How Chmod Permissions Work
Every Unix file or directory has three permission sets: owner (the user who owns it), group (members of the file's group), and others (everyone else). Each set can independently have read, write, and execute access. This calculator lets you toggle each of the nine permission bits directly, or type an octal value like 755 or a symbolic string like rwxr-xr-x, and keeps every representation in sync.
Octal vs. Symbolic Notation
Octal notation represents each permission set as a single digit 0-7, where read=4, write=2, and execute=1, summed together (so rwx = 4+2+1 = 7). Three digits cover owner, group, and others, like 755. Symbolic notation spells out each bit as a letter or dash across nine characters, like rwxr-xr-x: r for read, w for write, x for execute, and - for no permission. Both represent exactly the same information; which one you use is mostly a matter of convention (chmod 755 file and chmod u=rwx,g=rx,o=rx file do the same thing).
Special Bits Explained (setuid, setgid, sticky)
A fourth, optional octal digit controls three special bits. setuid (4000) makes an executable run with its owner's privileges rather than the invoking user's, shown as a lowercase s in the owner execute position (or uppercase S if execute isn't also set). setgid (2000) does the same for group privileges on files, and on directories makes new files inherit the directory's group instead of the creator's. The sticky bit (1000) restricts deletion in a shared, world-writable directory to each file's own owner, shown as t (or T) in the others position, best known from /tmp's default 1777 permissions.
Common Permission Values and When to Use Them
A handful of values cover the vast majority of real-world cases: 644 for regular files and configs (owner reads and writes, everyone else reads only), 755 for scripts and public directories (adds execute/traverse), 600 for private secrets like SSH keys, and 700 for private executables. The quick reference panel lists these alongside their symbolic form and a plain-English use case, click any row to apply it instantly.
Security Risks: What to Watch For
This calculator actively flags dangerous combinations as you build them, not just after the fact. World-writable files (chmod 777 or any "others: write" bit) let any user on the system modify the file, rarely intentional outside specific shared directories. SUID combined with world-executable is a classic privilege-escalation vector: the file runs with its owner's permissions and literally anyone can trigger that. These warnings appear in the "What this means" panel below the calculator as soon as a risky combination is set. As a file permissions calculator, it works as an octal permissions calculator, a unix permissions calculator, and a linux chmod calculator, doubling as a symbolic permissions converter, a reference for what chmod 755 meaning actually is, a chmod command generator, a guide to setuid setgid sticky bit behavior, and a file permission checker before you apply changes for real.