Glossary

File permissions

Who may read, write and execute — nine bits per file.

Every file carries a mode: read (r=4), write (w=2) and execute (x=1) for three audiences — the owner, the group, and everyone else. Written in octal, those become the familiar numbers: 755 is rwxr-xr-x, 644 is rw-r--r--.

For directories the letters shift meaning: r lists names, w creates and deletes entries inside, and x grants permission to traverse into it. That last one explains why a directory without x is sealed regardless of what it contains, and why deleting a file depends on the directory's permissions rather than the file's.

See it in practice

$ stat -c "%a %A %U:%G" script.sh

Prints the mode both numerically (755) and symbolically (rwxr-xr-x) plus owner and group — the whole permission picture in one line.

Where you'll meet it