Permissions and ownership

Linux is multi-user: the reactor controls shouldn’t be editable by the tourist who booked a week on the station. Every file has an owner, a group, and permission bits for three audiences. Here’s a line from ls -l:

1-rwxr-x---  1  ada  crew  512  Oct  2 09:00  launch.sh
2│└┬┘└┬┘└┬┘     │    │
3│ │  │  │      │    └── group
4│ │  │  │      └─────── owner
5│ │  │  └── others (everyone else): ---  nothing
6│ │  └───── group (crew):           r-x  read, execute
7│ └──────── owner (ada):            rwx  read, write, execute
8└────────── type: - file, d directory, l symbolic link

chmod (change mode) sets the bits. The symbolic form says who (u user/owner, g group, o others, a all), an operator (+ add, - remove, = set exactly) and which bits:

• chmod u+x launch.sh - let the owner run it
• chmod go-w notes.txt - take write away from group and others
• chmod a=r manual.txt - everyone may read, nobody may write

The octal form gives each audience one digit, adding r = 4, w = 2, x = 1. So rwx is 7, r-x is 5, r-- is 4, and chmod 755 launch.sh means rwxr-xr-x. You’ll see these all the time:

To run a file as ./name, it needs the execute bit. Without it the shell refuses with Permission denied and exit status 126. (./ means “in this directory” - the shell doesn’t search the current directory for commands, which protects you from running a booby-trapped ls someone left lying around.)

A few more permission tools you’ll meet:

• chown ada:crew file changes the owner and group (usually needs administrator rights).
• sudo command runs one command as root, the administrator, after checking you’re allowed. Use it for exactly the commands that need it - never as a reflex when something fails.
• umask sets which bits new files don’t get. The common 022 is why new files start as 644 and new directories as 755.