Chmod Calculator

A two-way calculator for Unix file permissions. Type an octal like 755 or 4755, or click the grid of read, write and execute boxes for owner, group and others, and the symbolic form updates live. The panel gives you a chmod command, a matching find -perm expression and the equivalent umask, so whichever one your shell session wants is there to copy.

The special bits are where the symbolic notation gets subtle, and the tool renders them the way ls does. Setuid (4000), setgid (2000) and the sticky bit (1000) each replace the execute character in their own column: lowercase s or t when execute is also set, uppercase S or T when it is not. That capital letter is almost always a mistake worth catching — 4644 shows rwSr--r--, a file marked setuid that nothing can execute, which does nothing except look alarming in an audit.

What each special bit actually does is worth keeping straight, because only one of the three means the same thing everywhere. Setuid on an executable runs it as its owner, and Linux deliberately ignores it on shell scripts, so a setuid .sh is not the security hole it looks like — it simply has no effect. Setgid on a directory is the genuinely useful one: new files inside inherit the directory's group rather than the creator's, which is how a shared project directory stays shared. The sticky bit on a directory, as on /tmp, means only a file's owner, the directory's owner or root can delete it, regardless of the directory's write permission.

The umask shown is the complement of the mode against 777, which is the number you type but not always the mode you get. Umask subtracts, and the starting point differs by kind: directories start from 777 and files from 666, because files are not given the execute bit by default. So a umask of 022 produces 755 directories and 644 files from the same setting. And the standing warning: chmod 777 is almost never the fix. It is the permission of last resort, it makes the files it is applied to readable, writable and executable by every user and process, and the problem it appears to solve is usually ownership or a missing execute bit on a parent directory.