the program runs with the file owner's privileges rather than the caller's — used by commands like sudo and passwd. SetGID (SGID, 2000): on executables, the program runs with the file's group privileges; on directories, new files created inside inherit the directory's group instead of the creator's. Sticky bit (1000): on a directory, only the file's owner (or root) can delete or rename their files — commonly set on /tmp. In ls -l, these appear as s (SUID/SGID replacing x) or t (sticky replacing x for others).

chmod followed by the octal value sets permissions directly: chmod 644 file.txt sets rw-r--r--, chmod 755 script.sh sets rwxr-xr-x, chmod 600 ~/.ssh/id_rsa sets rw------- (private key). For special bits, prefix a fourth digit: chmod 1755 /tmp sets sticky+755, chmod 4755 /usr/bin/sudo sets SUID+755. To apply recursively: chmod -R 755 /var/www. A safer recursive approach uses find: find /var/www -type f -exec chmod 644 {} ; and find /var/www -type d -exec chmod 755 {} ; so files and directories get appropriate separate values.