Chmod Calculator

Linux file permissions work the same way as on macOS and other Unix-like systems: every file has three sets of permissions, one for the owner (u), one for the file's group (g) and one for everyone else (o). Each set has three bits. Read is worth 4, write is worth 2 and execute is worth 1, and adding them gives a single digit from 0 to 7. Read plus write is 6, read plus execute is 5, and all three is 7. Put the three digits side by side and you have the octal permissions, so 755 means the owner has 7 (rwx) and group and others have 5 (r-x).

The symbolic form that ls -l prints is the same information spelled out: rwxr-xr-x is 755 and rw-r--r-- is 644. For directories, execute means permission to enter the directory and reach files inside it, and read means permission to list its contents, which is why directories are usually 755 and plain files 644.

An optional fourth digit in front holds the special bits. setuid (4) makes an executable run with its owner's privileges, which is how passwd can update a root-owned file. setgid (2) does the same for the group, and on a directory makes new files inherit the directory's group, which is handy for shared project folders. The sticky bit (1) on a directory, as on /tmp with mode 1777, means only a file's owner can delete or rename it. In ls output these replace the execute slot as s or t, or as S or T when the underlying execute bit is off.

chmod also accepts relative symbolic arguments like g+w or o-r, which change individual bits rather than setting them. As a Unix permissions calculator this tool works in absolute terms, so the symbolic command it shows, like u=rwx,g=rx,o=rx, sets each class exactly and works the same on GNU and BSD chmod.