About the Unix Permission (chmod) Calculator
Unix file permissions are written two ways: octal like 755 and symbolic like rwxr-xr-x. This calculator converts between them and spells out what the permission grants to the owner, the group and others. Enter an octal value and it shows the symbolic form and a plain description; enter a symbolic string and it gives the octal to use with chmod.
It is the tool to reach for when you remember that 644 is right for a file and 755 for a directory but want to confirm exactly what they allow, or when a tutorial gives one notation and your command needs the other. To convert the underlying numbers between bases, the Octal to Decimal tool helps.
Underneath, each triad is three bits worth 4 for read, 2 for write and 1 for execute, added together, so 7 is 4 plus 2 plus 1 and 5 is 4 plus 1. Three triads for owner, group and others fill nine bits, which is exactly three octal digits. A fourth leading digit carries the special bits, 4 for setuid, 2 for setgid and 1 for the sticky bit, which is how /tmp gets its 1777 mode where anyone may write but only the owner of a file may remove it; this calculator describes the last three digits. Execute means something different on a directory, where it grants the right to traverse into the directory at all, which is why 644 on a folder locks you out even though it looks generous. New files are masked on creation as well, since the umask removes permission bits before the mode is applied.
How to use
- Enter an octal value like 755, or a symbolic string like rwxr-xr-x.
- Read the other form and the description.
- Use the octal with chmod.
Common questions
- What does 755 mean?
- rwxr-xr-x: the owner can read, write and execute; group and others can read and execute. Common for directories and scripts.
- What is 644 for?
- rw-r--r--: owner can read and write; others can read. The usual permission for ordinary files.
- Can it read symbolic input?
- Yes, enter rwxr-xr-x and it returns 755.