chown and chgrp

Owner and group are part of the file

Every file carries a UID and a GID in its inode, right next to the permission bits. ls -l shows them as names in columns three and four. Permissions only make sense relative to them: -rw-r----- protects a file *from others*, but who counts as "owner" and "group" is exactly what chown and chgrp change. Typical reasons: you unpacked a project as root and want alice to own it; a directory should belong to the devs group so the team can share it; a user was deleted and left files behind.

chown: user, user:group, :group

~% cd /root/lab/l48
l48% chown alice notes.txt
l48% chgrp devs shared.txt
l48% chown alice:devs report.txt
l48% chown -R alice:devs project
l48% ls -l
-rw-r--r--    1 alice    root             0 Oct  4 19:20 notes.txt
drwxr-xr-x    2 alice    devs             0 Oct  4 19:20 project
-rw-r--r--    1 alice    devs             0 Oct  4 19:20 report.txt
-rw-r--r--    1 root     devs             0 Oct  4 19:20 shared.txt
l48% ls -l project
-rw-r--r--    1 alice    devs             0 Oct  4 19:20 a.c
-rw-r--r--    1 alice    devs             0 Oct  4 19:20 b.c

chown USER FILE changes only the owner. chown USER:GROUP FILE changes both at once. chown :GROUP FILE changes only the group, which makes chgrp GROUP FILE a convenience rather than a necessity; both exist because people think in both ways. A dot instead of the colon (alice.devs) is also accepted for historical reasons, but the colon is the portable spelling. -R recurses into directories; -c prints each file that changed, which is the right habit for recursive runs; -h changes a symbolic link itself instead of its target.

The rules

Since you are root in the lab you will never hit these limits; on a shared machine chown: Operation not permitted is what an ordinary user sees when trying to give a file to someone else. Note also that chown wipes the setuid and setgid bits on regular files as a safety measure, which you will appreciate after lesson 50.

When ls shows a number

The inode stores numbers; ls -l looks the names up in /etc/passwd and /etc/group at display time. If no entry matches, you see the raw number, as in drwxr-sr-x 2 1000 1000 … /home/bob after deluser bob. The files are intact; only the name is gone. Create a user with that UID, or chown -R the files to someone who exists, and the names come back. chown itself accepts numbers too: chown 1000:1000 file.