CHMOD Calculator - Octal and Symbolic Linux Permissions

Every file and directory on a Linux, macOS, or BSD system carries a mode: a small set of bits that decide who may read it, change it, and run or enter it. The bits are grouped into three classes. The owner is the single user account that owns the file. The group is one group the file belongs to, and any user who is a member of that group is judged by the group bits. Everyone else falls into the third class, usually called others or public. The system checks these in order, so if you are the owner you are judged by the owner bits alone, even if the group or others would be granted more.

Within each class there are three permissions, each with a numeric value that comes from its position in binary: read is 4, write is 2, and execute is 1. Because those three values never overlap, adding them can only produce one combination, which is why a single digit from 0 to 7 describes a whole class exactly. Read plus write is 6, read plus execute is 5, and all three is 7. Line up the owner digit, the group digit, and the others digit and you have the familiar three-digit octal number, 644 or 755, that people quote for permissions.

The symbolic string is the same information written out. A directory listing shows it as ten characters: the first marks the type, a dash for a regular file or a d for a directory, and the remaining nine are the three classes in order, each shown as r, w, and x or a dash where the permission is absent. So 644 on a file reads as -rw-r--r-- and 755 on a directory reads as drwxr-xr-x. This calculator shows both forms together so the number and the letters always agree.

The special bits do not get their own column in the symbolic string. Instead they take over the execute position of the class they affect, which is why the same slot can show four different letters. When setuid is set and the owner also has execute, the owner’s execute slot shows a lowercase s. When setuid is set but execute is off, it shows an uppercase S. The same pattern holds for setgid in the group column, and for the sticky bit in the others column, where the letters are t and T instead.

The uppercase form is a genuine warning sign, not a display quirk. A setuid or setgid bit on a file that nobody can execute accomplishes nothing, because the bit only takes effect when the file is run. Seeing an uppercase S usually means someone set a special bit and forgot the execute bit, or stripped execute later and left the special bit stranded. Many calculators quietly render a lowercase letter regardless, hiding the mistake. This one renders exactly what your choices produce, so if you get a capital S or T you can decide whether that is what you meant.

This tool computes the mode; it never changes a file. When you run the command it builds, two habits save trouble. First, be deliberate with the recursive -R flag: it applies one mode to files and directories alike, so a value that is correct for files will strip the execute bit directories need, and a value correct for directories will mark every file executable. When both need setting, target them separately with find . -type f and find . -type d, or use GNU chmod’s capital X, which adds execute only to directories and to files that already have it.

Second, remember that permissions are only half of access; ownership is the other half. If a file is owned by the wrong user or group, no chmod value will make the right person able to use it, and the fix is chownrather than a more permissive mode. Reaching for 777 to “make it work” almost always means the real problem is ownership or a missing directory execute bit, and the open mode just papers over it while creating a security hole.

If you are lining up several changes, the load box turns this into a two-way reference: paste a mode you found in a config file or a tutorial, read back exactly which bits it sets, and adjust from there. For related command-line work you can format and inspect other snippets with the JSON Formatter or decode tokens with the Base64 Converter.